Article

Physiological Effects of Salinity on the Osmotic Properties and Oxidative Stress Responses of the Razor Clam Solen regularis in Don Hoi Lot, Thailand

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Abstract

Estuaries are diverse coastal ecosystems that act as transitional zones between freshwater and seawater. The Don Hoi Lot tidal flat, located in the upper Gulf of Thailand, is one of Thailand's most important estuarine ecosystems. Nonetheless, the Don Hoi Lot area faces increasing environmental pressures due to human activities and natural changes. One of the most prominent species well-known in this area is the razor clam Solen regularis. This study investigated the effects of salinity fluctuations on the osmotic properties and antioxidant enzyme activities of the five tissues: foot, mantle, adductor muscle, gill, and digestive gland from the razor clam S. regularis collected from the Don Hoi Lot tidal flat. Razor clams were exposed to a range of salinity levels (0-35 PSU) for 7 days. The results indicated that the hemolymph osmolality of S. regularis increased with increasing salinity, demonstrating an osmoconforming pattern. Salinity changes significantly affected the activities of antioxidant enzymes (SOD, CAT, and GPX) in various tissues. In most tissues, SOD and CAT activities increased at higher salinities (30-35 PSU), while GPX activity generally increased across all tissues with increasing salinity. GST activity was not significantly affected by salinity changes. These findings suggest that S. regularis can activate antioxidant defense systems to reduce oxidative stress caused by salinity fluctuations. This study provides valuable insights into the physiological responses of S. regularis to environmental salinity changes, which can inform conservation efforts for this ecologically important species in estuarine ecosystems.

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We have measured the population density of razor clams, Solen regularis, as well as a related but economically less valuable species, Solen vitreus, at the same locations in the Don Hoi Lord mud flats in Samut Songkram, Thailand at monthly intervals from November, 1995 to March, 1997. The density of clams in each size class vs. time was fit by a simple population evolution model in terms of birth, growth, and death (e.g., by harvesting). The results indicate a growth rate of approximately 1 cm per month, the appearance of new clams during March-April and July-August, and intensive harvesting of larger clams during April-November. In addition to seasonal variations, a comparison with data from previous studies indicates a twelve-fold decline in the population of razor clams (S. regularis) since 1994-1995, while the population of S. vitreus declined only four-fold. We interpret this to indicate that environmental factors are responsible for a four-fold decrease in both clam populations, while intensive, selective harvesting of S. regularis has driven down its population by an additional factor of three.
Article
The razor clam Sinonovacula constricta is an economically important bivalve species in China, which inhabits the mudflat of intertidal and estuaries areas and frequently suffers the low salinity stress due to heavy rain events. The present study investigated the effects of hyposalinity stress (salinity 10, 5, 3, and 1) on osmoregulation and haemolymph microbiota in S. constricta after 7 days of exposure. The results showed that the survival rate of razor clams was only significantly reduced in the 1 ppt group compared to the control group (salinity 20) (P < 0.05). Haemolymph osmolality in razor clams reduced significantly with decreasing salinities (P < 0.05). Taurine, arginine, alanine and ornithine were the dominant free amino acids (FAAs) with higher contents in the haemolymph. Exposure to low salinities altered the composition of haemolymph FAAs, with alanine being the highest in the control group and taurine being the highest in the low salinity (LS) groups. Hyposalinity stress caused an increase in Na⁺/K⁺-ATPase (NKA) activity in gills, and histological analysis revealed severe hyperplasia in gills and a significant reduction in the length of gill filaments (P < 0.05). The microbial community variation in the haemolymph assessed by 16S rRNA sequencing explains that bacterial communities was shifted under low salinity stress. At the genus level, the relative abundance of Marinobacterium, Marinomonas and Pseudomonas changed significantly under hyposalinity stress (P < 0.05). These findings give further insight into the physiological responses and potential adaptation strategies of razor clams to hyposalinity stress and provide information for the development of inland low salinity culture.
Article
A numerical hydrodynamic modelling study has been implemented based on the seasonal salinity variations in a networked system (comprising creek and an estuary), which is the first of its kind attempted for the Indian subcontinent. Salinity variations in the estuaries and creeks exhibited unique characteristics caused by the combined effects of various external forces such as tidal flow, freshwater runoff, wind and geometric effects. Precise understanding of dynamical conditions in estuaries and creeks is necessary to address pertinent issues related to oceanography, water quality and ecosystem dynamics. In a broader perspective, it is noted that due to the influence of winds during monsoon, the salinity fields in the estuarine environment are not in a steady state. However, in creeks, tidal flow plays a major role in altering the salinity structure apart from runoff. The results from this study decipher the fact that the networked system was vertically homogenous during all seasons. However, a horizontal salinity gradient was observed in the system depending on the river runoff. The flushing time for the Ulhas estuary was about 1.5 and 2.57 days during the monsoon and non-monsoon seasons, respectively. Similarly, for the Thane creek, tide-driven flushing time was about 3.68 days. The low flushing time during the wet season provides a suitable dynamic environment for effluent discharge in the mid and upstream reaches of the estuary, wherein the freshwater influx is higher. On the contrary, during the dry season over this region, the low runoff and the highest flushing times can increase the pollution or can support the growth of phytoplankton biomass accumulation.
Chapter
This chapter provides an overview of the main drivers of change in estuarine systems, their expected causes and impacts on estuarine fish and fisheries. An analysis of global, regional and local patterns of estuarine fish and how climate‐induced change may impact estuarine systems and their fish communities is provided. We also examine the main environmental, climatic and biological stressors likely to impact estuarine fish and associated fisheries. A set of case studies is used to illustrate the differences in potential impacts associated with various global regions and types of estuaries. An understanding of climate change in estuaries will support estuarine ecosystem resilience, inform management and facilitate adaptation.
Article
Most marine mollusks exhibit osmoregulatory mechanisms to cope with hypo- or hyperosmotic stress. In the present study, we elucidated the effects of exposure to hyposmotic and hyperosmotic stress on bay scallop Argopecten irradians. The Na+, K+, and Cl− ions of bay scallop hemolymph traversed in a similar direction as that of surrounding salinity. Moreover, the gill Na+/K+-ATPase was affected by changes in salinity. We further evaluated the heat shock protein (HSP)70 mRNA expression in the digestive diverticula and glucose levels in the hemolymph of bay scallops. Sudden changes in salinity were found to affect the HSP70 mRNA expression and glucose levels. In 55% salt water (SW) at 48–72 h, HSP70 mRNA expression and glucose levels were reduced compared to the control. HSP70 mRNA was present in the cells of digestive diverticula tissue at higher levels in 120% SW at 72 h than in the control. A TUNEL assay revealed that salinity changes increased cell apoptosis. Thus, exposure to 55% SW or more for more than 24 h may threaten the survival of marine mollusks.
Article
Solen dactylus is one of the most common razor clams in the tidal zone in the western Indian Ocean and Persian Gulf. Habitats of these clams may be subject to salinity fluctuations due to high evaporation and heavy rainfall. To assess their adaptation, in the laboratory clams were placed in 50-litre tanks with 40 cm depth of natural substrate and were kept in salinity of 45 ppt for a week before subsequent experiments. They were then exposed to salinities of 5, 20, 35, 45 and 65 ppt for three weeks. Concentrations of Na⁺, Cl–, K⁺, Ca²⁺, and Mg²⁺, as well as osmolality in the haemolymph and tank water were measured at 1, 24, 72 h, one week and three weeks after the start of the experiment. Haemolymph Na⁺ and Cl– values followed the concentrations of the external medium so that they decreased at low salinities and increased at high salinities. The Ca²⁺, Mg²⁺ and K⁺ ions all showed a common pattern, and their trend was independent of the external environment. Histopathological analysis showed severe tissue damage at low salinities with expanded intercellular spaces, an increase of intracytoplasmic vacuoles in the digestive tubules, and necrosis and destruction of lamellae in gill tissues.
Chapter
Estuaries are commonly described as semi-enclosed bodies of water, situated at the interface between land and ocean, where seawater is measurably diluted by the inflow of freshwater (Hobbie, 2000). The term “estuary,” derived from the Latin word aestuarium, means marsh or channel (Merriam-Webster, 1979). These dynamic ecosystems have some of the highest biotic diversity and production in the world. Not only do they provide a direct resource for commercially important estuarine species of fishes and shellfish, but they also provide shelter and food resources for commercially important shelf species that spend some of their juvenile stages in estuarine marshes. For example, high fish and shellfish production in the northern Gulf of Mexico is strongly linked with discharge from the Mississippi and Atchafalaya rivers and their associated estuarine wetlands (Chesney and Baltz, 2001). Commercial fishing in this region typically brings in 769 million kg of seafood with a value of $575 million. Fisheries production and coastal nutrient enrichment, via rivers and estuaries, are positively correlated within many coastal systems around the world (Nixon et al., 1986; Caddy, 1993; Houde and Rutherford, 1993). The coupling of physics and biogeochemistry occurs at many spatial scales in estuaries (figure 1.1; Geyer et al., 2000). Estuarine circulation, river and groundwater discharge, tidal flooding, resuspension events, and exchange flow with adjacent marsh systems (Leonard and Luther, 1995) all constitute important physical variables that exert some level of control on estuarine biogeochemical cycles. There has been considerable debate about the definition of an estuary because of the divergent properties found within and among estuaries from different regions of the world. Consequently, there have been numerous attempts to develop a comprehensive and universally accepted definition. Pritchard (1967, p. 1) first defined estuaries based on salinity as “semi-enclosed coastal bodies of water that have a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.” A general schematic representation of an estuary, as defined by Pritchard (1967), and further modified by Dalrymple et al. (1992) to include more physical and geomorphological processes, is shown in figure 1.2.
Article
Molluscan shell formation is a complex energy demanding process sensitive to the shifts in seawater CaCO3 saturation due to changes in salinity and pH. We studied the effects of salinity and pH on energy demand and enzyme activities of biomineralizing cells of the Pacific oyster (Crassostrea gigas) and the hard-shell clam (Mercenaria mercenaria). Adult animals were exposed for 14 days to high (30), intermediate (18), or low (10) salinity at either high (8.0-8.2) or low (7.8) pH. Basal metabolic cost as well as the energy cost of the biomineralization-related cellular processes were determined in isolated mantle edge cells and hemocytes. The total metabolic rates were similar in the hemocytes of the two studied species, but considerably higher in the mantle cells of C. gigas compared with those of M. mercenaria. Cellular respiration was unaffected by salinity in the clams’ cells, while in oysters’ cells the highest respiration rate was observed at intermediate salinity (18). In both studied species, low pH suppressed cellular respiration. Low pH led to an upregulation of Na⁺/K⁺ ATPase activity in biomineralizing cells of oysters and clams. Activities of Ca²⁺ ATPase and H⁺ ATPase, as well as the cellular energy costs of Ca²⁺ and H⁺ transport in the biomineralizing cells were insensitive to the variation in salinity and pH in the clams and oysters. Species-specific variability in cellular response to low salinity and pH indicates that the disturbance of shell formation under these conditions has different underlying mechanisms in the two studied species.
Article
Mussels, such as the marine bivalve Mytilus galloprovincialis are sentinels for marine pollution but they are also excellent bioindicators under laboratory conditions. For that, in this study we tested the modulation of biochemical responses under realistic concentrations of the toxic metal Lead (Pb) in water for 28 days under different conditions of salinity and temperature, including control condition (temperature 17 ± 1.0 °C and salinity 30 ± 1.0) as well as those within the range expected to occur due to climate change predictions (± 5 in salinity and + 4 °C in temperature). A comprehensive set of biomarkers was applied to search on modulation of biochemical responses in terms of energy metabolism, energy reserves, oxidative stress and damage occurrence in lipids, proteins as well as neurotoxicity signs. The application of an integrative Principal Coordinates Ordination (PCO) tool was successful and demonstrated that Pb caused an increase in the detoxification activity mainly evidenced by glutathione S-transferases and that the salinities 25 and 35 were, even in un-exposed mussels, responsible for cell damage seen as increased levels of lipid peroxidation (at salinity 25) and oxidised proteins (at salinity 35).
Article
Bioturbation of sediments by burrowing organisms plays a key role in the functioning of the coastal ecosystems. Burrowing is considered an energetically expensive activity, yet the energy costs of burrowing and the potential impacts of multiple stressors (such as salinity stress and wave action) on bioenergetics and burrowing performance of marine bioturbators are not well understood. We investigated the effects of mechanical disturbance and salinity stress on the burrowing behavior, aerobic capacity and energy expense of digging in a common marine bioturbator, the soft clam Mya arenaria from the Baltic Sea (control salinity 15). M. arenaria showed large individual variability in the burrowing efficiency, with an average of ∼7% of the body energy reserves used per burial. Clams with higher mitochondrial capacity and lower energy expenditure per burial showed higher endurance. Acclimation for 3-4 weeks to low (5) or fluctuating (5-15) salinity reduced the burrowing speed and the number of times the clams can re-bury but did not affect the mitochondrial capacity of the whole body or the gill. Acclimation to the fluctuating salinity shifted the predominant fuel use for burrowing from proteins to lipids. Our data indicate that the reduced burrowing performance of clams under the salinity stress is not due to the limitations of energy availability or aerobic capacity but must involve other mechanisms (such as impaired muscle performance). The reduction in the burrowing capacity of clams due to salinity stress may have important implications for survival, activity and ecological functions of the clams in shallow coastal ecosystems.
Article
Osmoregulation is by no means an energetically cheap process, and its costs have been extensively quantified in terms of respiration and aerobic metabolism. Common products of mitochondrial activity are reactive oxygen and nitrogen species, which may cause oxidative stress by degrading key cell components, while playing essential roles in cell homeostasis. Given the delicate equilibrium between pro- and antioxidants in fueling acclimation responses, the need for a thorough understanding of the relationship between salinity-induced oxidative stress and osmoregulation arises as an important issue, especially in the context of global changes and anthropogenic impacts on coastal habitats. This is especially urgent for intertidal/estuarine organisms, which may be subject to drastic salinity and habitat changes, leading to redox imbalance. How do osmoregulation strategies determine energy expenditure, and how do these processes affect organisms in terms of oxidative stress? What mechanisms are used to cope with salinity-induced oxidative stress? This Commentary aims to highlight the main gaps in our knowledge, covering all levels of organization. From an energy-redox perspective, we discuss the link between environmental salinity changes and physiological responses at different levels of biological organization. Future studies should seek to provide a detailed understanding of the relationship between osmoregulatory strategies and redox metabolism, thereby informing conservation physiologists and allowing them to tackle the new challenges imposed by global climate change.
Article
The impacts of seawater acidification and salinity shifts on metabolism, energy reserves, and oxidative status of mussels have been largely neglected. With the aim to increase the current knowledge for the mussel Mytilus galloprovincialis a 28-day chronic test was conducted during which mussels were exposed to two pH (7.8 and 7.3; both at control salinity 28) and three salinity (14, 28 and 35, at control pH, 7.8) levels. After exposure to different conditions, mussels electron transport system activity, energy reserves (protein and glycogen content) carbonic anhydrase activity, antioxidant defences and cellular damage were measured. Results obtained showed that mussels exposed to seawater acidification presented decreased metabolic capacity that may have induced lower energy expenditure (observed in higher glycogen, protein and lipids content at this condition). Low pH condition induced the increase of carbonic anhydrase activity that was related to acid-base balance, while no significant activation of antioxidant defence mechanisms was observed resulting in higher LPO. Regarding the impacts of salinity, the present study showed that at the highest salinity (35) mussels presented lower metabolic activity (also related to lower energetic expenditure) and an opposite response was observed at salinity 14. Carbonic anhydrase slightly increased at stressful salinity conditions, a mechanism of homeostasis maintenance. Lower metabolic activity at the highest salinity, probably related to valves closure, helped to mitigate the increase of LPO in this condition. At low salinity (14), despite an increase of antioxidant enzymes activity, LPO increased, probably as a result of ROS overproduction from higher electron transport system activity. The present findings demonstrated that Mytilus galloprovincialis oxidative status and metabolic capacity were negatively affected by low pH and salinity changes, with alterations that may lead to physiological impairments namely on mussels reproductive output, growth performance and resistance to disease, with ecological and economic implications.
Article
Salinity is a major controlling factor in estuarine systems whose fast change, namely during the occurrence of extreme climatic events, causes drastic alterations on aquatic communities by promoting a physiologically stressful environment. The response of fatty acid (FA) and antioxidant enzymes’ activity (Glutathione S-transferase (GST) and Superoxide dismutase (SOD)) of Cerastoderma edule and Scrobicularia plana were investigated under a wide range of salinity. Species were sampled in Mondego estuary (Portugal). A set of organisms (namely “field”) were stored for biochemical analysis, whereas the remaining organisms collected in the field (namely “lab”) were exposed to a range of salinity concentrations. Organisms were fed daily. In general, results revealed a decrease on enzymatic activity along a set of salinity concentrations with an exception to the GST activity of C. edule where a trend of increase at the activity was observed at almost all treatments. S. plana presented a very low or null activity to both enzymes. Differences in the FA profiles of both groups were also observed, with “lab” organisms not presenting saturated FA of short chain. The diversity on FA and the quantity in unsaturated FA under different salinity concentrations presented the highest values at the extreme salinity treatments. C. edule directly stored from the field presented the highest diversity and quantity in polyunsaturated fatty acids (95.77%) whereas organisms of S. plana from the field showed the highest percentage of highly unsaturated fatty acids (20.93%). Results suggest that, under salinity stress, the consumption of food decreases and the physiological pathways are reduced. Still species can store FA recognized as of high physiological importance to animals, by reducing their activity and energy consumption. Therefore, under an extreme climatic event (e.g. drought or flood) these species may present a higher content of essential FA and, thus, a higher food quality, reducing, in general, the activity of the enzymes SOD and GST.
Article
The salinity and temperature tolerances of some burrowing bivalves which occur in the middle reaches of the Swartkops estuary have been studied. Five species (Solen caperats, S. comeus, Dosinia hepatica, Macoma litoralis and PsammotelUna capensis) have upper lethal temperatures of 41-46°C when heated at a rate of 1 °/10 minutes. Two species have upper lethal temperatures of 37°C and 39°C when heated at a rate of 1 C°/day. It has been concluded that they can tolerate much higher temperatures than they normally encounter in the estuary.The salinity tolerances of four species (S. capensis, S. comeus, Dosinia, Macoma) have been investigated. All four appear to be euryhaline osmoconformers and can tolerate a wider range of salinities than they normally encounter in nature. Most of them are, however, unable to survive very low salinities, such as occur during floods, for long periods. It has been noted that species with a large gape to their valves come rapidly into osmotic equilibrium with the external medium, while those with no gape can remain closed, and thus survive low salinities for long periods. A strong shell with little gape also has a noticeable insulating effect when the animals are rapidly heated.
Article
The 30(th) birthday of a central concept in biomedicine, such as oxidative stress (OS) is a good time for re-evaluation of its contribution to science and particularly to the field of redox biology. In his recent communication, Sies described the history of the concept as well as the benefits and pitfalls of the term OS. In this mini-review, we discuss the problems associated with the still common perception of "bad OS, good antioxidants". Specifically, the term OS is an intuitively understood term originally used to describe an imbalance between pro-oxidative factors and anti-oxidative factors. It has no units, its level is dependent on the way it is measured and there is no correlation between various criteria of OS, which indicates that there are sub-classes (types) of OS (other than the classifications presented by Sies). In spite of these limitations, it is commonly regarded a measure of a person's probability to suffer from oxidative damages and is being held responsible for many diseases and antioxidants are predicted to be good to us. In fact, a "Basal OS" is vital and antioxidants may interfere with the mechanisms responsible for maintaining the oxidative status. We also discuss the linkage of OS to the outcome of antioxidant supplementation and comment on the importance of kinetic studies in evaluation of OS and on the ranking of antioxidants. Copyright © 2015. Published by Elsevier Inc.
Article
Changes in water temperature and salinity are responsible for a variety of physiological stress responses in aquatic organisms. Stress induced by these factors was recently associated with enhanced reactive oxygen species (ROS) generation, which caused oxidative damage. In the present study, we investigated the time-related effects of changes in water temperature and salinity on mRNA expression and the activities of antioxidant enzymes (SOD and CAT) and lipid peroxidation (LPO) in the gills and digestive glands of the ark shell, Scapharca broughtonii. To investigate physiological responses, hydrogen peroxide (H(2)O(2)), lysozyme activity, aspartate aminotransferase (AspAT), and alanine aminotransferase (AlaAT) were measured in the hemolymph. Water temperature and salinity changes significantly increased antioxidant enzyme mRNA expression and activity in the digestive glands and gills in a time-dependent manner. H(2)O(2) concentrations increased significantly in the high-temperature and hyposalinity treatments. LPO, AspAT and AlaAT levels also increased significantly in a time-dependent manner, while lysozyme activity decreased. These results suggest that antioxidant enzymes play important roles in reducing oxidative stress in ark shells exposed to changes in water temperature and salinity.
Article
Given their global importance, coastal marine environments are a major focus of concern regarding the potential impacts of climate change, namely due to alterations in seawater salinity. It is known that environmental characteristics, such as salinity, affect immune and physiological parameters of bivalves. Nevertheless, scarce information is available concerning the biochemical alterations associated to salinity alterations. For this reason, the present work aimed to evaluate the biochemical responses of three venerid clam species (Venerupis decussata, V. corrugata, V. philippinarum) submitted to salinity changes. The effects on the native (V. decussata and V. corrugata) and invasive (V. philippinarum) species collected from the same sampling site and submitted to the same salinity gradient (0 to 42 g/L) were compared. The results obtained demonstrated that V. corrugata is the most sensitive species to salinity changes and V. decussata is the species that can tolerate a wider range of salinities. Furthermore, our work showed that clams under salinity associated stress can alter their biochemical mechanisms, such as increasing their antioxidant defenses, to cope with the higher oxidative stress resulting from hypo and hypersaline conditions. Among the physiological and biochemical parameters analysed (glycogen and protein content; lipid peroxidation levels, antioxidant enzymes activity; total, reduced and oxidized gluthatione) Catalase (CAT) and especially superoxide dismutase (SOD) showed to be useful biomarkers to assess salinity impacts in clams.
Article
Solen cylindraceus (Hanley, 1843) is an infaunal bivalve that in the St Lucia Estuary is currently restricted to the southern part of its South Lake, having disappeared from the northern reaches due to persisting hypersaline conditions (>70 ‰) and air exposure at low water levels. The system experiences marked fluctuations in salinity due to quasi-decadal changes from wet to dry periods. In this study, the salinity tolerance of S. cylindraceus is determined using both shock and gradual change tests. Animals were collected at Catalina Bay (eastern shores of South Lake) and acclimated under laboratory conditions to naturally occurring salinities of 50 ‰ and 45 ‰ for the shock and gradual test, respectively. Mortalities were recorded for animals exposed to a sudden change in salinity, using eight different treatments ranging from 0 to 80 ‰. The second test involved exposing bivalves to a gradual change in salinity, using eight different treatments from 0 to 85 ‰. In the shock test, the lower salinity tolerance limit for S. cylindraceus was 30 ‰ and the upper 60 ‰, while in the gradual test, these limits were 15 and 65 ‰, respectively. The time it took for 50 % of animals to die increased from the shock to the gradual test for 10, 20 and 70 ‰, and decreased for 0 and 80 ‰. This knowledge may be useful towards predicting major crises in the S. cylindraceus populations, as drought and flood events alternate in the region. Major losses will be expected when salinities exceed 65 ‰ during dry phases or drop below 15 ‰ during flood events.
Article
The objective of this study was to determine whether Solen marginatus could be cultured to commercial size. Fertilized eggs measured 156.2 µm in diameter and they were surrounded by a 30-µm-thick chorionic envelope. Settlement occurred on day 9 when a length of 302.6 µm was reached. The percentage of settlement varied between 28.2 and 80.9%. Growth of post-set razor clam spats was described by the equation L=0.006x1.644, where L=length in mm and x is the number of days of culture. The razor clam seed reached a length of 19.1 mm at 4 months. One-year-old juveniles measured 38.5 mm and commercial size was reached with three-year-old individuals (80.1 mm). Survival in the ongrowing experiment ranged between 50 and 83%. The life cycle of this species was closed, obtaining larvae from the individuals reared in the ongrowing experiment.
Article
Recent research on estuarine and coastal marine systems has revealed two particularly interesting things about nutrients and productivity. First is the observation that these areas are among the most intensively fertilized environments on earth. Second is the common finding that much of the characteristically high primary productivity of these shallow waters is supported by nutrients released or recycled by pelagic and benthic microheterotrophs. Since nutrient inputs to coastal areas have probably been increasing and are likely to continue to do so, it is particularly important to understand the relationship between nutrient loading and nutrient cycling and the extent to which their interactions may set the levels of primary and secondary production in coastal systems.
Article
Mactra veneriformis, Ruditapes philippinarum, and Meretrix lusoria are dominant clams in Japanese tidal flats. Juveniles and adults of these species were reared in the laboratory and the effects of salinity on their sand burrowing activity and growth- and clearance-rates were examined. Juveniles of each species (shell length 10–16 mm) filtered water actively in the salinity range of 11.8–34.6 psu, with clearance rates not affected by salinity. The clams showed positive growth in the above salinity range, but growth rates were retarded at 11.8 psu, probably because of increased respiration at low salinity levels. In 6.1-psu seawater, adult Ma. veneriformis dug into the sand after a few days acclimation, Me. lusoria did not and R. philippinarum died. At 10.8 psu, adult Ma. veneriformis filtered water actively, Me. lusoria dug into the sand but did not filter water, and R. philippinarum neither dug into sand nor filtered water. These results indicate that the clam species examined are euryhaline but that the response to low-salinity water (≤11 psu) by adult clams (shell length 31–37 mm) differed among species: Ma. veneriformis is the most adaptable to low-salinity water, followed (in order) by Me. lusoria and R. philippinarum. These differences, however, are not consistent with the distribution patterns of these clam species in the Shirakawa tidal flat (Japan), where salinity varies spatially and temporally.
Article
Sinonovacula constricta is representative of two genera of deep burrowing bivalves hitherto placed in the Novaculininae of the Tellinacea. This species, widely cultured in China, has not previously been critically examined. Aspects of its anatomy show that it should more properly be placed in the Solenacea. The presently accepted classification of this superfamily suggests that it should be placed close to Pharella in the Cultellidae but it is clear that the group requires more detailed re-examination. Sinonovacula is somewhat posteriorly elongate with long separate siphons. The anterior pedal gape acts as a valve to the piston-like foot enabling rapid reburrowing to occur. Sensory tentacles around the pedal gape respond to mechanical disturbance and water can be squirted from the gape to, presumably, flush away the source of irritation. Modifications of this kind are rare in the Bivalvia. The anatomy of Sinonovacula is described and related to its life style as a suspension feeder in soft intertidal muds. The ciliary currents of the mantle cavity are powerful, the labial palps in particular being large as compared with the relatively small ctenidia. The stomach is adapted for dealing with large amounts of food, though the sorting areas are not extensive; principal sorting is clearly the function of the labial palps. The style sac and mid-gut are separate (unlike the conjoined style sac and mid gut of the Tellinacea), and ensure separation of the style from the sediment loaded mid gut. In many ways Sinonovacula foreshadows the highly modified true razor shells and conditions in these bivalves can be accounted for by further posterior elongation of a bivalve essentially similar to Sinonovacula.
Article
The concentrations of Rare Earth Elements and yttrium (REY) were measured in dissolved phase, in suspended particulate matter (SPM) and in sediments in seven sampling stations in the Mae Klong estuarine system (Inner Thailand Gulf) in order to study their behaviour and distribution pattern. The analysed samples generally show high Rare Earth Element (REE) content in the dissolved phase, with high Medium Rare Earth Elements (MREEs) and Y enrichments in the shale-normalized pattern (versus PAAS). These chemical features are interpreted in terms of direct influences of weathering processes of REE-rich minerals (e.g., phosphates), which abundantly out-crop in the Mae Klong watershed. These large MREE enrichments also suggest that dissolved sulphate complexes play an important role in the dissolved REE fate close to the marine coastal system. The shale normalized REEs and Y/Ho ratios additionally indicate a direct REY co-precipitation with Fe-oxyhydroxides along the river path and consequent adsorption, as coating, onto FeOOH surfaces. The REY behaviour in SPM appears strongly influenced by Al, Fe and Mn content, suggesting that the formation of an authigenic Fe, Mn oxyhydroxides and aluminosilicate particulate leads to REY adsorption and/or co-precipitation. The observed variations in the Y/Ho ratios suggest that different Y–Ho fractionation processes take place in the studied estuarine system. Strong similarities among chondrite-normalized REE patterns in both sediments and granitic rocks out-cropping in the surrounding regions suggest a generic relationship between the two groups of materials.