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Osmoregulation in elasmobranchs: A review for fish biologists, behaviourists and ecologists
Abstract
This article provides a broad review of osmoregulation in elasmobranchs for non-specialists, focusing on recent advances. Marine and euryhaline elasmobranchs in seawater regulate urea and other body fluid solutes (trimethylamine oxide (TMAO), Na þ , Cl À) such that they remain hyper-osmotic to their environment. Salt secretions of the rectal gland and excretions in the urine compensate for continuous inward diffusion of environmental salts. Freshwater and euryhaline elasmobranchs in fresh water synthesise less urea and retain less urea and other body fluid solutes compared to marine elasmobranchs and thus have relatively lower osmolarity. Electrolyte uptake at the gills and kidney reabsorption of salts maintain acid–base balance and ionic consistency. The role of the gills, kidney, liver and rectal gland in elasmobranch osmoregulation is reviewed. The ontogeny of osmoregulatory systems in elasmobranchs and the contribution of drinking and eating processes in maintaining osmotic consistency are discussed. Recommendations for future research are presented.
... To generate osmoregulatory capacity and homeostasis maintenance metrics, urea and phosphorus concentrations were determined in serum. Sharks are ureotelic and urea plays a key role in osmoregulation (Hammerschlag, 2006). When allostatic overload occurs and homeostatic balance is lost, alterations in urea concentrations are observed (Wosnick et al., 2017). ...
... The rectal gland plays an important role in shark osmoregulation and associated homeostatic balance (Hammerschlag, 2006;Ballantyne and Fraser, 2012). The accumulation of Hg in the rectal gland found herein (Table 1) may be related to its high vascularization, suggesting target organ concerning metal toxicity. ...
... One of the main roles of gills in sharks is to maintain the acid-basic balance and prevent systemic acidosis or alkalosis (Shuttleworth, 1988). Additionally, gills play a major role in urea regulation, the main osmolyte for elasmobranch homeostatic balance (Hammerschlag, 2006). Traditionally, Na + /K + ATPase activity and expression are used to evaluate the sublethal effects of exposure to metals in the gills and rectal gland (De Boeck et al., 2001Grosell et al., 2003;Eyckmans et al., 2013). ...
Contamination by metals is among the most pervasive anthropogenic threats to the environment. Despite the ecological importance of marine apex predators, the potential negative impacts of metal bioaccumulation and biomagnification on the health of higher trophic level species remains unclear. To date, most toxicology studies in sharks have focused on measuring metal concentrations in muscle tissues associating human consumption and food safety, without further investigating potential impacts on shark health. To help address this knowledge gap, the present study evaluated metal concentrations in the gills, muscle, liver, and rectal gland of coastal sharks opportunistically sampled from Brazilian waters and tested for potential relationships between metal bio-accumulation and general shark health and homeostatic balance metrics. Results revealed high metal concentrations in all four tissue types, with levels varying in relation to size, sex, and life-stage. Metal concentrations were also associated with serum biomarkers (urea, lactate, ALT, triglycerides, alkaline phosphatase, and phosphorus) and body condition, suggesting potential negative impacts on organismal health.
... They normally maintain their blood osmolarity slightly higher than their environment. This is accomplished by retaining high levels of solutes, such as sodium, chloride, urea and trimethylamine oxide (TMAO) (Evans et al. 2004;Hammerschlag 2006;Anderson et al. 2007). By remaining hyperosmotic compared to the environment, they have less water loss and thus avoid dehydration (Hammerschlag 2006). ...
... This is accomplished by retaining high levels of solutes, such as sodium, chloride, urea and trimethylamine oxide (TMAO) (Evans et al. 2004;Hammerschlag 2006;Anderson et al. 2007). By remaining hyperosmotic compared to the environment, they have less water loss and thus avoid dehydration (Hammerschlag 2006). Because they have the ability of regulating their electrolyte and urea plasma concentrations based on their environment, to higher salinities. ...
... Initially being in an environment with a higher salinity may have resulted in higher concentrations of these ions until the rectal gland could excrete adequate amounts to regulate to the new environment. The kidney is involved with sodium and chloride movement, although to a lesser extent, as well as urea reabsorption and clearance (Evans et al. 2004;Hammerschlag 2006). Some marine elasmobranchs seem to acclimate to lower salinities, not only by increasing urine flow (thus eliminating urea, sodium and chloride), but also possibly by decreasing urea synthesis in the liver (Hazon et al. 2003;Tam et al. 2003;Anderson et al. 2005). ...
... Physiology limits animal distributions (McNab, 2002;Pearse, 1922;Somero, 2012), including stenohaline sharks in coastal ecosystems due to salinity thresholds (Hammerschlag, 2006;McCandless et al., 2007;Whitfield et al., 2012). Juvenile blacktips exhibit a preference for salinities >20 ppt across the Gulf of Mexico (e.g., McCandless et al., 2007;Ward-Paige et al., 2015) and >25 ppt in our study area . ...
... Thus, the distribution of age 0 blacktips should reflect their habitat preferences. Yet, it is unclear if the primary benefit afforded to age 0 blacktips by environmental predictability was physiological homeostasis (Evans et al., 2004;Hammerschlag, 2006;Steiner & Michel, 2007). Comparatively, other factors, like predictability in prey availability or risk attributed to stable salinities, could be more important if prey or predators prefer environmentally stable habitats, thus juvenile blacktips may prefer stable salinities because conditions ecologically optimize growth and survival (Drymon et al., 2013;Lima & Dill, 1990;Speed et al., 2010;Werner & Gilliam, 1984). ...
Coevolution with predators leads to the use of low‐risk habitats by many prey species, which promotes survival during early developmental phases. These nurseries are valued by conservation and management agencies because of their contributions to adult populations. However, the physical and geographic characteristics, like shallow depths and isolation from other marine habitats, that restrict access to predators and thereby reduce risk to juvenile animals can also limit scientific research. Consequently, many nursery habitats are still unidentified and understudied. Here, we used gillnet monitoring from 1982‐2018 to delineate blacktip shark (Carcharhinus limbatus) nurseries in the northwestern Gulf of Mexico, and elucidate their physical, environmental, and biological characteristics. Nursery habitats within estuaries (<2% of spatial area) were proximate to the Gulf of Mexico, and exhibited significantly lower variability in salinity than non‐nurseries. However, relative abundances of predators and prey were not significant delineators of nursery habitats. As such, food and risk may not influence juvenile blacktip habitat use as expected. Alternatively, reduced osmoregulatory stress attributed to predictable environments likely provides advantageous conditions for blacktips to develop foraging and anti‐predator tactics, which is vital prior to the winter migration of juvenile sharks into the Gulf of Mexico. This article is protected by copyright. All rights reserved.
... They use organic osmolytes (urea and trimethylamine oxide) to elevate plasma osmolality to values usually slightly above seawater, and sodium and chloride have a lesser role as the principal extracellular osmolytes. Thus, elasmobranch fishes are typically isosmotic or slightly hyperosmotic to surrounding seawater, primarily attributable to retention of urea (Hammerschlag 2006;Ballantyne 2016). Only the coelacanth among other fishes accumulates urea and TMAO to maintain isosmotic plasma similarly to elasmobranchs (Takei 2000). ...
... Although marine elasmobranchs are not characteristically regarded to drink seawater (and the same could be assumed for coelacanths), euryhaline species drink upon returning to more concentrated seawater as with teleosts (Anderson et al. 2007;Evans and Claiborne 2009). In all sharks, the rectal gland is the principal site of NaCl secretion (Epstein and Silva 2005), the kidneys retain urea, and TMAO (Cohen et al. 1958;Janech et al. 2006), and the gills are the site of acid-base regulation as well as salt uptake in dilute seawater or fresh water (Hammerschlag 2006;Evans and Claiborne 2009;Reilly et al. 2011). ...
Acquisition of fresh water is important to animals, and is both difficult and critical for species residing in marine environments. Adaptive radiations to fully marine habitats were constrained by the need for fresh water and the capacity of various taxa to adapt physiology to reliance on sources of water other than free drinking water. Here, we review the water relations of marine vertebrates, with an emphasis on drinking and the need to procure fresh water. Numerous marine teleost fishes drink seawater, but some do not, and drinking is more variable and complex than suggested by textbooks. The mechanisms by which fishes and other vertebrates regulate water balance involve the renin–angiotensin and aldosterone endocrine systems, but plasma osmotic and ionic concentrations as well as other chemical signals can also be involved. Multiple mechanisms for stimulation of drinking are operative and diverse among species. Clearly, evolutionary adaptations to environmental salinities can alter drinking behaviors. Marine elasmobranchs do not characteristically drink seawater, but euryhaline species drink upon returning to more concentrated seawater, as with teleosts. Hagfish are osmoconformers, and there is no evidence for drinking. In general, marine reptiles and most marine mammals and seabirds do not drink seawater. Exceptions include sea turtles, cetaceans, and some pinnipeds. Some marine species (e.g., sea snakes) require fresh water that can be acquired from ephemeral rainwater lenses, while others are adapted to utilize dietary and metabolic water. Regardless of drinking behaviors, numerous forms have evolved varied strategies for conserving water while reducing its losses to the surrounding sea.
... Finally, the Mediterranean diet group significantly increased fish intake after intervention. It is possible that this elevated fish consumption could have contributed to the lack of effect on TMAO concentration because salt-water fish produce TMAO for use in osmotic regulation 49 . This elevated fish consumption could have caused an increase in pre-formed TMAO consumption in this group, assuming that the types of fish predominantly consumed were salt-water varieties. ...
... Another area worthy of exploration is the impact of fish consumption as part of a Mediterranean diet on TMAO levels. It was not possible to study the effects of salt-water fish consumption in this study, but given that this is a potential source of pre-formed TMAO49 , it would be beneficial to investigate the effects of Mediterranean diets with and without fish on TMAO concentrations.CONCLUSIONThis study supports previous findings that serum TMAO concentrations are increased by age, metabolic syndrome risk factors and systemic exposure to lipopolysaccharide. Six months of dietary intervention with either Healthy Eating or Mediterranean goals, however, did not alter fasting TMAO concentration in a population of subjects at elevated risk for colon cancer. ...
Elevated circulating levels of trimethylamine N-oxide (TMAO) has been identified as a risk factor for numerous diseases, including cardiovascular disease (CVD) and colon cancer. TMAO is formed from trimethylamine (TMA)-precursors such as choline via the combined action of the gut microbiota and liver. We conducted a Mediterranean diet intervention that increased intakes of fiber and changed intakes of many other foods containing fat to increase the relative amount of mono-unsaturated fats in the diet. The Mediterranean diet is associated with reduced risks of chronic diseases and might counteract the pro-inflammatory effects of increased TMAO formation. Therefore, the purpose of this study was to determine if the Mediterranean diet would reduce TMAO concentrations. Fasting TMAO concentrations were measured before and after six-months of dietary intervention in 115 healthy people at increased risk for colon cancer. No significant changes in plasma TMAO or in the ratios of TMAO to precursor compounds were found in either the Mediterranean group or the comparison group that followed a Healthy Eating diet. TMAO concentrations exhibited positive correlations with age and markers of metabolic health. TMAO concentrations were not associated with circulating cytokines, but the relative abundance of Akkermansia mucinophilia in colon biopsies was modestly and inversely correlated with baseline TMAO, choline, and betaine serum concentrations. These results suggest that broad dietary pattern intervention over six months may not be sufficient for reducing TMAO concentrations in an otherwise healthy population. Disruption of the conversion of dietary TMA to TMAO should be the focus of future studies.
... Trimethylamine oxide (TMAO) is a small compound, accumulated as an osmolyte by a variety of marine organisms Bickel, 1969; see Seibel and Walsh, 2002;Yancey, 2005 for reviews). It serves an additional role as a universal protein stabilizer (Yancey and Siebenaller, 1999;Yancey et al., 2001Yancey et al., , 2004, counteracting perturbations due to salinity (Pillans et al., 2005;Hammerschlag, 2006;Deck et al., 2016), temperature (Raymond and DeVries, 1998;, hydrostatic pressure (Yancey et al., 2002;Bockus and Seibel, 2016) and urea (Somero, 1986;Baskakov et al., 1998;Yancey, 2001;Zou et al., 2002). TMAO is held at exceptionally high levels in elasmobranchs (sharks, skates and rays) that retain urea, a nitrogenous waste compound and macromolecular destabilizer, as their primary osmolyte (Smith, 1929;Forster and Goldstein, 1976;Withers, 1998;Trischitta et al., 2012). ...
... The body fluids of marine elasmobranchs are generally maintained slightly hyper-osmotic to the surrounding seawater. However, TMAO is preferentially accumulated inside the cell and contributes to a larger portion of the total osmotic pressure in the muscle and liver than it does in the plasma, which exhibits high concentrations of inorganic salts, such as Na + and Cl − (Hammerschlag, 2006). In contrast, urea readily diffuses across the cell membrane and is found at similar concentrations in both the intraand extracellular compartments. ...
... In comparison, the salinity range of bull sharks has been studied in depth because bull sharks are euryhaline and can tolerate a wide range of salinities (reviewed in Hammerschlag, 2006). Overall, bull sharks have been observed in waters from 0 -42 ppt (Snelson & Williams, 1981;Thorburn & Rowland, 2008) though specific sex and lifestage specific trends can influence habitat choice within this range. ...
... In comparison to great hammerhead and tiger sharks, the environmental tolerances of bull sharks have been very well studied because bull sharks are euryhaline and are able to survive in a wide range of salinities (Hammerschlag, 2006). Their ability to withstand a wide salinity range means that bull sharks can move into environments that are either hyper-or hyposaline to the open ocean, including estuary and mangrove habitats (e.g., Werry et al., 2012). ...
Aquatic highly migratory species (HMS) are economically and ecologically important, however, their highly migratory nature makes them difficult to study and thus there are knowledge gaps relating to their movement and habitat use patterns. Highly migratory sharks are likely to interact with commercial longline fishing gear and be caught as target or bycatch, which can threaten their populations. Understanding the environmental factors that influence and drive the movements of highly migratory sharks may help researchers better predict their presence and subsequently identify areas where they are vulnerability to fisheries. Here I evaluated the overlap between habitat suitability and gear restricted zones for three co-occurring apex predatory sharks in the Southwest Atlantic Ocean and Gulf of Mexico (great hammerhead Sphyrna mokarran, tiger Galeocerdo cuvier, and bull sharks Carcharhinus leucas) to identify areas in this region where these species are vulnerable to and protected from commercial longline fishing. This research was accomplished in three integrated steps. First, I reviewed and summarized what is known about the environmental drivers of great hammerhead, tiger,
and bull shark habitat use and movement patterns. Second, I used the results of this review to parameterize and subsequently generate habitat suitability models for these three species. Third, I used these models to spatially compare where each species’ highly suitable habitat overlaps with longline gear restricted areas within the Southwest Atlantic Ocean and Gulf of Mexico, to identify regions where these species were both vulnerable to and protected from longline fishing gear. The results of this thesis have implications to the management of these species as well as for the conservation of other highly migratory aquatic species.
... comm.). However, chondrichthyan blood salinity is 35-37 ppt so decreasing salinity is not advised (Hammerschlag 2006). ...
... Osmolarity is of primary importance for all sperm holding media, with inappropriate media resulting in hypoosmotic membrane swelling, or hyperosmotic crenation, of sperm cells. For sharks, it is expected that seminal fluids would have a similar osmolarity as serum and the surrounding environment, and therefore, isotonic solutions with osmolarities between 800 and 1000 mOsm would be better suited for liquid storage of sperm [39,[52][53][54]. We clearly saw negative effects of hypoosmotic stress as indicated by significantly decreased normal morphology with decreased tonicity of the media. ...
Three experiments were performed to evaluate the effects of osmolarity, temperature, media and cryoprotectants on white spotted bamboo shark (Chiloscyllium plagiosum) spermatozoa. Semen was collected from 7 different males using a massage technique with the animals held in tonic immobilization with gills submerged. A Computer Assisted Sperm Analysis (CASA) system was used to assess sperm motility. Experiment one evaluated the effects of Hank’s balanced salt solution (HBSS) with different osmolarities (200, 400, 800 and 1000 mOsm Kg¹) and incubation temperatures (5 °C and 24 °C [room temperature]) over time (6 h, 12 h and 24 h). Membrane integrity and morphology were degraded (P < 0.05) over time; however, sperm extended with HBSS 1000 and HBSS 800 were least affected. Higher total motility (TM) and average path velocity (VAP, P > 0.05) sperm parameters were observed with HBSS 1000 and HBSS 800, respectively. In addition, TM, VAP, and curvilinear velocity (VCL) were all reduced (P < 0.05) compared to sperm held at 5 oC. Experiment two compared the effects of sperm diluted in crystal reef water (CRW), elasmobranch ringers (ER) or raw held at 5 oC. Results indicated that CRW had improved viability, TM, VAP, VSL, VCL, and straightness (STR) when compared to sperm diluted in ER or Raw at 5 oC. The final experiment compared different cryoprotectants (dimethyl sulfoxide [DMSO] 10%, DMSO 20%, Ethylene-glycol [EG] 10%, EG 20%, and Trehalose [Tre] 10%, Tre 20%) during incubation at 5 °C. Across all time points, sperm incubated in Tre (10 or 20%) had increased (P < 0.05) membrane integrity and normal morphology compared to all other media and raw. Across all time points, sperm exposed to Tre (10% and 20%) and 10% DMSO had significantly improved TM (P < 0.05) compared to EG (10% and 20%) and 20% DMSO which were similar to raw. Although sperm collected from WSBS are robust and appear to tolerate a wide range of osmotic conditions and media, media made at 1000 mOsm Kg¹ should provide the best in-vitro osmotic pressure, while storing at cooler temperatures should improve storage survival. For future cryopreservation studies, media using TRE and DMSO as cryoprotectants should be evaluated.
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
... Approximately 50% of osmotic pressure on blood cells and tissues is induced by the salt content. In blood of sharks, accumulation of significant amount of urea and trimethylamine oxide, diffusing poorly through the cells of kidneys and gills, which increases the osmotic pressure, is observed, (Hammerschlag, 2006). ...
The results of hematological, cytochemical, and biochemical surveys for fish blood in the specimen Ocellate river stingray Potamotrygon motoro and two specimens of cat sharks including the Halmahera epau-lette shark Hemiscyllium halmahera and the grey bambooshark Chiloscyllium griseumare have been reported. A comparison in the parameters analyzed in the survey between the common carp Cyprinus carpio, the wels catfish Silurus glanis, and the sterlet Acipenser ruthenus has been performed. A high percentage of eosinophils involved in antiparasite immunity in the Cartilaginous fishes is recorded. A high content of cationic protein in lysosomes of neutrophils in these fish species can indicate a considerable potent phagocytic activity of the latter. A low amount of alanine aminotransferase in the analyzed shark and ray specimens compared to the bony fish was recorded, which indicate the strength of membranes in hepatocytes and cardiomyocytes.
... Marine and euryhaline elasmobranchs in SW regulate urea and other body fluid solutes [trimethylamine oxide (TMAO), Na + , Cl − ] such that they remain iso-or slightly hyperosmotic to their environment (Hammerschlag, 2006). While little information is available, it seems that the thyroid axis may contribute to elasmobranch osmoregulation. ...
This thesis aimed to understand the role that the hypothalamus-pituitary-thyroid (HPT) axis plays in appetite regulation of goldfish (Carassius auratus). I altered nutritional and thyroid statuses to measure the response of thyroid axis components and appetite-regulating peptides. I predicted that fasting would downregulate the thyroid axis and trigger an orexigenic response, while overfeeding would upregulate the thyroid axis and trigger an anorexigenic response. Additionally, I predicted that hyperthyroid conditions would lead to negative feedback of the thyroid axis and an orexigenic response, whilst opposite under hypothyroid conditions. I uncovered for both experiments that the thyroid axis in goldish is most responsive to overfeeding and hyperthyroidism. Overfeeding led to a time-dependent increase in central thyroid transcripts while fasting decreased thyroid hormone degradation peripherally with no central response, no treatment altered levels of thyroid hormone in circulation. Hyperthyroidism resulted in negative feedback to the pituitary, but not hypothalamus, and did not lead to an increase in food intake despite an increase in the levels of thyroxine. The thyroid inhibitor, propylthiouracil, did not induce hypothyroidism or alter the expression of any thyroid axis transcript. Appetite-regulating peptides correlated weakly to changes in the thyroid, suggesting an overall poor association in goldfish between appetite regulation and thyroid status.
... Ni is moderately toxic to aquatic organisms, and exerts its toxicity through different cellular pathways, mainly disruption of essential metal homeostasis (e.g., Ca, Mg, and Fe) and the generation of Reactive Oxygen Species (ROS) (Brix et al., 2017), similarly noted for V, also a redoxactive metal. Vanadium contamination, in particular, may be of significant concern in elasmobranchs, as the main excretion route for this element is urine, and elasmobranch osmoregulation is carried out by increasing or decreasing blood urea concentrations (Hammerschlag, 2006). No toxicity thresholds or detoxification mechanisms have been established for these elements in elasmobranchs. ...
This study comprises a novel report on subcellular metal partitioning and metallothionein (MT) metal detoxification efforts in lesser numbfish (Narcine brasiliensis) electric ray specimens, as well as the first assessment on MT contents in any ray electric organ. Individuals sampled from an area in Southeastern Brazil affected by the Mariana dam rupture disaster were assessed concerning subcellular metal partitioning and MT metal-detoxification in the liver, gonads, electric organ and muscle of both adults and embryos. Yolk was also assessed when available. Relative total and heat-stable (bioavailable) metal and metalloid comparisons between adults and embryos in different developmental stages demonstrates maternal transfer of both total and bioavailable metals and significant MT associations demonstrate the detoxification of As, Ag, Mn, Ni, Cd, Co, Cu, Se and V through this biochemical pathway. Our findings expand the lacking ecotoxicological assessments for this near-threatened species and indicates significant ecological concerns, warranting further biomonitoring efforts.
... In large rivers, C. leucas is able to cover distances of many thousands of kilometers upriver, due to urea-based osmoregulation (PanG et al. 1977;hazon et al. 2003;anderson et al. 2005a;hammerschlaG 2006;trischitta et al. 2012). This large-scale freshwater migration leads to extended periods in freshwater during the early stages of the natural life cycle of C. leucas (see Table 11). ...
The bull shark (Carcharhinus leucas Valenciennes, 1839) is a large, primarily coastally distributed shark famous for its ability to penetrate far into freshwater bodies in tropical, subtropical, and warm-temperate climates. It is a cosmopolitan species with a geographical range that includes the coastlines of all major ocean basins (Atlantic Ocean, Indian Ocean, Pacific Ocean). As a consequence, freshwater occurrences of C. leucas are possible everywhere inside its geographic range. Carcharhinus leucas is a fully euryhaline, amphidromous species and possibly the widest-ranging of all freshwater tolerating elasmobranchs. This species is found not only in river systems with sea access that are not interrupted by human impediments but in hypersaline lakes as well. Rivers and estuaries are believed to be important nursery grounds for C. leucas, as suggested by observations of pregnant females in estuaries and neonates with umbilical scars in rivers and river mouths. Due to the physical capability of this species to enter riverine systems, the documentation of its occurrence in fresh and brackish water is essential for future conservation plans, fishery inspections, and scientific studies that focus on the link between low salinity habitats, shark nurseries, and feeding areas. The author’s review of the available literature on C. leucas revealed the absence of a comprehensive overview of fresh and brackish water localities (rivers and associated lakes, estuaries) with C. leucas
records. The purpose of this literature review is to provide a global list of rivers, river systems, lakes, estuaries, and lagoons with records and reports of this species, including a link to the used references as a base for regional, national, and international conservation strategies. Therefore, the objective of this work is to present lists of fresh and brackish water habitats with records of C. leucas as the result of an extensive literature review and analysis of databases. This survey also took into account estuaries and lagoons, regarding their function as important nursery grounds for C. leucas. The analysis of references included is not only from the scientific literature, but also includes semi-scientific references and the common press if reliable. The result of 415 global fresh and brackish water localities with evidence of C. leucas highlights the importance of these habitats for the reproduction of this species. Moreover, gaps in available distribution maps are critically discussed as well as interpretations and conclusions made regarding possible reasons for the distribution range of C. leucas, which can be interpreted as the result of geographic circumstances, but also as a result of the current state of knowledge about the distribution of this species. The results of the examination of available references were used to build a reliable and updated distribution map for C. leucas, which is also presented here.
... Lower osmolality was probably caused by concomitant lower urea levels, given the relevance of this osmolyte in elasmobranchs (Ballantyne & Robinson, 2010). Furthermore, given that urea is regulated by the kidneys (Hammerschlag, 2006), it is possible that the stress experienced by Shortnose guitarfish individuals harmed their capacity to ensure urea retention. This effect reflects osmoregulatory dynamics during stressful situations. ...
The assessment of physiological responses to capture provides useful information for elasmobranch conservation. In particular, understanding these impacts in species caught as bycatch that are released after capture can predict their “post‐release” fate and aid in management plans. Although well studied in several species, capture‐stress is nonetheless frequently assessed without considering putative variability between sexes. Given the high mortality of pregnant Zapteryx brevirostris females reported in the literature, the present study aimed at determining greatest vulnerability period for males. Our results demonstrate that, when caught during the reproductive period, males display higher mortality (68%) than when caught outside the reproductive period (zero mortality). Seven out of 11 markers (i.e. sodium, chloride, potassium, lactate, phosphorus, triglycerides and condition factor) differed significantly following capture‐induced stress between males caught during the reproductive period and those caught outside this period. Taken together, these results demonstrate the need for specific protocols during the reproductive period, to improve Shortnose guitarfish management and conservation.
... Some studies also report that excess exposure to this element causes cardiovascular alterations [72]. Interestingly, the main excretion route for this element is urine, which may of significance to elasmobranch health, as osmo-regulation in this group is performed by increasing or decreasing blood urea concentrations [73]. Again, not many studies are available concerning this element in elasmobranchs. ...
Background
Elasmobranchs are particularly vulnerable to environmental metal contamination, accumulating these contaminants at high rates and excreting them slowly. The blue shark Prionace glauca L. is one of the most heavily fished elasmobranchs, although information regarding metal contamination and detoxification in this species is notably lacking.
Methods
Blue sharks were sampled in the western North Atlantic Ocean, in offshore waters adjacent to Cape Cod, Massachusetts. Total liver and muscle metal concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS), metallothionein detoxification and oxidative stress endpoints were determined by UV-Vis spectrophotometry.
Results
Metallothionein detoxification occurred for As, Cd, Cs, Cu, Hg, Pb, Se, Ti and Zn in liver, and for As, Cd, Cs, Pb, Se, and Zn in muscle, while reduced glutathione defenses seem to be related to Co and Zn exposure.
Conclusion
This is the first report for several metals (Ag, Co, non-radioactive Cs, Sb, Ti and V) for this species, which will aid in establishing baseline elemental data for biomonitoring efforts, health metrics, and conservation measures.
... Elasmobranch liver is a highly complex tissue due to its role in osmotic regulation and energy storage (Hamlett, 1999;Hoffmayer et al., 2006). The synthesis of urea (CO(NH 2 ) 2 ) and the presence of trimethylamine n-oxide (TMAO, C 3 H 9 NO; to counteract the fact that urea inhibits protein binding and folding; Yancey, 2005) is key to maintain osmotic balance (Hazon et al., 2003;Hammerschlag, 2006). Acclimatization to reduced salinity environments by lemon sharks (Negaprion brevirostris), for example, results in increased urea and extracellular solute (e.g., TMAO) excretion (Goldstein et al., 1968). ...
Stable-isotope analysis (SIA) provides a valuable tool to address complex questions pertaining to elasmobranch ecology. Liver, a metabolically active, high turnover tissue (~166 days for 95% turnover), has potential to reveal novel insights into recent feeding/movement behaviours of this diverse group. To date, limited work has used this tissue, but ecological application of SIA in liver requires consideration of tissue preparation techniques given the potential for high concentrations of urea and lipid that could bias δ13C and δ15N values (i.e. result in artificially lower δ13C and δ15N values). Here we investigated the effectiveness of: i) deionized water washing (WW) for urea
removal from liver tissue and ii) chloroform-methanol for extraction of lipids from this lipid rich tissue. We then; i) established C:N thresholds for deriving ecologically relevant liver isotopic values given complications of removing all lipid and ii) undertook a preliminary comparison of δ13C values between tissue pairs (muscle and liver) to test if observed isotopic differences correlated with known movement behaviour. Tests were conducted on four large shark species; the dusky (DUS; Carcharhinus obscurus), sand tiger (RAG; Carcharias taurus), scalloped hammerhead (SCA; Sphyrna lewini) and white shark (GRE; Carcharodon carcharias). There was no significant difference in δ15N values between lipid-extracted (LE) liver and lipidextracted/water washed (WW) treatments, however, WW resulted in significant increases in %N, δ13C and %C. Following lipid extraction (repeated three times), some samples were still biased by lipids. Our species-specific ‘C:N thresholds’ provide a method to derive ecologically viable isotope data given the complexities of this lipid rich tissue (C:N thresholds of 4.0, 3.6, 4.7 and 3.9) for DUS, RAG, SCA and GRE liverLEWW tissue, respectively). The preliminary comparison of C:N
threshold corrected liver and muscle δ13C values corresponded with movement/habitat behaviours for each shark; minor difference in δ13C values were observed for known regional movements of DUS and RAG (δ13CDiffs = 0.24 ± 0.99‰ and 0.57 ± 0.38‰, respectively), while SCA and GRE showed greater differences (1.24 ± 0.63‰ and 1.08 ± 0.71‰, respectively) correlated to large scale movements between temperate/tropical and pelagic/coastal environments. These data provide an approach for the successful application of liver δ13C and δ15N values to examine elasmobranch ecology.
... Marine and euryhaline elasmobranchs in SW regulate urea and other body fluid solutes [trimethylamine oxide (TMAO), Na + , Cl − ] such that they remain iso-or slightly hyperosmotic to their environment (302). While little information is available, it seems that the thyroid axis may contribute to elasmobranch osmoregulation. ...
In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.
... Similarly, fossil coelacanth specimens were found in freshwater sediments (15), although extant coelacanths live in marine environments. Extant marine elasmobranchs (sharks and rays), holocephalan fishes (e.g., the elephant shark), as well as L. chalumnae are ureosmotic regulators (36,40,44). They are ionic regulators that maintain concentrations of ions in the extracellular fluid (including sodium and chloride) below levels found in seawater, while osmolarity is adjusted to match or slightly exceed seawater levels by accumulation of osmolytes such as urea and trimethylamine N-oxide (TMAO). ...
The conquest of freshwater and terrestrial habitats was a key event during vertebrate evolution. Occupation of low-salinity and dry environments required significant osmoregulatory adaptations enabling stable ion and water homeostasis. Sodium is one of the most important ions within the extracellular liquid of vertebrates and molecular machinery for urinary reabsorption of this electrolyte is critical for the maintenance of body osmoregulation. Key ion channels involved in the fine-tuning of sodium homeostasis in tetrapod vertebrates are epithelial sodium channels (ENaC) which allow the selective influx of sodium ions across the apical membrane of epithelial cells lining the distal nephron or the colon. Furthermore, ENaC-mediated sodium absorption across tetrapod lung epithelia is crucial for the control of liquid volumes lining the pulmonary surfaces. ENaCs are vertebrate-specific members of the degenerin/ENaC family of cation channels, however, there is limited knowledge in the evolution of ENaC within this ion channel family. This review outlines current concepts and hypotheses on ENaC phylogeny and discusses the emergence of regulation-defining sequence motifs in the context of osmoregulatory adaptations during tetrapod terrestrialisation. Based on the distinct regulation and expression of ENaC isoforms in tetrapod vertebrates we discuss the potential significance of ENaC orthologs in osmoregulation of fish as well as the putative fates of atypical channel isoforms in mammals. We hypothesize that ancestral proton-sensitive ENaC orthologs might have aided the osmoregulatory adaptation to freshwater environments whereas channel regulation by proteases evolved as a molecular adaptation to lung liquid homeostasis in terrestrial tetrapods.
... We hypothesised that chl-a and SST would influence shark occurrence and expected to find pelagic sharks offshore in less turbid waters. We also hypothesised salinity would limit these species to higher and more stable salinities offshore (Hammerschlag, 2006;Bernal et al., 2012) due to the high cost of osmoregulation (Pang et al., 1977), and that geophysical and geomorphological parameters would influence shark occurrence in different locations rather than sediment characteristics. ...
Many of the world’s shark populations are in decline, indicating the need for improved conservation and management. Well managed and appropriately located marine parks and marine protected areas (MPAs) have potential to enhance shark conservation by restricting fisheries and protecting suitable habitat for threatened shark populations. Here, we used shark occurrence records collected by commercial fisheries to determine suitable habitat for pelagic sharks within the Australian continental Exclusive Economic Zone (EEZ), and to quantify the amount of suitable habitat contained within existing MPAs. We developed generalised linear models using proportional occurrences of pelagic sharks for three families: Alopiidae (thresher), Carcharhinidae (requiem), and Lamnidae (mackerel) sharks. We also considered aggregated species from the Lamnidae and Carcharhinidae families (‘combined sharks’ in the models). Using a set of environmental predictors known to affect shark occurrence, including chlorophyll-a concentration, salinity, sea surface temperature, and turbidity, as well as geomorphological, geophysical, and sedimentary parameters, we found that models including sea surface temperature and turbidity were ranked highest in their ability to predict shark distributions. We used these results to predict geographic regions where habitat was most suitable for pelagic sharks within the Australian EEZ, and our results revealed that suitable habitat was limited in no-take zones within MPAs. For all shark groupings, suitable habitats were found mostly at locations exposed to fishing pressure, potentially increasing the vulnerability of the pelagic shark species considered. Our predictive models provide a foundation for future spatial planning and shark management, suggesting that strong fisheries management in addition to MPAs is necessary for pelagic shark conservation.
... The unique physiology of elasmobranchs leads to additional challenges in interpreting stable isotope values due to the retention of urea in body tissues for osmoregulatory purposes ( Hazon et al. 2003;Hammerschlag 2006). Urea is enriched in 14 N relative to pure protein due to lighter isotopes reacting faster and being preferentially selected during amination processes ( Gannes et al. 1998). ...
The presence of lipids and urea in elasmobranch tissues can affect carbon (δ13C) and nitrogen (δ15N) stable isotope values, introducing bias in food web interpretations. Information on how lipids and urea affect δ13C/δ15N is only available for < 5% of ~ 1150 described elasmobranch species and results are highly variable among existing studies. Here, we investigate the effects of lipid and urea extraction on δ13C, δ15N, and C:N ratios in muscle and blood plasma of two batoids (Pastinachus ater and Himantura australis) and examine the influence of lipid/urea extraction on isotopic niche size and overlap. Lipid extraction (LE) did not affect δ13C, δ15N, or C:N ratios, suggesting low lipid content. Urea extraction (UE), however, increased δ15N and C:N in muscle. The ~ 1‰ δ15N increase represents a shift of ~ 0.5 trophic levels relative to bulk samples, highlighting the importance of UE to accurately assess trophic positions. Although there was no effect of any treatment on niche size, the probability of P. ater occurring within the niche of H. australis increased following UE. Overall, results suggest that urea should be removed from muscle prior to analysis, but LE is not required. Given the interspecific variability in the effects of lipid/urea on elasmobranch δ13C/δ15N, more studies are needed to assess the effects of lipid and urea on a broader range of species to produce a generalized understanding. Where no species‐specific data are available, we recommend pilot samples are analyzed to determine if LE is needed prior to preparation of the overall sample set.
... Whaler shark attacks were found to increase closer to a river, with 47% of attacks occurring within 100 m of a river mouth. This coincides with the preferred habitat zone for bull sharks, i.e. coastal zones with high freshwater inflow (Carlson et al. 2010), as they can osmoregulate in both salt and fresh water (Pillans & Franklin 2004, Hammerschlag 2006. Acoustically tagged young bull sharks spend most of their time within 11 km of the river mouth (Heupel & Simpfendorfer 2008), and bull sharks breed and feed in nearshore coastal areas and rivers (Carlson et al. 2010, Werry et al. 2011. ...
Shark attacks are rare but traumatic events that generate social and economic costs and often lead to calls for enhanced attack mitigation strategies that are detrimental to sharks and other wildlife. Improved understanding of the influence of environmental conditions on shark attack risk may help to inform shark management strategies. Here, we developed predictive models for the risk of attack by white Carcharodon carcharias, tiger Galeocerdo cuvier, and bull/ whaler Carcharhinus spp. sharks in Australian waters based on location, sea surface temperature (SST), rainfall, and distance to river mouth. A generalised additive model analysis was performed using shark attack data and randomly generated pseudo-absence non-attack data. White shark attack risk was significantly higher in warmer SSTs, increased closer to a river mouth (<10 km), and peaked at a mean monthly rainfall of 100 mm. Whaler shark attack risk increased significantly within 1 km of a river mouth and peaked in the summer months. Tiger shark attack risk increased significantly with rainfall. We performed additional temporal and spatio-temporal analyses to test the hypothesis that SST anomaly (SSTanom) influences white shark attack risk, and found that attacks tend to occur at locations where there is a lower SSTanom (i.e. the water is relatively cooler) compared to surrounding areas. On the far north coast of eastern Australia — an attack hotspot — a strengthening of the East Australian Current may cause white sharks to move into cooler upwelling waters close to this stretch of the coast and increase the risk of an attack.
... Following defatting, all elasmobranch muscle samples were rinsed in distilled water three times to remove urea [72]. This compound is found at high concentrations in marine elasmobranch tissues [97,98], and may lower apparent 15 N/ 14 N ratios of muscle tissue [99,100]. ...
An extensive ecological literature applies stable isotope mixing models to derive quantitative dietary reconstructions from isotope ratios of consumer tissues. While this approach works well for some organisms, it is challenging for consumers with complex, varied diets, including humans; indeed, many archaeologists have avoided the use of mixing models because uncertainties in model outputs are sufficiently large that the findings are not helpful in understanding ancient lifeways. Here, we exploit an unparalleled opportunity to evaluate the feasibility of dietary quantification in a nutritionally and isotopically complex context on the Cape Peninsula, South Africa. Delta values (δ¹³C and δ¹⁵N) of 213 indigenous food samples enable us to characterise four food groups: terrestrial plants, terrestrial vertebrates, marine invertebrates and marine vertebrates. A recent study of baboons that consumed marine and terrestrial foods provides insight into the relationship between such foods and consumer tissue isotopes. We use this information to refine our interpretation of δ¹⁵N and especially δ¹³C in bone collagen from 35 archaeological hunter-gatherers, achieving better estimates of the relative importance of marine and terrestrial foods in the diet than has hitherto been possible. Based on Bayesian stable isotope mixing model (SIMM) outputs, we infer that the trophic enrichment factor (TEF) for δ¹³Cbone collagen in these coastal humans is closer to +3 than +5‰. In the most ¹³C- and ¹⁵N-rich individuals, 65–98% of bone collagen (95% credible intervals) derived from marine foods. Conversely, in ¹³C and ¹⁵N-poor individuals, 7–44% of bone collagen derived from marine foods. The uncertainties discussed here highlight the need for caution when implementing SIMMs in studies of consumers with complex diets. To our knowledge, this work constitutes the most detailed and most tightly constrained study of this problem to date.
... The blood of elasmobranchs (sharks, skates, and rays) and of non-cartilaginous fishes represent highly divergent food sourceselasmobranch blood often contains 2 to 3 orders of magnitude more urea than that of non-cartilaginous fishes (Wells et al., 1986;Hammerschlag, 2006). Nonetheless, these strongly denaturing conditions (Zou et al., 1998;Bennion and Daggett, 2003) seem to present no impediment to those piscicolid species that effectively parasitize elasmobranchs. ...
Leeches (Annelida: Hirudinea) possess powerful salivary anticoagulants and, accordingly, are frequently employed in modern, authoritative medicine. Members of the almost exclusively marine family Piscicolidae account for 20% of leech species diversity, and feed on host groups (e.g., sharks) not encountered by their freshwater and terrestrial counterparts. Moreover, some species of Ozobranchidae feed on endangered marine turtles and have been implicated as potential vectors for the tumor-associated turtle herpesvirus. In spite of their ecological importance and unique host associations, there is a distinct paucity of data regarding the salivary transcriptomes of either of these families. Using next generation sequencing, we profiled transcribed, putative anticoagulants and other salivary bioactive compounds that have previously been linked to bloodfeeding from 7 piscicolid species (3 elasmobranch-feeders; 4 non-cartilaginous fish-feeders) and 1 ozobranchid species (2 samples). In total, 149 putative anticoagulants and bioactive loci were discovered in varying constellations throughout the different samples. The putative anticoagulants showed a broad spectrum of described antagonistic pathways, such as inhibition of factor Xa and platelet aggregation, that likely have similar bioactive roles in marine fish and turtles. A transcript with homology to ohanin, originally isolated from king cobras, was found in Cystobranchus vividus but is otherwise unknown from leeches. Estimation of selection pressures for the putative anticoagulants recovered evidence for both positive and purifying selection along several isolated branches in the gene trees and positive selection was also estimated for a few select codons in a variety of marine species. Similarly, phylogenetic analyses of the amino acid sequences for several anticoagulants indicated divergent evolution.
... The analysis conducted here shows that urea has a major role in assuring homeostasis in the species, followed by plasma sodium. Plasma osmolality in elasmobranchs is essentially the sum of NaCl and urea (and TMAO, not assayed here) (Marshall and Grosell 2006;Hammerschlag 2006;Ballantyne and Robinson 2010;Wright and Wood 2015), and these were very different in the dead animals, when compared to both groups of living animals. The result was expected and entirely consistent with cessation of salt secretion by the rectal gland, causing plasma NaCl levels to increase, and breakdown of urea reabsorption by the kidneys, causing urea titers to decrease in plasma. ...
Sharks are very sensitive to stress and prone to a high mortality rate after capture. Since approximately 50 million of sharks are caught as bycatch every year, and current recommendations to reduce the impact of commercial fishing strongly support immediate release, it is imperative to better understand post-release mortality caused by the stress of capture and handling. Blood samples allow the assessment of stress levels which are valuable tools to reduce mortality in commercial, recreational and scientific fishing, being essential for the improvement in those conservation measures. Biochemical analyses are widely used for sharks as stress indicators, with secondary plasma parameters (lactate, glucose and ions) being the most often employed assays. However, it is virtually impossible to determine baseline plasma parameters in free-ranging sharks, since blood withdrawal involves animal capture and restrain, which are stressful procedures. This study aims at analyzing secondary parameters of five healthy tiger sharks captured with circular hooks and handlines in Fernando de Noronha (Northeastern Brazil) and comparing them with secondary parameters of three dead tiger sharks caught off Recife (also Northeastern Brazil). The results showed that the analysis of some plasma constituents in dead animals may be an efficient tool to assess stress and lethality. However, traditional parameters such as glucose and calcium, need to be used with caution. The results also demonstrated the extreme importance of urea and phosphorus for assessing stress response and mortality in tiger sharks, both parameters frequently neglected and of utmost importance for shark’s homeostasis.
... The last route that may have an impact on elasmobranch water balance involves drinking or swallowing of SW during eating. Little has been done to investigate the effect of these actions on water balance, but evidence exists that shows that elasmobranchs do drink when needed and probably imbibe SW when eating (Hazon et al., 2003;Hammerschlag, 2006). This action would bring water and salts into the gastrointestinal tract where both may be absorbed (Anderson et al., 2007). ...
Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.
© 2015 Marine Biological Laboratory.
... Marine teleost fish overcome this problem by actively drinking sea water and then excreting the excess electrolytes into specialised structures called chloride cells that are primarily located in the gills ( Figure 2b). Marine elasmobranchs, sharks and rays, have evolved a different solution to osmoregulation (Hammerschlag, 2006). Like marine teleost fish, the salt concentration of their blood is 1/3 that of sea water. ...
Animals must maintain their internal environment relatively constant with respect to water, electrolytes and temperature. The process by which they regulate the concentration of their internal water and solute concentrations is called osmoregulation. Osmoregulation is accomplished by physiological, anatomical and behavioural adaptations. These include adaptations that modify the permeability of external surfaces and the development of specialised structures that actively transport solutes and metabolites. Animals face the greatest osmoregulatory challenge in terrestrial environments because concentration gradients there are the greatest. As evaporative cooling is used for thermoregulation in terrestrial environments, animals are often faced with the dilemma of maintaining either water or thermal balance, but not both.
... Vanderklift and Ponsard (2003) reviewed nitrogen fractionation values for consumers with different forms of nitrogen excretion and found that ureotelic consumers exhibit the largest mean fractionation values (3.11‰). Despite being ureotelic, the shark renal system reabsorbs up to 95% of urea into body fluids for osmoregulatory purposes (Evans et al. 2004;Hammerschlag 2006), which could explain the low nitrogen discrimination factors observed in this study. Fisk et al. (2002) used Hobson's et al. (1995 average fractionation value of 3.8‰ to estimate the trophic level of Greenland sharks (Somniosus microcephalus). ...
Los tiburones se clasifican como depredadores tope (Cortés, 1999) y pueden ejercer una importante influencia en la estructura y funcionamiento del ecosistema marino (Heithaus et al. 2008). El uso del análisis de contenido estomacal es una primera aproximación para evaluar los hábitos alimentarios en estos organismos. Sin embargo, se considera que esta técnica solo aporta información del último evento de alimentación del organismo y puede no ser representativo de la dieta y el nivel trófico en un periodo de tiempo más amplio. Para entender la importancia ecológica de los tiburones dentro de las tramas tróficas, es necesario caracterizar sus hábitos alimentarios. El análisis de las razones de los isótopos estables de elementos ligeros como el carbono (13C/12C) y el nitrógeno (15N/14N) ha contribuido sustancialmente al entendimiento de estructuras tróficas (Vander Zanden & Rasmussen, 2001; Michener & Kaufman, 2007). Los isótopos estables atraviesan procesos de fraccionamiento y mezcla que permiten discriminar niveles tróficos (15N/14N; DeNiro & Epstein 1981) y áreas con diferentes fuentes de producción primaria (13C/12C; Fry & Sherr, 1984). Procesos metabólicos producen diferencias en las razones isotópicas del consumidor en relación a sus presas, lo que se conoce como fraccionamiento trófico (Tieszen et al. 1983). Así mismo, posterior a un cambio de dieta o a un evento de migración a diferentes zonas de alimentación, la composición isotópica de los tejidos de los organismos cambiará hasta equilibrarse con la composición isotópica de su nueva fuente de alimento (Herzka 2005, Phillips & Eldridge, 2006). Para una apropiada interpretación de datos de poblaciones naturales, es necesario conocer la tasa de recambio isotópico y el tiempo que integra la composición isotópica de los tejidos de los organismos, así como conocer los valores de fraccionamiento trófico (Gannes et al. 1997). El objetivo de este trabajo fue realizar un experimento de cambio de dieta en condiciones de laboratorio en el tiburón leopardo (Triakis semifasciata) y evaluar el cambio en la composición isotópica y los valores de fraccionamiento trófico de carbono y nitrógeno (δ15N y δ13C) en sangre, hígado, músculo, aleta y cartílago. Los resultados del experimento en laboratorio se usaron para interpretar datos de campo y evaluar la potencial aplicación de esta técnica al análisis de hábitos alimentarios de tiburones pelágicos.
... Elasmobranch fish achieve plasma ionic homeostasis and maintain a plasma osmolarity equal or slightly higher than sea water by actively excreting excess NaCl by rectal gland (Figure 1) and retaining large amounts of urea. Many works have been performed to study the osmoregulatory strategies adopted by elasmobranch fish and the results obtained have been reviewed by many authors [1][2][3][4][5][6][7][8]. ...
Marine elasmobranchs maintain their body fluid isoosmotic or slightly hyperosmotic to the exter-nal medium by the retention of large urea con-centrations. This review focuses on the strate-gies adopted by these fishes to maintain a large outwardly direct concentration gradient of this osmolyte minimizing the loss across the main interfaces between body fluid and the external medium such as the gills, the kidney and the rectal gland, thus reducing the cost of making urea. The high plasma osmolarity, mainly main-tained by urea retention, is a challenge to vol-ume homeostasis when fish move from sea-water to water with a low salinity, since the high water permeability of branchial epithelium would cause a net flux of water into the animal. Since the renal regulation of urea retention in habitat with different salinities is crucial for the osmotic homeostasis of these species, the regulation of the activity and/or the expression of urea trans-porters in renal tubules will be also discussed. In addition attention will be paid on the urea– methylamine system involved in maintaining the stability and functioning of many proteins since it is known that the high urea concentration found in marine elasmobranch fish, similar only to that found in mammalian kidney, has a desta-bilizing effect on many macromolecules and inhibits functions such as ligand binding.
... In addition to urea, elasmobranchs have trimethylamine N-oxide (TMAO) in their tissues to prevent protein destabilization due to urea. Concentrations of urea and TMAO are regulated concurrently according to ambient salinity and habitat depth, further complicating the urea effect on nitrogen isotope fractionation between diet and elasmobranch tissues (Hammerschlag, 2006;Laxson et al., 2011;Wood et al., 2007). The effect of TMAO on elasmobranch isotope and TEF values is not understood, as it is uncertain whether elasmobranchs biosynthesize TMAO or acquire it through diet (Ballantyne, 1997). ...
... Elasmobranch fishes in seawater regulate concentrations of urea and other body fluid solutes (trimethylamine oxide, Na ? , and Cl -) in their body fluid to remain hyper-osmotic to their environment (reviewed in Neil 2006). As gnathiid larvae suck and digest fishes' body fluid, the digestive system between third-and first/second-stage larvae of G. trimaculata must be physiologically different between the third-and the first/ second-stage larvae of G. trimaculata. ...
The life cycle of the fish ectoparasitic isopod Gnathia trimaculata is described based on both field samplings and laboratory observations. Species identification of the larvae was based on morphological observation and supported by molecular analysis. As the results of field samplings in several sites of southwestern and central Japan (24–34°N, 124–139°E) from 2005–2011, approximately 900 third-stage larvae of G. trimaculata were found on 25 elasmobranch species, and 220 first- and second-stage larvae were found on three teleost species. No third-stage larvae were found on the teleosts, and the larvae of younger stages never infested elasmobranchs. Therefore, G. trimaculata is supposed to shift its host from teleosts to elasmobranchs as it develops. We discuss the developmental periods, life span, distribution, and predation risk of the present species compared with other gnathiid species.
... There are 3 commonly cited sources of urea: (1) zooplankton and fish excretion (Eppley et al. 1973, Conover & Gustavson 1999, Hammerschlag 2006, (2) bacterial degradation of dissolved organic matter (Price et al. 1985, Cho et al. 1996, Berman et al. 1999 and (3) microheterotrophic regeneration (Slawyk et al. 1990, L'Helguen et al. 2005, though the relative contribution from each process is far from clear and appears highly variable across aquatic systems. Early studies on urea uptake suggested that phytoplankton were the major consumers of urea (e.g. ...
We investigate the distribution of urea and its uptake by phytoplankton during 3 meridional transects of the Atlantic Ocean between 50 degrees N and 500 S. Significant relationships were identified between Urea uptake and Prochlorococcus abundance (p < 0.01) in the northern sub tropical Atlantic, where Prochlorococcus appears likely to dominate urea uptake, and between urea concentration and the <200 mu m microplankton biomass fraction (p < 0.005) in the South Atlantic, which may be associated with the production of urea. These results suggest that the distribution of urea in the subtropical ocean may be controlled by regional imbalances between Urea consumption and urea production, In parallel with these simple relationships significant spring-autumn seasonal changes in the distribution of urea were identified in southern subtropical and tropical latitudes. Urea was twice as abundant during local spring than during local autumn in the subtropical South Atlantic but 2.5 times more abundant in equatorial waters during the boreal autumn period. Euphotic zone integrated urea uptake rates also varied seasonally, being considerably higher in the North Atlantic temperate and subtropical latitudes during the boreal spring whilst in the subtropical South Atlantic Urea uptake peaked in local autumn.
... Vanderklift and Ponsard (2003) reviewed nitrogen fractionation values for consumers with different forms of nitrogen excretion and found that ureotelic consumers exhibit the largest mean fractionation values (3.11‰). Despite being ureotelic, the shark renal system reabsorbs up to 95% of urea into body fluids for osmoregulatory purposes (Evans et al. 2004;Hammerschlag 2006), which could explain the low nitrogen discrimination factors observed in this study. Fisk et al. (2002) used Hobson's et al. (1995 average fractionation value of 3.8‰ to estimate the trophic level of Greenland sharks (Somniosus microcephalus). ...
There are very few studies reporting isotopic trophic discrimination factors and turnover rates for marine elasmobranchs. A controlled laboratory experiment was conducted to estimate carbon and nitrogen isotope trophic discrimination factors and isotope turnover rates for blood, liver, muscle, cartilage tissue, and fin samples of neonate to young-of-the-year leopard sharks (Triakis semifasciata). Trophic discrimination factors varied (0.13‰–1.98‰ for δ13C and 1.08‰–1.76‰ for δ15N). Tissues reached or were close to isotopic equilibrium to the new diet after about a threefold biomass gain and 192 days. Liver and blood exhibited faster isotope turnover than muscle, cartilage tissue, and fin samples, and carbon isotopes turned over faster than those of nitrogen. Metabolic turnover contributed substantially to isotopic turnover, which differs from most reports for young marine teleosts. We modeled the relationship between muscle turnover rates and shark size by coupling laboratory results with growth rate estimates for natural populations. Model predictions for small, medium, and large wild leopard sharks indicate the time to isotopic equilibrium is from one to several years.
A novel thermally-responsive chloride-anions rich polyionic liquid (TPIL) hydrogel was synthesized and its potential in membrane-free osmotic desalination for saline and seawater was explored. The tunable Lower Critical Solution Temperature...
The basic mechanisms of iono/osmoregulation, acid-base regulation, and homeostasis and excretion of nitrogenous wastes (ammonia and urea) are reviewed for freshwater fish, marine and euryhaline fish, and for special cases (marine hagfish and chondrichthyans). Six different examples are then explored where physiological understanding of environmental impacts on these processes has already informed regulatory and conservation strategies, or should do so in the future. These include: (i) the acid-rain crisis; (ii) survival of fishes in the acidic, ion-poor blackwaters of the Rio Negro; (iii) development of the Biotic Ligand Model for environmental regulation of metals; (iv) survival of fishes in highly alkaline lakes; (v) the commercial hagfish fishery; and (vi) the critical importance of feeding for osmoregulation in chondrichthyans. These lessons argue for a greater awareness of the wide-spread variation in the physical chemistry of natural waters, and the associated physiology of the fish that live there. This knowledge should be incorporated into regulatory strategies for both environmental protection and conservation of resident species at risk.
Acute-stress situations in vertebrates induce a series of physiological responses to cope with the event. While common secondary stress responses include increased catabolism and osmoregulatory imbalances, specific processes depend on the taxa. In this sense, these processes are still largely unknown in ancient vertebrates such as marine elasmobranchs. Thus, we challenged the lesser spotted catshark (Scyliorhinus canicula) to 18 min of air exposure, and monitored their recovery after 0, 5, and 24 h. This study describes amino acid turnover in the liver, white muscle, gills, and rectal gland, and plasma parameters related to energy metabolism and osmoregulatory imbalances. Catsharks rely on white muscle amino acid catabolism to face the energy demand imposed by the stressor, producing NH4+. While some plasma ions (K+, Cl− and Ca2+) increased in concentration after 18 min of air exposure, returning to basal values after 5 h of recovery, Na+ increased after just 5 h of recovery, coinciding with a decrease in plasma NH4+. These changes were accompanied by increased activity of a branchial amiloride-sensitive ATPase. Therefore, we hypothesize that this enzyme may be a Na+/H+ exchanger (NHE) related to NH4+ excretion. The action of an omeprazole-sensitive ATPase, putatively associated to a H+/K+-ATPase (HKA), is also affected by these allostatic processes. Some complementary experiments were carried out to delve a little deeper into the possible branchial enzymes sensitive to amiloride, including in vivo and ex vivo approaches, and partial sequencing of a nhe1 in the gills. This study describes the possible presence of an HKA enzyme in the rectal gland, as well as a NHE in the gills, highlighting the importance of understanding the relationship between acute stress and osmoregulation in elasmobranchs.
Because the salt concentration of body fluids in aquatic vertebrates differs from that of their environment, they face net influx or efflux of water and salt across their permeable skin or exposed membranes. In fishes, these diffusional movements of both salts and water largely involve the gills. Other vertebrates have less permeable body surfaces, but water and salts are gained in drinking that is incidental to eating prey, which also can be an important source of salt intake. Kidneys function importantly in freshwater fishes to remove excess water, and ions are actively transported inwards across gill tissue as well as acquired in food. Marine fishes tend to dehydrate in seawater, so they drink seawater to make up a water deficit and secrete excess salt across the gills. Marine elasmobranch fishes accumulate urea in body fluids to minimise the gradient for osmotic exchange and secrete excess ions via a specialised rectal gland that functions accessory to the kidney. In other vertebrates, inhabitants of fresh water must counteract a key problem of acquiring sufficient ions while excreting nitrogenous waste as ammonia or urea, whereas marine species must avoid excess salt intake while avoiding dehydration. Vertebrates without gills have evolved either extrarenal salt glands that excrete excess salt, or in the case of mammals, a specialised kidney that can excrete highly concentrated urine. Relatively few, non-fish taxa drink seawater as a route for water gain, while most others have dependency on dietary, metabolic or free drinking water.
• Osmoregulation involves the maintenance of volume, distribution and ionic composition of body fluids in organisms.
• With the exception of hagfish, extant vertebrates maintain osmotic concentrations of body fluids at levels that are roughly one-third that of seawater.
• Aquatic vertebrates living in fresh water must counteract excessive influx of water and losses of ions to the surrounding environment, whereas those living in marine environments tend to dehydrate and gain excess salt.
• Freshwater fishes eliminate excess water by means of producing a copious flow of dilute urine, while acquiring ions from surrounding water by transporter mechanisms located in the gills.
• Marine fishes acquire needed water by drinking seawater and excreting excess salt gained by diffusion and drinking by transport mechanisms in the gills.
• Marine vertebrates other than fishes and mammals have evolved extrarenal salt glands that excrete excess salts, and water is gained largely from dietary, metabolic and freshwater drinking sources.
• Marine mammals excrete salt loads and urea from kidneys that have effective concentrating abilities, while gaining water from diet and metabolism.
This chapter is intended to help maximize the information gained from diagnostic tests in fish. The text focuses on selecting appropriate tests, understanding their limitations, and evaluating sample quality. Specific topics include laboratory medicine (wet mounts, cytology, histopathology, hematology, and biochemistry), toxicologic and nutritional analyses, microbiology, and molecular and antibody-based diagnostics (PCR, DNA sequencing, immunohistochemistry, in situ hybridization, fluorescent antibody, and ELISA).
The aim of this study is to investigate the effect of three daily fish feeding frequencies, two, four and eight times per day (FF2, FF4, and FF8, respectively) on growth performance of sea bass ( Dicentrarchus labrax )and lettuce plants ( Lactuca sativa ) reared in aquaponics. 171 juvenile sea bass with an average body weight of 6.80 ± 0.095 g were used, together with 24 lettuce plants with an average initial height of 11.78 ± 0.074 cm over a 45-day trial period. FF2 fish group showed a significantly lower final weight, weight gain and specific growth rate than the FF4 and FF8 groups. Voluntary feed intake was similar for all the three feeding frequencies treatmens ( p > 0.05). No plant mortality was observed during the 45-day study period. All three aquaponic systems resulted in a similar leaf fresh weight and fresh and dry aerial biomass. The results of the present study showed that the FF4 or FF8 feeding frequency contributes to the more efficient utilization of nutrients for better growth of sea bass adapted to fresh water while successfully supporting plant growth to a marketable biomass.
In heterogeneous environments, mobile species should occupy habitats in which their fitness is maximized. Mangrove rivulus fish inhabit mangrove ecosystems where salinities range from 0-65 ppt but are most often collected at ∼25 ppt. We examined rivulus' salinity preference in a lateral salinity gradient, in the absence of predators and competitors. Fish could swim freely for 8 hours throughout the gradient with chambers containing salinities from 5-45 ppt (or 25 ppt throughout, control). We defined preference as the salinity in which the fish spent most of their time, and also measured preference strength, latency to begin exploring the arena, and number of transitions between chambers. To determine whether these traits were repeatable, each fish experienced three trials. Rivulus spent a greater proportion of time in salinities lower (5-15 ppt) than they occupy in the wild. Significant among-individual variation in the (multivariate) behavioral phenotype emerged when animals experienced the gradient, indicating strong potential for selection to drive behavioral evolution in areas with diverse salinity microhabitats. We also showed that rivulus had a significantly greater probability of laying eggs in low salinities compared to control or high salinities. Eggs laid in lower salinities also had higher hatching success compared to those laid in higher salinities. Thus, although rivulus can tolerate a wide range of salinities, they prefer low salinities. These results raise questions about factors that prevent rivulus from occupying lower salinities in the wild, whether higher salinities impose energetic costs, and whether fitness changes as a function of salinity.
Serum protein electrophoresis (EPH) is used to assess relative
concentrations of blood proteins in clinical and biological studies.
Serum EPH fractions have been determined for elasmobranchs using
mammalian albumin, alpha 1-, alpha 2-, beta-, and gamma-globulin
fractions, and have been deemed fractions 1 through 5, respectively.
However, serum EPH fraction concentration reference intervals
(RIs) have not been widely established for different elasmobranch
species. In this study, RIs for fractions 1 through 5 were determined
from 45 wild-caught Nurse Sharks Ginglymostoma cirratum (27
females and 23 males) in South Florida. Serum samples were isolated
from whole blood following caudal venipuncture. Body condition was
also measured in the field to assess the relative health of the individuals
sampled. There was no relationship between body condition and
serum EPH fraction concentrations. In addition, there was no difference
in body condition or serum EPH fraction concentrations
between females and males. Total solids and total protein values
were significantly different (P < 0.001). Nurse Shark serum EPH
fraction 1 was found within the mammalian albumin migrating band
distance and was negligible. Fraction 2 showed no peak in the mammalian
alpha 1-globulin range. A thin, medium peak in the mammalian
alpha 2-globulin range represented fraction 3. In the
mammalian beta-globulin range, fraction 4 consisted of the majority
of protein observed. It was represented by a smooth, broad peak. A
short, medium broad peak in the mammalian gamma-globulin range
represented fraction 5. The Nurse Shark serum EPH fraction RIs
provided in this study may be utilized to clinically evaluate the health
of Nurse Sharks in captivity and in the wild, and to compare the
health of their populations around the world experiencin
The antibody repertoire of cartilaginous fish comprises an additional heavy-chain-only antibody isotype that is referred to as IgNAR (immunoglobulin novel antigen receptor). Its antigen-binding site consists of one single domain (vNAR) that is reportedly able to engage a respective antigen with affinities similar to those achieved by conventional antibodies. While vNAR domains offer a reduced size, which is often favorable for applications in a therapeutic as well as a biotechnological setup, they also exhibit a high physicochemical stability. Together with their ability to target difficult-to-address antigens such as virus particles or toxins, these shark-derived antibody domains seem to be predestined as tools for biotechnological and diagnostic applications. In the following chapter, we will describe the isolation of anti-idiotypic vNAR domains targeting monoclonal antibody paratopes from semi-synthetic, yeast-displayed libraries. Anti-idiotypic vNAR variants could be employed for the characterization of antibody-based therapeutics (such as antibody-drug conjugates) or as positive controls in immunogenicity assays. Peculiarly, when using semi-synthetic vNAR libraries, we found that it is not necessary to deplete the libraries using unrelated antibody targets, which enables a fast and facile screening procedure that exclusively delivers anti-idiotypic binders.
With the aim of reducing the ammonia content and improving the quality of meat from the blue shark, Prionace glauca, and salmon shark, Lamna ditropis, we evaluated the effect of soaking solutions containing 10% sucrose and 5% NaCl, with or without the addition of an aqueous extract suspension (AES) of rice bran (RB-AES), or fermented RB (FRB-AES) generated using Lactobacillus plantarum Sanriku-SU8 and Saccharomyces cerevisiae Misaki-1 (FRB-AES-Misaki-1), or S. cerevisiae Sanriku- SUY2 (FRB-AES-SUY2). RB is a major by-product of rice polishing. Although the effect of the soaking treatments on ammonia content and preference ranking were not clear in the fresh shark meats, there was a significant effect for meat refrigerated at 10 °C for 2 and 4 days (not-so-fresh meat). In particular, FRB-AES-Misaki-1 treatment raised the preference ranking for chub mackerel, Pacific cod and chicken breast meat. These results suggest that FRB-AES-Misaki-1 can be used as a potential starter for the pretreatment of meat from blue sharks and other fish species.
1. Introduction2. Osmoconformers Versus Osmoregulators 2.1. Osmoconforming: Elasmobranchs in the Oceans2.2. Osmoregulating: Elasmobranchs at Low Salinities3. Properties of Organic Osmolytes 3.1. Inorganic Ions Versus Compatible Solutes3.2. Urea Versus Methylamines: Counteracting Solutes3.3. Salts Versus Methylamines: Counteracting Solutes3.4. TMAO Versus Pressure: A Piezolyte in the Deep Sea?3.5. Physicochemical Mechanisms of (De)stabilization3.6. Urea and Methylamines: Buoyancy3.7. Other Cytoprotective/Compensatory Properties3.8. Implications for Food Industry4. Metabolism and Regulation 4.1. Osmolyte Synthesis Versus Dietary Intake4.2. Retention of Osmolytes4.3. Development4.4. Cell Volume Regulation4.5. Hormonal Regulation5. Evolutionary Considerations6. Knowledge Gaps and Future DirectionsMarine elasmobranchs are hypoionic osmoconformers. Their extra- and intracellular fluids accumulate organic osmolytes for osmotic balance, mainly urea and methylamines such as trimethylamine N-oxide (TMAO), higher intracellularly at about 2:1 in shallow species, even in embryos and starvation. The relatively few euryhaline species reduce osmolytes only partially, and become hyperosmotic regulators, while permanent freshwater species have almost no organic osmolytes. Urea binds to and unfolds proteins while methylamines promote folding, thermodynamically counteracting each other at 2:1. Both also provide buoyancy. Deep-sea elasmobranchs increase TMAO and reduce urea, possibly because TMAO also counteracts protein-destabilizing effects of hydrostatic pressure. Recent studies show TMAO coordinates water molecules in a complex excluded from protein backbones (“osmophobicity”). This entropically favors folded protein conformations, and conteracts urea and pressure unfolding effects. Osmolytes are made in the liver or obtained from diets and retained by adaptations of gill, kidney, intestine, rectal gland. Regulatory mechanisms and evolutionary history are incompletely understood.
1. Introduction2. Cardiovascular Function and Energetics 2.1. Oxygen Transport by the Cardiovascular System2.2. Responses to Hypoxia2.3. Elasmobranch Cardiac Anatomy2.4. Cardiac Metabolic Biochemistry3. Factors Controlling and Effecting Cardiovascular Function 3.1. Heart Rate and Stroke Volume3.2. Body Fluid Volume and Blood Pressure Regulation4. Signaling Mechanisms Effecting Blood Vessel Diameter 4.1. Gasotransmitters4.2. Endothelins and Prostaglandins (Prostacyclin)4.3. Autonomic Nervous System Signaling Mechanisms (Adrenaline and Noradrenaline)4.4. Other Vascular Signaling Mechanisms (Acetylcholine, Adenosine, CNP, Serotonin, Vasoactive Intestinal Polypeptide, Bombesin, and Neuropeptide Y)4.5. Substances Affecting Gill Blood Flow Patterns5. The Action Potential and Excitation–Contraction (EC) Coupling in Elasmobranch Hearts: The Influences of Environmental, Biochemical, and Molecular Factors 5.1. The Action Potential5.2. EC Coupling5.3. Effects of Catecholamines and Acetylcholine5.4. Effects of Temperature and Acidosis6. Practical Applications: Physiology in the Service of Elasmobranch Conservation 6.1. Global Climate Change and Ocean Acidification6.2. Surviving Interactions with Fishing Gear7. SummaryThe functional characteristics of elasmobranch and teleost cardiovascular systems are similar at routine metabolic rates. Differences do become apparent, however, in cardiovascular function of high-energy-demand species (e.g., mako shark and yellowfin or skipjack tunas) at maximum metabolic rates. Elasmobranchs have an autonomic nervous system separable into parasympathetic and sympathetic components. The vagus nerve has a major role in controlling heart rate, although sympathetic innervation of the heart and blood vessels is absent. Elasmobranchs increase cardiac output primarily by increasing stroke volume which, in turn, is primarily determined by ventricular end diastolic volume. End diastolic volume is determined by filling time and venous filling pressure; with the latter being effected by venous tone and venous capacitance. Blood volume and pressure in elasmobranchs are controlled by endocrine (e.g., renin-angiotensin, kallikrein-kinin, and natriuretic peptides) and paracrine (e.g., endothelins, prostaglandins, the gasotransmitters NO and H2S,) mechanisms. Excitation-contraction (EC) coupling in elasmobranch hearts largely fits the accepted model for vertebrates, although the rise in cytoplasmic calcium is primarily from trans-sarcolemmal sources, which includes Na⁺-Ca²⁺ exchanger (NCX). Some elasmobranchs populations are severely depleted due to the intersection of life history characteristics, unsustainable rates of fisheries-associated mortality, and environmental degradation. To address these issues effectively will require a better understanding of the elasmobranch cardiovascular physiology – including the ability of various species to withstand the physiological consequences of the increasing temperature and expanding hypoxic zones accompanying global climate change, and the severe acidosis and the plasma ionic imbalances resulting from interactions with fishing gear.
1. Introduction2. Ionoregulation 2.1. Overview2.2. Seawater2.3. Freshwater and Intermediate Salinities3. Acid–Base Balance 3.1. Overview3.2. Seawater3.3. Freshwater and Intermediate Salinities4. Nitrogenous Wastes 4.1. Overview4.2. Seawater4.3. Freshwater and Intermediate Salinities5. Concluding RemarksElasmobranch fishes osmoconform to the marine environment by synthesizing and retaining substantial levels of urea (∼350 mM) and other organic osmolytes. Urea is reabsorbed at the kidney and although renal urea transport proteins have been isolated and characterized there is still a limited understanding of the reabsorptive processes involved. Marine elasmobranches are nitrogen-limited and attention has now turned to ammonia retention mechanisms. Elasmobranch fishes regulate plasma ions below seawater levels but above what is typical of marine teleosts. The gill plays a primary role in ion regulation, with the kidney and rectal gland contributing as well. The gill also has a phenomenal capacity to eliminate acid or base loads. Recent discoveries of solute transporters in the gill, kidney, and gut of elasmobranchs bring an enhanced understanding of ion and acid–base regulation. There have been a series of exciting papers on the elasmobranch gut that demonstrate a dynamic tissue with multiple roles. This chapter discusses recent and earlier research on ion, acid–base, and nitrogen regulation in these magnificent fishes inhabiting marine, fresh water, and intermediate salinity environments. There is tremendous scope for novel discoveries in this captivating group of fishes and we have identified compelling research questions for fish biologists.
MicroRNAs (miRNAs) are a class of short, evolutionary conserved non-coding RNA molecules, which are shown as the key regulators of many biological functions. External stress can alter miRNA expression levels, thereby changing the expression of mRNA target genes. Here, we show that miR-21 is involved in the regulation of alkalinity tolerance in Nile tilapia. Alkalinity stress results in a marked reduction in miR-21 levels. miR-21 loss of function could affect ion balance regulation, ROS production, and antioxidant enzyme activity in vivo. Moreover, miR-21 knockdown protects cell against alkalinity stress-induced injury in vitro. miR-21 directly regulates VEGFB and VEGFC expression by targeting the 3'-untranslated regions (UTRs) of their mRNAs, and inhibition of miR-21 significantly increases the levels of VEGFB and VEGFC expression in vivo. Taken together, our study reveals that miR-21 knockdown plays a protective role in alkalinity tolerance in tilapia.
It is thought that the elasmobranch corticosteroid hormone 1α-hydroxycorticosterone (1α-B) functions as both a glucocorticoid (GC) and mineralocorticoid (MC). Classical antinatriuretic MC activities would run counter to the osmoregulatory strategy of euryhaline elasmobranchs acclimating to fresh water (FW). Therefore we hypothesize that FW acclimation will be accompanied by a decrease in plasma corticosteroids in these animals. However, events that activate the "fight-or-flight" response could mask changes associated with acclimation to lower salinities. To better define the MC role of corticosteroids in elasmobranchs, we designed a transfer system that allows the acclimation of Atlantic stingrays (Dasyatis sabina) from seawater (SW) to FW over 12h while minimizing other extraneous stressors. Blood and interrenal glands were sampled from one group of stingrays 24h after FW transfer, while another group was sampled two weeks after FW transfer. Two other groups served as mock-transfer controls in that they were treated and sampled in the same way, but remained in SW for the entire period. Plasma corticosteroids, osmolality, chloride, and urea were significantly lower in FW-acclimated stingrays (compared to mock-transfer stingrays) 24h after FW transfer. This pattern remained after two weeks in FW, with the exception that plasma corticosteroids returned to pre-acclimation levels. There were no significant differences between experimental groups in interrenal levels of mRNAs encoding key steroidogenic proteins (steroidogenic acute regulatory protein and cholesterol side chain cleavage enzyme). Temporally decreased corticosteroid levels during FW acclimation are consistent with the unique strategy of euryhaline elasmobranchs, whereby lower plasma osmolality is maintained in FW vs. SW environments to reduce hydromineral gradients.
Copyright © 2015. Published by Elsevier Inc.
Elasmobranchs represent 96% of the cartilaginous fishes, while the holocephalans represent only 4%, and as such, elasmobranchs are the primary focus in this chapter. The elasmobranchs housed in public aquaria share large systems with mixed species. Catching these animals can be challenging and even dangerous to both the people and animals involved. Tonic immobility (TI) or hypnosis is one form of physical restraint used in a variety of elasmobranchs. Access to the elasmobranchs vasculature is often determined by the size and position of the animal rather than which vessel is most ideal. Ideally, preanesthetic assessment will include a preanesthetic assessment including a thorough history of the fish, information on water quality conditions and husbandry practices, remote observation and familiarization with any unique characteristics of the species or individual. The chapter lists anesthetic drugs used in the elasmobranchs, and discusses field immobilization of the elasmobranchs.
The white-edge freshwater whip ray Himantura signifer can survive in freshwater (0.7‰) indefinitely or in brackish water (20‰) for at least two weeks in the laboratory. In freshwater, the blood plasma was maintained hyperosmotic to that of the external medium. There was approximately 44mmoll - 1 of urea in the plasma, with the rest of the osmolality made up mainly by Na + and Cl-. In freshwater, it was not completely ureotelic, excreting up to 45% of its nitrogenous waste as urea. Unlike the South American freshwater stingray Potamotrygon motoro, H. signifer has a functional ornithine-urea cycle (OUC) in the liver, with hepatic carbamoylphosphate synthetase III (CPS III) and glutamine synthetase (GS) activities lower than those of the marine blue-spotted fan tail ray Taeniura lymma. More importantly, the stomach of H. signifer also possesses a functional OUC, the capacity (based on CPS III activity) of which was approximately 70% that in the liver. When H. signifer was exposed to a progressive increase in salinity through an 8-day period, there was a continuous decrease in the rate of ammonia excretion. In 20‰ water, urea levels in the muscle, brain and plasma increased significantly. In the plasma, osmolality increased to 571 mosmol kg - 1 , in which urea contributed 83mmoll - 1 . Approximately 59% of the excess urea accumulated in the tissues of the specimens exposed to 20‰ water was equivalent to the deficit in ammonia excretion through the 8-day period, indicating that an increase in the rate of urea synthesis de novo at higher salinities would have occurred. Indeed, there was an induction in the activity of CPS III in both the liver and stomach, and activities of GS, ornithine transcarbamoylase and arginase in the liver. Furthermore, there was a significant decrease in the rate of urea excretion during passage through 5‰, 10%o and 15‰ water. Although the local T. lymma in full-strength sea water (30‰) had a much greater plasma urea concentration (380 mmoll - 1 ), its urea excretion rate (4.7 μmol day - 1 g - 1 ) was comparable with that of H. signifier in 20‰ water. Therefore, H. signifer appears to have reduced its capacity to retain urea in order to survive in the freshwater environment and, consequently, it could not survive well in full-strength seawater.
A review of past behavioral ultrasonic telemetry studies of sharks and rays is presented together with previously unpublished material on the behavior of the lemon shark, Negaprion brevirostris, around the Bimini Islands, Bahamas. The review, focusing on movement behaviors of 20 shark and three ray species, reveals that elasmobranchs exhibit a variety of temporal and spatial patterns in terms of rates-of-movement and vertical as well as horizontal migrations. The lack of an apparent pattern in a few species is probably attributable to the scarcity of tracking data. Movements are probably governed by several factors, some still not studied, but data show that food, water temperature, bottom type, and magnetic gradient play major roles in a shark's decision of where and when to swim. A few species exhibit differences in behavior between groups of sharks within the same geographical area. This interesting finding warrants further research to evaluate the causes of these apparent differences and whether these groups constitute different subpopulations of the same species. The lack of telemetry data on batoids and some orders of sharks must be addressed before we can gain a more comprehensive understanding of the behavior of elasmobranch fishes. Previously unpublished data from 47 smaller and 38 larger juvenile lemon sharks, collected over the decade 1988–1998, provide new results on movement patterns, habitat selection, activity rhythms, swimming speed, rate-of-movement, and homing behavior. From these results we conclude that the lemon shark is an active predator with a strong, apparently innate homing mechanism. This species shows ontogenetic differences in habitat selection and behavior, as well as differences in movements between groups of individuals within the same area. We suggest three hypotheses for future research on related topics that will help to understand the enigmatic behavior of sharks.
The field of “elasmobranch endocrinology” began at the same time as the field of vertebrate endocrinology itself, when Bayliss and Starling (1903) used extracts from the intestines of sharks and skates to demonstrate the actions of secretin, the first described vertebrate hormone. Although perhaps coincidental, a pivotal role for sharks and their relatives in the birth of this field is prophetic to some extent, given that many vertebrate hormones appear to have first appeared in the cartilaginous fishes. Because sharks and their relatives occupy such a critical position in the evolution of the vertebrate endocrine system, studies on endocrinology of these fishes contribute to a better understanding of the roles that hormones exert in all higher vertebrates. Furthermore, because hormones regulate virtually all aspects of elasmobranch physiology, knowledge concerning the function of the elasmobranch endocrine system is essential for developing a full comprehension of how these fishes develop, grow, reproduce, and survive.
Thesis (Ph. D.)--University of Delaware, 1964. Photocopy.
Publisher Summary This chapter describes the role that salt-secreting organs play in fishes that are living in a hyperosmotic medium. Hypoosmoregulation is evidenced by those animals that can maintain the salt concentration of their extracellular fluid below that of the environment in which they live. Many hypoosmoregulating species inhabit marine or brackish waters, whereas others are located inland in saline lakes. Hyperosmoregulators on the other hand occupy terrestrial and/or freshwater habitats. The development of diverse anatomical structures within the various vertebrate classes that deal with salt and water balance suggest that at least two, and possibly more, independent solutions to the problem of osmoregulation have arisen. The evolution and development of the mammalian renal system appears to be an example of a system that couples the mechanism of regulating deviations of extracellular solutes and excess solvent within the same structure. The chapter reviews recent studies on cellular differentiation and its related biochemical changes in regard to the development of extrarenal osmoregulation. It also presents physiological studies of hypoosmoregutation in myxinoids morphological studies of salt secretion in petromyzonids
Since the discovery of the phenomenally high urea content of the body fluids of cartilaginous fishes by Staedeler and Frerichs (1858), the unique osmoregulatory system of this vertebrate class has been studied by many investigators. In broad outline, the mechanisms are essentially similar in all three of the major subtaxa: the selachians, the batoids, and the holocephalans. A single exception is the genus Potamotrygon (fresh-water stingrays of South America and Africa), which has apparently lost the ability to concentrate urea even when transferred to salt water.
The gill of fishes is modified for gas exchange, thereby providing a site for net movement of salts and water down their respective gradients. Specialized cells in the gill epithelium are joined by tight junctions of variable depth and express a variety of transporters and channels. These cells mediate NaCl extrusion in marine fishes and NaCl uptake in freshwater fishes. These transport steps also provide pathways for the extrusion of ammonia and acid vs. base equivalents. (C) 1999 Wiley-Liss, Inc.
Study of urea transport in teleosts offers systems that are in a sense “stripped down” from the elasmobranch situation, such that facilitated diffusion, rather than active transport, may predominate. Likewise, different tissues within teleosts also appear to offer heterogeneity of function and additional model systems. The toadfish system appears to offer an advantage in studies of the temporal dynamics of urea transport regulation. In addition, the chapter focuses on urea transport in adult fish because urea transport during development. Earlier stages in development may in fact continue to offer novel and simpler systems for the study of urea transport. Finally, with the rapid progress in identifying urea transporters at the molecular level in both mammals and fish, phylogenetic perspectives of transporter evolution will be most informative.
In general, marine elasmobranch fishes ( sharks, skates, and rays) maintain body fluid osmolality above seawater, principally by retaining large amounts of urea. Maintenance of the high urea concentration is due in large part to efficient renal urea reabsorption. Regulation of renal urea reabsorption also appears to play a role in maintenance of fluid homeostasis of elasmobranchs that move between habitats of different salinities. We identified and cloned a novel 2.7- kb cDNA from the kidney of the euryhaline Atlantic stingray Dasyatis sabina ( GenBank accession no. AF443781). This cDNA putatively encoded a 431- amino acid protein ( strUT- 1) that had a high degree of sequence identity ( 71%) to the shark kidney facilitated urea transporter ( UT). However, the predicted COOH- terminal region of strUT- 1 appears to contain an additional sequence that is unique among cloned renal UTs. Injection of strUT- 1 cRNA into Xenopus oocytes induced a 33- fold increase in [ (14)C] urea uptake that was inhibited by phloretin. Four mRNA bands were detected in kidney by Northern blot: a transcript at 2.8 kb corresponding to the expected size of strUT- 1 mRNA and bands at 3.8, 4.5, and 5.5 kb. Identification of a facilitated UT in the kidney of the Atlantic stingray provides further support for the proposal that passive mechanisms contribute to urea reabsorption by elasmobranch kidney.
Marine elasmobranch fishes maintain high urea concentrations and therefore must minimize urea loss to the environment in order to reduce the energetic costs of urea production. Previous studies have identified a facilitated urea transporter in the kidney of the dogfish. We examined mechanisms of urea transport in the kidney of the little skate Raja erinacea using an isolated brush-border membrane vesicle preparation. Urea uptake by brush-border membrane vesicles is by a phloretin-sensitive, non-saturable uniporter in the dorsal section and a phloretin-sensitive, sodium-linked urea transporter ( K m=0.70 mmol l–1, V max=1.18 μmol h–1 mg–1 protein) in the ventral section of the kidney. This provides evidence for two separate urea transporters in the dorsal versus ventral sections of the kidney. We propose that these two mechanisms of urea transport are critical for renal urea reabsorption in the little skate.
Marine elasmobranchs retain urea and other osmolytes, e.g. trimethylamine oxide (TMAO), to counterbalance the osmotic pressure of seawater. We investigated whether a renal urea transporter(s) would be regulated in response to dilution of the external environment. A 779 bp cDNA for a putative skate kidney urea transporter (SkUT) was cloned, sequenced and found to display relatively high identity with facilitated urea transporters from other vertebrates. Northern analysis using SkUT as a probe revealed three signals in the kidney at 3.1, 2.8 and 1.6 kb. Upon exposure to 50% seawater, the levels of all three SkUT transcripts were significantly diminished in the kidney (by 1.8- to 3.5-fold). In response to environmental dilution, renal tissue osmolality and urea concentration decreased, whereas water content increased. There were no significant differences in osmolyte and mRNA levels between the dorsal–lateral bundle and ventral sections of the kidney. Taken together, these findings provide evidence that the downregulation of SkUT may play a key role in lowering tissue urea levels in response to external osmolality.
Hochachka P.W., Somero G.N. (2002) Biochemical adaptation: mechanism and process in physiological evolution. New York: Oxford University Press. 466 p.
1. The major body fluid compartments were measured in two species of fresh-water Chondrostei, two species of fresh-water Holostei, three species of fresh-water Teleostei and seven species of marine teleosts. These were compared with previous measurements of an agnathan species and four species of Chondrichthyes.2. A general correlation was shown between the relative rates of respiratory movements and pulse rates, but neither of these appeared to be related to the taxonomic series. A faster pulse was more characteristic of marine than of fresh-water species.3. Plasma volume was measured by the dye dilution method, using T-1824. Whole blood volume was calculated from plasma volume and hematocrit. A progressive reduction in plasma and whole blood volume was noted, proceeding from the primitive to the more advanced groups. This is true both among the three classes of aquatic vertebrates and also among the three groups within the Osteichthyes. These volumes were remarkably similar in fresh-water and marine tel...
The bacterial flora of 28 neritic sharks, comprising five species, was examined. Representatives of the genus Vibrio and related species were isolated, including V. alginolyticus, V. harveyi, V. furnissii, V. damsela, V. carchariae, and Vibrio spp. Vibrios accounted for 59 of 78 pure cultures identified to genus. The remaining 19 isolates comprised Photobacterium and Proteus spp., one strain of Escherichia coli, and three unidentified strains. Vibrios were found in all the organs and tissues that were cultured, indicating that these bacteria are part of the natural flora of sharks. Surprisingly, vibrios were frequently isolated from the kidneys, liver, and spleen of the sharks, strongly suggesting a bacterial-host relationship. Implications of this relationship are that vibrios may act as opportunistic pathogens for sharks and that vibrios may affect shark physiology, for example urea homeostasis.
The gill of fishes is modified for gas exchange, thereby providing a site for net movement of salts and water down their respective gradients. Specialized cells in the gill epithelium are joined by tight junctions of variable depth and express a variety of transporters and channels. These cells mediate NaCl extrusion in marine fishes and NaCl uptake in freshwater fishes. These transport steps also provide pathways for the extrusion of ammonia and acid vs. base equivalents. J. Exp. Zool. 283:641–652, 1999.
This chapter discusses the molecular and mitochondrial studies of the urea cycle in fish. A significant proportion of energy production in fish involves the catabolism and oxidation of proteins and amino acids. Consistent with their water habitat, the major end product of nitrogen metabolism in most fish is ammonia. Carbamoyl phosphate is the precursor for two major metabolic pathways: the urea cycle and pyrimidine nucleotide biosynthesis. The first step of the urea cycle in mammalian and amphibian ureotelic species is catalyzed by carbamoyl-phosphate synthetase I. The function of CPSase II is to catalyze carbamoyl phosphate formation as the first step in pyrimidine nucleotide biosynthesis. The utilization of the amide group of glutamine for the biosynthesis of carbamoyl phosphate in the glutamine-dependent CPSases involves the reaction of glutamine with a cysteine SH group on the enzyme to form a γ-glutamyl thioester intermediate, releasing ammonia, which reacts with an activated intermediate common to all CPSase. The unique co-functioning of glutamine synthetase and CPSase III in ammonia assimilation in the mitochondrial matrix probably reflects the adaptation of urea synthesis for the dual role of ureoosmotic and ureotelic functions.
The evolutionary aspects of nitrogen metabolism and excretion in fishes continue to be an exciting and fruitful area for research, hypothesis generation, and testing. The scenario proposed by Griffith can be evaluated more completely because several significant findings of the past decade: (1) Although larval lampreys (ammocoetes) do not appear to express O-UC activity as predicted by Griffith, the O-UC occurs in embryos of teleosts that are typical ammonoteles as adults; (2) urea transport appears to be governed in fish by very specific transport molecules; (3) CPSase activities with some “I” isozyme characteristics, that is, enzymes with at least equal preference for ammonia as substrate, are found in higher teleostean fish; and (4) it is fully understand the cost of osmoregulation in ureosmotic versus hypoosmotic regulators, and the teleostean strategy may prove to be more economical.
Peripheral plasma concentrations, metabolic clearance rates (MCR) and blood production rates (BPR) of 1α-hydroxycorticosterone (1-OH-B) were determined in female dogfish (Scyliorhinus canicula) under varying environmental conditions. The constant-infusion technique, using high specific activity tritiated 1-OH-B, was applied to measure the MCR, and BPR were derived from the product of plasma concentration and MCR at equilibrium. Urea plasma clearances and apparent BPR were assessed in a similar manner. Fish were adapted stepwise to 140, 120, 90, 80, 70, 60 and 50% normal sea water (about 1000 mosmol/l). In all cases 1-OH-B was the major corticosteroid, cortisol and corticosterone were sought but never detected.
In environments of reduced osmolarity, plasma osmolarity, sodium, chloride and urea concentrations all declined, alongside increases in plasma concentrations, MCR and BPR of 1-OH-B. In fish held in environments at concentrations greater than normal sea water, plasma osmolarity, sodium, chloride and urea concentrations all increased. Plasma clearance of urea increased in fish held in environments more dilute than sea water, while it decreased in the more hyperosmotic waters. It is tentatively concluded that homeostasis of plasma composition, with particular respect to urea, is in part regulated by 1-OH-B in the dogfish.
J. Endocr. (1984) 103, 205–211
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South American freshwater stingrays, which lack the high urea content characteristics of all marine and euryhaline elasmobranchs, fail to build up their urea concentration appreciably even when exposed to a saline environment gradually increased to a concentration approaching that of full strength sea water over a period of as much as six days. In a hypertonic external medium, osmolality of body fluid rises somewhat, but largely by an increase of sodium chloride rather than by urea accumulation.
Urea and trimethylamine oxide (TMAO) have long been recognized as important osnuotic constituents of elasmobranch body fluids. Together, these organic solutes make up roughly one-half of the osmotically active constituents of elasmobranchs, allowing them to remain osmotically superior to the surrounding sea water. Eggs of elasmobranchs are nearly isosmotic to sea water ( Dakin, 191 1) , and like the adults, they contain significant quantities of urea (Kruken burg, 1888) . Smith ( 1936) , in reviewing urea retention and modes of repro duction in elasmobranchs, concluded that the tendency of elasmobranchs toward intrauterine development may be related to its adaptive advantage in protecting early embryos fronu urea loss. More recently, Price and Daiber ( 1967) , in their study of the intrauterine environment of ovoviviparous and viviparous forms, have concurred with Smith's conclusion. In what appears to be the only detailed study of the amounts of urea at various developmental stages of elasmobranchs, Needham and Needham (1930) found that in the dogfish Scyllium canicula, low amounts of urea are present when the eggs are laid, and that this urea is added to by the embryos as development proceeds (see also Needham, 1931) . Although Needham and Needham did not measure urea concentrations directly, they suggested that urea levels are lower in the undeveloped eggs than in the maternal fluids. They found that only a minimal amount of urea was lost from the developing embryo, and concluded that the increased urea content during development must be due to the excretion of urea by the embryo into the yolk. Their conclusions were rendered uncertain, however, by the lack of a good series of weighings for Scyllium embryos, and more importantly by their failure to measure urea in the embryos and yolks separately. In adult elasmobranchs, the major pathway of urea synthesis appears to be via the ornithine-urea cycle (Schooler et al., 1966). Recently (Read, 1968), evidence was presented of an ornithine-urea cycle early in elasmobranch develop ment. Embryos of the oviparous Raja binoculata and the ovoviviparous Squalus suckleyi were found to contain ornithine carbamoyltransferase and arginase, and the specific activities of these ornithine-urea cycle enzymes underwent marked increases early in development. Neither of these enzymes was found in the yolk, suggesting that any urea formed during development must be supplied by the embryo. This work thus supported the finding of Needham and Needham (1930) thatelasmobranchembryos are capableof producingurea.
This chapter examines the hormonal control of gill Na+, K+-ATPase and chloride cell function. The secretion of excess sodium and chloride by teleosts in seawater is carried out by gill chloride cells. These highly specialized cells are characterized by a large, columnar appearance, numerous mitochondria, an extensive tubular system, an apical crypt, and mucosal–serosal exposure. The breakdown of ATP is detected in the presence and absence of ouabain. Herndon found that prolactin treatment of seawater-adapted tilapia resulted in a dramatic reduction in chloride cell size without changing chloride cell density. Chloride cell height and the proportion of cells spanning the opercular membrane were reduced, suggesting that these cells were effectively removed as chloride secretory cells. Based on reductions in opercular membrane conductance and short-circuit current following prolactin the treatment of seawater-adapted tilapia, Foskett postulated that prolactin reduces chloride cell numbers and active transport in the remaining chloride cells. Working with the closely related but stenohaline Nile tilapia (Oreochromis niloticus), Pisam found that prolactin injection caused the appearance of the smaller β-chloride cells that were previously absent from these fish in seawater.
In all but 3 of 16 species of Amazon crustaceans and fishes, the activities of inorganic ions in the blood are 15–27% lower than expected. In most of the fishes, blood pH is higher than expected. In two of the fishes both departures are reversed by adding NaCl or KCl, but not H+, to the medium. Ammonia excretion varies inversely with blood and water salinity, and changes in blood ammonia induced by experimental exposure to air or high salinity can explain the changes in blood pH. Preliminary data suggest that the (Na+ + K+)-ATPase prepared from the gill of one teleost has a higher affinity for NH4+ than K+. It is suggested that at least some of the Na+ absorbed from the medium is exchanged for NH4+, and that binding to the ammonia molecule is responsible for at least some of the reduction of H+ activity in the blood.
1. Juvenile freshwater stingrays of the family Potamotrygonidae are unable to survive in salinities in excess of 20.6‰ when gradually acclimated.2. No differences were observed in blood pH or hematological parameters when fish adapted to a salinity of 14.5‰ were compared with freshwater controls.3. Significant increases were found in serum sodium (21%), chloride (21%), calcium (48%), and magnesium (51%). Increases in total CO2 (16%), potassium (21%), and inorganic phosphorus (35%) were not significant on account of high variance. Serum osmolarity increased 69%.4. There was no apparent effect of salinity on serum total cholesterol, organically bound phosphorus, or total carbohydrates. Glucose contributed only 25% of the latter.5. Serum urea was low (1.1 mM/1) as previously reported, and the trend to increase in a saline environment was not osmotically significant.6. Freshwater stingrays are unique among elasmobranchs in possessing significant amounts of a protein with the electrophoretic mobility of human ...
1. Water, urea and glucose reabsorption have been studied simultaneously using the inulin clearance method.2. Variations in filtration rate appear to be due to changes in the number of functional glomeruli, rather than to changes in the rate of function of individual units.3. Glucose reabsorption is not correlated in any way with that of urea.4. The main factor controlling the urea reabsorption is the concentration of urea occurring normally in the plasma. Attempts to increase the reabsorption of urea by raising the plasma level result in a depression of reabsorption not only of urea but of glucose and water as well. To some extent this reduced water reabsorption is offset by a decreased filtration rate, with the result that only a very moderate diuresis ensues.5. The percentage of the filtered urea which is reabsorbed varies with the concentration in the plasma, ranging from 70% to 99.5%, comparing quite favorably with the reabsorption of glucose.6. On the average, the actual amount of unabsorbed urea le...
Comprehensive analyses have been made of the plasma and parietal muscle of the marine cyclostome Myxine glutinosa and of the chimaeraoid Chimaera monstrosa, and the sum of some 17 constituents compared with the directly determined osmotic concentration of plasma. Both Myxine and Chimaera plasmas are isosmotic with sea water within 2%. Ions account for at least 96% of the total concentration of the plasma in Myxine and about 60% in Chimaera, but the blood of the latter contains 332 mM urea, compared with 3 mM in Myxine. In muscle a similar divergence exists, the urea concentrations per kilogram muscle water being practically the same as in the respective plasmas of the two species. The mean values in millimoles of the other nitrogenous constituents of muscle are for Myxine and Chimaera (figures for latter in brackets), trimethylamine oxide 87 (189), betaine 65 (38), creatine and creatinine 30 (37), amino acids 291 (43). Using osmotic coefficients the calculated osmotic concentration of muscle exceeds that of plasma in Myxine and possible reasons for this are discussed.
Estimates of inulin space in Myxine muscle have enabled intracellular concentrations of ions to be calculated. Analysis of Myxine bile shows it to have much higher concentrations of sodium, calcium and magnesium, and much lower chloride, than plasma. The resulting large anion deficit is presumably made up chiefly by the bile salt myxinol disulphate.
In undiluted sea water, the lip-shark Hemiscyllium plagiosum excretes urine high in Mg++ and rectal-gland fluid with high concentrations of Na+ and Cl−. Urea is excreted largely by the gills (87%), but also by kidney (5%) and rectal gland (6%).
In 50% sea water, the urine flow rate increases while the rate of excretion of Mg++ decreases. The rectal gland continues to secrete a fluid with Na+ and Cl− hypertonic to those of plasma. Branchial loss of urea is diminished as a result of a decline in plasma-urea concentration so that the clearance rate is unaffected. The U/P ratio for urea increases and the kidney excretes 48.4% of the total urea loss. However, this change in excretion of urea is too small to account for the concomitant decline in body urea content indicating that a reduction in the rate of biosynthesis of urea occurs.
Published data on the diversity, life history, ecology, and status of freshwater and euryhaline elasmo-branchs was reviewed in the context of anthropogenic threats and principles of conservation biology. At least 171 species of elasmobranch, representing 68 genera and 34 families, are recorded from fresh or estuarine waters. Of these, over half are marginal in estuaries, less than one-tenth are euryhaline, and one-¢fth are obligate in fresh water. Obligate freshwater elasmobranchs are dominated by myliobatoid stingrays, of which two-thirds are potamotrygonids endemic to Atlantic drainages of South America. Freshwater and euryhaline elasmobranchs adhere to strongly K-selected life histories and feed at high trophic levels, similar to those of their marine relatives. However, freshwater and euryhaline elasmo-branchs are also subject to habitat constraints, notably more limited volume and physicochemical variability than the ocean, that may render them more vulnerable than marine elasmobranchs to the e¡ects of human activities. The greatest diversity and abundance of freshwater and euryhaline elasmo-branchs occur in tropical countries with enormous and rapidly increasing human populations, notably South America, West Africa, and south-east Asia. Knowledge of the biology, distribution, ecology, and status of freshwater and euryhaline elasmobranchs is frustrated by unresolved taxonomic problems, which are brie£y summarized. To clarify selected issues in the conservation of freshwater and euryhaline elasmo-branchs, special attention is given to sharks of the genus Glyphis, pristids, and potamotrygonids. To foster live release when possible as well as prevent discard of specimens and loss of data, an illustrated key to di¡erentiate Carcharhinus from Glyphis sharks is provided. Obligate freshwater elasmobranchs with limited geographic ranges are deemed most vulnerable to extinction, but euryhaline elasmobranchs that require access to the sea to breed are also at signi¢cant risk. Based on the foregoing data and principles of conser-vation biology, suggested action plans for the conservation of freshwater and euryhaline elasmobranchs and the conservation of freshwater habitats are provided.
There is increasing evidence that urea movement across many epithelia involves more than passive diffusion. Of particular interest is the observation that urea transport across the erythrocyte membrane and across the vasopressin-stimulated urinary bladder of the toad occurs by facilitated diffusion, and can be selectively inhibited by phloretin and chromate. These inhibitory agents have been employed in studies of renal urea reabsorption by the spiny dogfish Squalus acanthias. Both agents inhibit urea reabsorption; the effect of chromate is of particular interest, since it blocks urea reabsorption to a proportionately greater extent than sodium reabsorption, and does so irreversibly.
Under laboratory conditions, the gills of the striped dogfish Poroderma africanum (Gmelin) were studied as a possible site of ion and water transfer between internal and external media. Haematocrits showed that a drop in the external osmalarity produced increase in blood volume, measured as a decrease in blood pcv's (packed cell volume). Similarly, when fish were exposed to increased external osmolarity, the converse occurred, with resultant rise in pcv's. The use of phenol red showed that normally-fed fish did not drink the medium, and hence dilution of the blood was most probably due to water influx at the gills. Hypoosmotic fish (due to underfeeding) drank the medium in appreciable quantities, and dilution and concentration of external medium had a more pronounced effect on blood pcv's in these individuals. The pcv's of normally fed fish returned to initial values within 7 days after transfer to new medium is most cases, but hypo-osmotic specimens took longer to adjust to the new state of water balance. Surgical closure of rectal gland and urinary systems produced initial rises in serum sodium and chloride levels, but these reached equilibrium after 5 to 7 days, indicating compensatory regulation by some other organ, such as the gills. After removal of the sutures to the urinary systems of 3 fish, there was a noticeable drop in sodium and chloride levels of the serum in these individuals. All fish were kept for 14 days in the laboratory, with little change in blood composition (as measured) and with only the gills as regulatory organs in two of them. By using the radioisotopes Chloride-36 and Sodium-22, it was shown that both ions are lost at the gills, against the concentration gradient. Histochemical examination of gill tissue from several fish indicated that many cells contain high concentrations of chloride and are probably the site of chloride excretion. The number of such cells increased with increase in external salinity, and they were also abundant in tissue from hypo-osmotic specimens and those with inoperative urinary and rectal gland systems. From these findings it was concluded that the gills have a definite role in the ion and water balance of P. africanum.
Pyjama sharks (Poroderma africanum) were exposed to a wide range of salinities, over which blood serum was analysed for osmolarity, chloride and urea concentrations. Fish were divided into two groups, those fed twice weekly (high intake), and those fed once a month (low intake). Both groups were exposed to the same salinity range. High intake fish showed the characteristic elasmobranch osmolarity picture, with serum values slightly hyper-osmotic at all times. Low intake fish, however, showed a degree of hypo-osmotic regulation. Serum values for both groups overlapped at very low salinities. Serum urea was also affected by diet, so that again two distinct sets of values were produced, again with overlap at the lower salinities. When previously well-fed fish were starved over a period of one month, serum urea and osmolarity decreased simultaneously. Consequently, it is felt that serum osmolarity is directly related to serum urea levels. Serum chloride was not found to be affected by diet, both groups showing the same change in blood values when exposed to the same change in salinity. It is shown, however, that a reduction in food intake, over a period of more than a fortnight, can reduce metabolic urea to the extent of depressing serum osmolarity and, hence, shift the ionic and osmotic equilibrium between the fish and the sea water. This may result in varying degrees of hypoosmotic regulation.
1.1. Freshwater stingrays, Potamotrygon sp., were slowly acclimated to 40% sea water.2.2. Saline acclimation had no significant effect on the rate of urea biosynthesis.3.3. Rates of urea excretion were low and did not differ significantly in freshwater and saline acclimated rays. Total ammonia-n + urea-n was excreted primarily as ammonia-n in both groups.4.4. The rate of simultaneous total-body clearance of urea-14C and thiourea-14C in saline acclimated rays did not indicate that saline acclimation was accompanied by renal conservation of urea.5.5. Hematocrits and plasma urea concns did not differ significantly in fresh water and saline acclimated rays.6.6. Plasma sodium and chloride concns were significantly increased by saline acclimation.
1.1. Activity levels of ornithine-urea cycle enzymes in the liver of Potamotrygon, a fresh-water ray, were one-half to one-twentieth those found in marine rays. The rate of incorporation of 14C-bicarbonate into urea by liver slices was also markedly reduced.2.2. In contrast to marine elasmobranchs, Potamotrygon did not actively reabsorb 14C-urea in its renal tubules.
The focus of this chapter is on recent developments in our understanding of the expression of the urea cycle and the roles of glutamine synthetase (GSase) and glutamine-dependent carbamoyl-phosphate synthetase (CPSase III) in ammonia detoxification in fish, with emphasis on teleosts. Marine elasmobranch fishes have an active urea cycle, synthesizing and retaining urea for the purpose of osmoregulation. Well-characterized biochemical features uniquely different from the urea cycle in ureotelic terrestrial vertebrates that have served as a reference for studies on teleost fishes are reviewed. The majority of teleost fishes are ammonotelic, that is, ammonia is simply excreted directly across the gills into the surrounding aqueous environment. But, a functional urea cycle and ureotelism have been documented in a few adult species as adaptations to unusual environmental circumstances (stress, air exposure, high pH, exposure to high concentrations of ammonia). These adaptations are reflected in altered (1) CPSase III amino acid sequences and kinetic properties, (2) regulation of the levels of gene expression, and (3) specificity of expression with respect to organ localization (e.g., expressed in muscle instead of liver) and life-cycle stage. The emerging view is that all fish likely have genes for the urea cycle enzymes; teleost fish normally do not express the urea cycle enzymes (except perhaps during embryogenesis); fish are opportunistic in variations of adapting expression of the urea cycle to local environments; and fish are highly individualistic in the mechanisms they employ for adapting to varying environmental challenges, that is, expression of the urea cycle is only one of several possible strategies.
The physiological effects of waterborne silver exposure (added as AgNO3) on spiny dogfish, Squalus acanthias, were evaluated at 30, 200 and 685 μg silver per l in 30‰ seawater. These concentrations cover the toxic range observed for freshwater teleosts, where silver is extremely toxic, to seawater teleosts which tolerate higher silver concentrations. However, these levels are considerably higher than those that occur in the normal environment. At 685 μg l−1, dogfish died within 24 h. Causes of death were respiratory as well as osmoregulatory failure. Arterial PaO2 rapidly declined below 20 Torr, and blood acidosis (both respiratory and metabolic) occurred. Urea excretion increased dramatically and plasma urea dropped from 340 to 225 mM. There were pronounced increases in plasma Na+, Cl−, and Mg2+, indicative of ionoregulatory failure due to increased diffusive permeability as well as inhibited NaCl excretion. At 200 μg l−1, fish died between 24 and 72 h of silver exposure. The same physiological events occurred with a small time delay. At 30 μg l−1, effects were much less severe, although slight mortality (12.5%) still occurred. Respiratory alkalosis occurred, together with moderate elevations in plasma Na+ and Cl− levels. Silver accumulated to the highest concentrations on gills, with only low levels in the intestine, in accord with the virtual absence of drinking. Na+/K+-ATP-ase activities of gill and rectal gland tissue were impaired at the highest silver concentration. Normal gill function was impaired due to swelling and fusion of lamellae, lamellar aneurism and lifting of the lamellar epithelium. Our results clearly indicate that this elasmobranch is much more sensitive (about 10-fold) to silver than marine teleosts, with silver's toxic action exerted on the gill rather than on the intestine, in contrast to the latter.
Single tubules, dissected from the peritubular sheath of the dorsal bundle zone of kidney of the dogfish shark, Squalus acanthias, were perfused in vitro at 17-18 degrees C. This segment is the largest of the five in the peritubular sheath and had average inner and outer diameters of 46.9 +/- 1.2 and 74.4 +/- 2.1 microns, respectively (n = 32). These values suggest that this is the intermediate IV segment. When perfused with symmetrical buffered elasmobranch saline, intermediate IV segments exhibited high rates of Cl- absorption (JCl, pmol.s-1.cm-2): 1,696 at an average perfusion rate (Vo) of 8.2 nl/min. Cl- absorption was highly flow dependent [1/JCl = 57.95(1/Vo) + 1.75; r = 0.71, P less than 0.01]. Maximal rates of Cl-absorption, calculated from reciprocal transformation of the flow dependence of JCl, yielded a value of 5,714 pmol.s-1.cm-2. In the presence of a 200-mosmol/kg transepithelial osmotic gradient, fluid absorption was negligible. The spontaneous transepithelial voltage (Vte, mucosal with respect to serosal compartment) averaged 8.0 +/- 1.0 mV (n = 26). Such active transport of Cl- in the absence of fluid movement and in the presence of a lumen-positive transepithelial voltage is characteristic of amphibian and mammalian diluting segments. Na(+)-to-Cl- permeability ratios (PNa/PCl) averaged 2.5 +/- 0.5, indicating that, as in mammalian thick ascending limbs, this segment is Na+ (cation) permselective. Vte was dependent on the presence of Na+ and Cl- in the external solutions and was reversibly abolished by isosmotic replacement with N-methyl-D-glucamine or with isethionate, respectively. Ouabain inhibited Vte but was not reversible within the time course of these experiments. Furosemide (10(-4) M), but not equimolar concentrations of amiloride or hydrochlorothiazide, added to the luminal perfusate inhibited both Vte and JCl. These results suggest that apical membrane Na+ entry in intermediate IV segments is mediated by Na(+)-K(+)-Cl- cotransport and is consistent with the existence of a functional role of urinary dilution in the reabsorption of urea in the elasmobranch kidney.