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A preliminary investigation into the morphology of oral papillae and denticles of blue sharks (Prionace glauca) with inferences about its functional significance across life stages
Abstract
Sensory organs in elasmobranchs (sharks, skates, rays) detect and respond to a different set of biotic and/or
abiotic stimuli, through sight, smell, taste, hearing, mechanoreception and electroreception. Although
gustation is crucial for survival and essential for growth, mobility, and maintenance of neural activity and the
proper functioning of the immune system, comparatively little is known about this sensory system in
elasmobranchs. Here we present a preliminary investigation into the structural and dimensional characteristics
of the oral papillae and denticles found in the oropharyngeal cavity of the blue shark (Prionace glauca) during
embryonic development through adulthood. Samples were obtained from the dorsal and ventral surface of the
oropharyngeal cavity collected from embryos at different development stages as well as from adults. Our
results suggest that development of papillae occurs early in ontogeny, before the formation of the oral
denticles. The diameter of oral papillae gradually increases during development, starting from 25 lm in stage I
embryos, to 110 lm in stage IV embryos and 272–300 lm in adults. Embryos exhibit papillae at early
developmental stages, suggesting that these structures may be important during early in life. The highest
density of papillae was observed in the maxillary and mandibular valve regions, possibly related to the ability
to identify, capture and process prey. The oral denticles were observed only in the final embryonic stage as
well as in adults. Accordingly, we suggest that oral denticles likely aid in ram ventilation (through reducing the
hydrodynamic drag), to protect papillae from injury during prey consumption and assist in the retention and
consumption of prey (through adhesion), since these processes are only necessary after birth.
... Blue sharks are occasionally described as having a series of knob-like protrusions along the anterior margins of all gill arches which has been suggested to help facilitate handling of invertebrate prey (Compagno, 1984, Rangel et al., 2017. They are also known for having unusually broad pectoral fins, allowing them to swim very moving prey, such as jellyfish . ...
... In addition to the evidence provided by biochemical tracers, we provide qualitative data in support of the possibility that blue sharks feed on gelatinous or planktonic prey. In their study of the morphology of oral papillae, Rangel et al. (2017) described the presence of "gill rakers" in the oropharyngeal cavity of embryonic blue sharks. Our observations confirm that these knob-like soft tissue structures along the gill arches persist at least throughout the early lifetime of the shark (Figure 4-8). ...
... Our observations confirm that these knob-like soft tissue structures along the gill arches persist at least throughout the early lifetime of the shark (Figure 4-8). They have been hypothesised to be used to improve prey retention by preventing small, perhaps planktonic, prey from slipping out of the gill slits (Compagno, 1984, Rangel et al., 2017 and could be worth comparing to turtle oesophageal papillae or basking shark gill rakers Parker, 1950, Vogt et al., 1998) but appear to be most similar to spiny dogfish gill rakers (Gudo and Homberger, 2002). Gelatinous prey such as jellyfish have been found to make up 30-40% of the stomach volume of the latter (Purcell and Arai, 2001). ...
As the world faces the threats of multiple compounded and worsening crises, scientists are racing to gather the knowledge necessary to safeguard entire ecosystems and species. Technological advances are continuously facilitating more in-depth studies to understand the mechanisms driving species functioning and variations among and within populations, communities, and individuals. New methods are providing insights into difficult to access environments and species such as the open ocean. The following thesis uses theoretical and empirical approaches to understand the drivers of variation in ecological niches of large marine predators. One of the methods commonly used to study a species resource use is stable isotope analysis (SIA). However, the relationship between variation in stable isotope values in the tissues of consumers and their diet is often misconstrued or over-simplified. Here, we lay out the underlying factors that influence stable isotope ratios and how these can be accounted for when designing an ecological study. I then review new advances in stable isotope technologies and how compound-specific SIA can be used to ask questions about the life-history of a broad range of species. Having shown the value in combining biochemical methods with other disciplines, I apply this approach to study the ecology of large marine vertebrates in Ireland. By using fatty acid analysis in combination with reproductive hormone analysis, biologging, morphometrics and observations, I investigate the ecology and physiology of blue sharks, Prionace glauca, in Ireland. This population of predominantly female individuals in varying stages of maturity show indications of a seasonal change in resource use between the June and November, possibly indicating opportunistic foraging on abundant gelatinous or planktonic prey. Finally, I present the results from our tagging efforts on mature female porbeagle sharks, Lamna nasus, caught in Donegal (Ireland) in April 2022. As I have followed these two individuals over the past nine months, both have crossed many environmental and jurisdictional boundaries but have displayed high inter-individual variation. I argue the need for increased and continued cross-country collaboration in the Northeast Atlantic to study and manage this critically endangered species. The work carried out here emphasises the value in moving our field from uni- and multidisciplinary approaches towards more holistic interdisciplinary approaches, particularly when our aim is the sustainable preservation of large oceanic migrators.
... Oropharyngeal dermal denticles (usually referred as "oral denticles") are structurally similar to external body dermal denticles, composed of a crown, a single basal plate and a pedicel (Kemp, 1999;Marshall, 2011;Reif, 1985). They are distributed on the upper and lower portions of the mouth cavity, from the region immediately posterior to teeth to pharyngeal cavity, including maxillary and mandibular valves and cover the inner surface of gill slits (Atkinson & Collin, 2012;Cook & Neal, 1921;Nelson, 1970;Poscai et al., 2021;Rangel et al., 2017;Rangel et al., 2018;Reif, 1985), providing protection against abrasion during food ingestion to increase friction and grip on prey items. In addition, they may limit the density of oral papillae (Atkinson & Collin, 2012;Poscai et al., 2021). ...
... That is, the low density of oral papillae is related to the high number of oral denticles covering the oral epithelium. The same pattern was observed in shark taxa within Carcharhinidae (Atkinson & Collin, 2012;Poscai et al., 2021;Rangel et al., 2017). ...
... Regarding the analysis of dermal denticles of elasmobranchs, recent studies have been employing micro CT scan to describe their morphological aspects (e.g., Tomita et al., 2020) or to investigate their biomimetic properties (Ankhelyi et al., 2018;Domel et al., 2018;Wen et al., 2015). Although histology and SEM are the most usual tools for this kind of work (Compagno, 1988;Dillon et al., 2017Dillon et al., , 2021Popp et al., 2020;Poscai et al., 2017Poscai et al., , 2021Rangel et al., 2016Rangel et al., , 2017Rangel et al., , 2018Reif, 1974Reif, , 1985, CT scan method works as a complement to elucidate the morphology of dermal denticles, which is not possible with the use of traditional techniques. It is worth mentioning that the same tissue sections (dorsal and ventral) used in CT scan were analysed under SEM. ...
Oral denticles of sharks are composed by a crown, dentine covered by a layer of enameloid and pulp cavity, the same structure of the dermal denticles found across the body surface of most elasmobranchs. In addition, oral papillae and taste buds are distributed among denticles within the oropharyngeal cavity, playing a fundamental role for tasting as part of the chemosensory system of fishes. Scanning electron microscopy (SEM) has been employed as an important tool for the study of dermal denticles and other structures, as well as histology and more recently computed tomography (CT) scan analysis. Herein, the authors used two methods for the study of the morphology of the oropharyngeal cavity of Lamna nasus (Lamniformes), an oceanic and pelagic shark: SEM and CT scan. The general morphology of oral denticles studied herein is related to abrasion strength as they are diamond‐shaped, lack lateral cusps and have less pronounced ridges. In addition, smooth ridges and broad rounded denticles could be related to prevent abrasion during food consumption and manipulation. Oral papillae had a round shape and were observed only under SEM. The densities of papillae were estimated in 100 per cm², whereas denticles were 1760 and 1230 cm² over the dorsal and ventral regions, respectively. The high numbers of denticles are inversely proportional to papillae density; denticles seem to restrict papillae distribution. Regarding the differences between methodologies, under SEM, only the crown was visualized, as well the papillae, allowing the estimation of size and density of both structures. Nonetheless, under CT scan, the whole components of denticles were clearly visualized: different views of the crown, peduncle, basal plate, and pulp cavity. On the contrary, oral papillae were not visualized under CT due to the tissue preparation. Furthermore, both methods are complementary and were important to extract as much information as possible from denticles and papillae.
... Taste papillae density decreased throughout development, whereas taste papillae diameter increased, and total numbers of taste papillae remained relatively constant for all stages examined (Atkinson et al. 2016). Similar developmental trends were seen in the Blue Shark (Prionace glauca) and the Shortnose Guitarfsh (Zapteryx brevirostris) (Rangel et al. 2016(Rangel et al. , 2017. Taste papillae density is variable among the oropharyngeal regions, with higher densities often observed in the oral cavity, especially near the teeth, likely refecting their importance during prey handling (Atkinson et al. 2016;Rangel et al. 2016Rangel et al. , 2017. ...
... Similar developmental trends were seen in the Blue Shark (Prionace glauca) and the Shortnose Guitarfsh (Zapteryx brevirostris) (Rangel et al. 2016(Rangel et al. , 2017. Taste papillae density is variable among the oropharyngeal regions, with higher densities often observed in the oral cavity, especially near the teeth, likely refecting their importance during prey handling (Atkinson et al. 2016;Rangel et al. 2016Rangel et al. , 2017. ...
... They are found in nearly all selachians; however, among batoids, oral denticles are found only in Rhinobatidae, refecting the overall reduction of dermal denticles in this taxon (Atkinson and Collin 2012). Functionally, oral denticles are hypothesized to reduce hydrodynamic drag during ram ventilation and protect the epithelium and taste papillae from mechanical damage while increasing friction during prey manipulation (Atkinson and Collin 2012;Atkinson et al. 2016;Rangel et al. 2016Rangel et al. , 2017Rangel et al. , 2019. In several species, it was noted that the high density of oral denticles seems to spatially limit taste papillae presence (Atkinson and Collin 2012;Rangel et al. 2016). ...
... Such sensory systems are especially important for behavioral activities such as feeding, avoiding predators, spatial orientation, social interactions, navigation, among others which play a significant role in their survival and longevity (Collin 2012). Although the sensory biology of sharks is well studied, the gustatory system has received relatively less attention, despite its likely importance in prey selection and capture (Atkinson and Collin 2010;Collin 2012;Rangel et al. 2017). ...
... Through direct contact, oral papillae allow sharks to evaluate the suitability of potential prey, leading to ingestion or rejection of the item (Atkinson and Collin 2010;Collin 2012;Kirino et al. 2013;Atkinson et al. 2016). The oral papillae appear to develop early in shark embryos, for example, Prionace glauca and Chiloscyllium punctatum, and are functional before birth or on emergence from the egg case (Atkinson et al. 2016;Rangel et al. 2017). These papillae increase in size and decrease 1 3 in density as the animal grows (Rangel et al. 2017); however, the total number of papillae is maintained throughout the ontogeny (Atkinson et al. 2016;Rangel et al. 2017). ...
... The oral papillae appear to develop early in shark embryos, for example, Prionace glauca and Chiloscyllium punctatum, and are functional before birth or on emergence from the egg case (Atkinson et al. 2016;Rangel et al. 2017). These papillae increase in size and decrease 1 3 in density as the animal grows (Rangel et al. 2017); however, the total number of papillae is maintained throughout the ontogeny (Atkinson et al. 2016;Rangel et al. 2017). Overall, the oral papillae seem to have the same morphological configuration in all species of elasmobranchs studied (Whitear and Moate 1994;Rangel et al. 2016Rangel et al. , 2017 and conserved throughout vertebrate evolution (Atkinson et al. 2016). ...
The microstructures of the oral cavity in sharks have received relatively little study, despite their potential functional importance for gustation, feeding, and ventilation. Accordingly, here we conducted a preliminary comparative investigation into the structure and organization of oral papillae and denticles found on the ventral surface of the oral cavity in four species of shark (bigeye thresher, shortfin mako, scalloped hammerhead, and smooth hammerhead). Despite a limited sample size, differences in complexity and ornamentation of oral papillae and denticles were found across the four species. The scalloped hammerhead shark exhibited the largest oral papillae compared to the bigeye thresher and the shortfin mako. The most complex oral denticles, in terms of number of ridges and microstructures, were found in the scalloped hammerhead, followed by the bigeye thresher, smooth hammerhead and shortfin mako. For smooth hammerheads, in which samples were available from both juveniles and adults, differences in denticle microstructures were found suggesting possible ontogenetic variations. These results suggest that shape, size and arrangement of oral papillae and denticles may be related to ecology and phylogeny of the species studied. Based on these emerging patterns we discuss several plausible hypotheses relating to the function of these structures for consideration in future studies.
... All photographs were analyzed in triplicate, and the average was used to estimate the density of papillae and denticles per cm 2 by selecting random areas (four per photography). These procedures were adapted from Rangel et al. (2017). ...
... In C. signatus which presented the highest number of denticles (which are also overlapping), we observed the lowest density of oral papillae on both dorsal and ventral regions. Rangel et al. (2017) observed an increase in size of the papillae throughout the ontogeny of the blue shark Prionace glauca, but the overall density of papillae decreased as the animal grew up. ...
Oropharyngeal dermal denticles and oral papillae are present throughout the oropharyngeal cavity, and incorporate the use of taste buds to orally process and evaluate the food items, whereas oral denticles are thought to provide a form of protection against abrasion during food consumption and improve ventilation efficiency. Herein, are compared the microstructure of the oropharyngeal denticles and papillae of large predatory requiem sharks (Carcharhinidae) (Carcharhinus brevipinna, Carcharhinus leucas, C. limbatus, Carcharhinus obscurus, Carcharhinus signatus, and Galeocerdo cuvier), under scanning electron microscopy. The results revealed that the largest oral denticles were found in adults of C. signatus, followed by juveniles of G. cuvier, C. leucas, and C. obscurus, respectively. Oral papillae were found to be larger in G. cuvier, C. signatus, and in C. leucas, and all these specimens presented round-shaped papillae. The higher denticles densities were found in the oral cavity of C. signatus, however, this species presented the lowest density of papillae. Carcharhinus limbatus presented the second highest rate of denticles density, followed by G. cuvier, C. obscurus, C. leucas, and C. brevipinna. The highest density of papillae was found in C. brevipinna, indicating that the density of denticles is inversely proportional to the papillae distribution, the same as we observed in C. signatus. The denticles density seems to be higher as the animal increases in size, as we observed in adult specimens of C. signatus, and this shark presented two different morphologies of denticles, different from the other species studied here. This may suggest that densities and sizes of these structures differ as the animals grow, expressed by the prey spectrum availability and the dietary shifts due to the distinct habitat which the species are associated during their life cycles.
... Nelson (1970) described macroscopic features, such as shape, distribution and abundance of oropharyngeal denticles in Rhizoprionodon terraenovae. Later, Ciena et al. (2016) and Rangel et al. (2017) described the ultrastructure of oropharyngeal denticles and their disposition amongst dermal papillae in Rhizoprionodon lalandii and Prionace glauca, respectively. In all of these species, the denticles are distributed on the entire ventral surface of the oropharyngeal cavity. ...
The genus Scyliorhinus is part of the family Scyliorhinidae, the most diverse family of sharks and of the subfamily Scyliorhininae along with Cephaloscyllium and Poroderma. This study reviews the phylogenetic relationships of species of Scyliorhinus in the subfamily Scyliorhininae. Specimens of all Scyliorhinus species were examined as well as specimens of four of the 18 species of Cephaloscyllium, two species of Poroderma, representatives of almost all other catshark (scyliorhinid) genera and one proscylliid (Proscyllium habereri). A detailed morphological study, including external and internal morphology, morphometry and meristic data, was performed. From this study, a total of 84 morphological characters were compiled into a data matrix. Parsimony analysis was employed to generate hypotheses of phylogenetic relationships using the TNT 1.1. Proscyllium habereri was used to root the clado-gram. The phylogenetic analysis, based on implied weighting (k = 3; 300 replications and 100 trees saved per replication), resulted in three equally most parsimonious cladograms with 233 steps, with a CI of 0.37 and an RI of 0.69. The monophyly of the subfamily Scyliorhininae is supported as well as of the genus Scyliorhinus, which is proposed to be the sister group of Cephaloscyllium. The phylogenetic relationships amongst Scyliorhinus species are presented for the first time.
... Some particularities of the denticles that make them interesting tools include: (a) their morphological aspects (i.e., shape, size, and arrangement) are highly variable intra and interspecifically; (b) they are correlated with shark ecology; (c) are very abundant across the body of sharks, including oral regions and nictitating membrane; and (d) are continually exchanged (e.g., Dillon et al., 2017;Poscai et al., 2017;Rangel et al., 2017). Although recent studies have addressed extensively about morphology, taxonomy, and function of denticles (e.g., Dillon et al., 2017), more studies are needed considering both the high shark diversity (~500 living species; Weigmann, 2016) and intra and interspecifically variability of denticles across the body and oral cavity (e.g., Ankhelyi, Wainwright, & Lauder, 2018;Dillon et al., 2017;Rangel, Salmon, Poscai, Kfoury Jr., & Rici, 2019). ...
The dermal denticles are among the unique morphological adaptations of sharks, which have been acquired throughout their long evolutionary process of more than 400 million years. Species-specific morphological characteristics of these structures has been applied specially as tools for functional and taxonomic (family-level) studies. Nevertheless, few studies have explored the diversity of denticle structure in different around the body and oral cavity. In the present study, we described the morphological differences observed in skin and oral cavity of sharpnose sevengill shark Heptranchias perlo, using scanning electron microscopy. Our findings demonstrate substantial variation in morphological structure of the denticles of the body and oral cavity. Overall, the dermal denticles observed across body surface were overlapped, tricuspid, with the central cuspid being more pronounced, pointed, and triangular in shape compared with lateral ones. Unlike, the denticles on the tip of the nose had a smooth crown, with rounded edges, being compact, and overlapped. The oral denti-cles were found in the ventral and dorsal region of the oral cavity. They also were tricuspid, but with differences in arrangement and ridges. These results suggest a strict functional relationship with the morphological characteristics observed. Such morphological diversity body-region-dependent highlights the need for comparative studies that include oral denticles, since this structure has an important functional role in sharks and can be found in fossil and recent records.
As they grow, sharks both replace lost denticles and proliferate the number of denticles by developing new (de novo) denticles without prior denticle shedding. The loss and replacement of denticles has potential impacts on the energetic cost of maintaining the skin surface, the biomechanical functions of shark skin, as well as our ability to predict shark abundance from fossil denticle occurrence in sediment cores. Here, we seek to better understand patterns of denticle loss and to show how denticles are being replaced in mature sharks. We illustrate shark skin surfaces with missing denticles and quantify both within‐species and between‐species patterns of missing denticles using images from across regions of the body for two species and images at similar body regions for 16 species of sharks. Generally, sharks are missing similar numbers of denticles (0%–6%) between species and regions. However, there are exceptions: in the smooth dogfish, the nose region is missing significantly more denticles than most posterior‐body and fin regions, and the common thresher shark is missing significantly more denticles than the smooth dogfish, leopard shark, angel shark, bonnethead, and gulper shark. Denticle regrowth starts with crown development and mineralization beneath the epidermis, followed by eruption of the crown, and finally the mineralization of the root. The pulp cavity of replacement denticles is initially large and surrounded by a thin shell of enameloid upon eruption of the denticle. After eruption of the denticle, the deposition of dentine continues internally after the denticle reaches its final position. Replacement of missing denticles, representing less than 6% of the skin surface at any one time, may not compromise hydrodynamic function, but by constantly updating the skin surface throughout life, sharks may reduce surface fouling and maintain a functional complex skin surface by repairing local damage to individual denticles.
The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia.
Most species of chondrichthyes have lecithotrophic yolk sac viviparity where yolk provides the majority of nutrients for embryonic growth, but some degree of histotrophy is probably expressed in all viviparous species. Whilst the quality and quantity of uterine secretions varies considerably, species exhibiting matrotrophic histotrophy exhibit morphological modification of the uterus through increased vascularity and reduced cell layering separating maternal and foetal blood.
Elasmobranchs have an impressive range of highly specialized sensory systems shaped over 400 million years of evolution. The morphological analysis of oral papillae and denticle in elasmobranchs elucidates the biological role that these structures play during feeding and ventilation, bringing important descriptive information about ecological implications in an evolutionary context. The present study provides descriptions of the distribution patterns, histological characteristics and three-dimensional aspects of oral papillae and denticles in the lesser guitarfish Zapteryx brevirostris, through light microscopy and scanning electron microscopy. The presence of oral denticles in the oropharyngeal cavity suggests that this structure may have the following functions: protect against abrasion and parasites, increase the ability to grasp and hold prey and assist in reduction in hydrodynamic drag. The denticles in Z. brevirostris are similar to those found in pelagic sharks with forced ventilation (RAM). The structural conformity of denticles observed in the gill slits may facilitate water flow during prey grasp and food processing. This study supports the hypothesis that these structures may be an adaptive reflection shaped by feeding habits, capture strategies and processing prey.
Significance
Shark populations are declining worldwide because of overexploitation by fisheries with unknown consequences for ecosystems. Although the harvest of oceanic sharks remains largely unregulated, knowing precisely where they interact with fishing vessels will better aid their conservation. We satellite track six species of shark and two entire longline fishing vessel fleets across the North Atlantic over multiple years. Sharks actively select and aggregate in space-use “hotspots” characterized by thermal fronts and high productivity. However, longline fishing vessels also target these habitats and efficiently track shark movements seasonally, leading to an 80% spatial overlap. Areas of highest overlap between sharks and fishing vessels show persistence between years, suggesting current hotspots are at risk, and arguing for introduction of international catch limits.
Background
Knifing is a behaviour whereby a shark swims directly at the surface with its dorsal fin out of the water. While this behaviour has been reported in a number of species, information on the frequency and timing of such behaviour could provide insights on how sharks use the ocean–atmosphere interface.
Findings
Our analysis of the timing of the reception of satellite (Argos) messages from SPOT-tagged blue sharks has revealed important insights on knifing behaviour in one of the ocean’s most abundant large predators. We found that knifing behaviour was common in all tagged sharks and occurred during 54–76 % of days tracked, with a mean (and SD) of 4.7 ± 0.4 knifing events per day when observed. The frequency of knifing behaviour increased during the dawn period in all sharks and was supported by analysis of high-resolution depth data from a recovered archival tag. One shark also had a pronounced peak in knifing activity at dusk.
Conclusions
We suggest that blue sharks may be using surface waters during twilight periods to maximise foraging opportunities. Light conditions at dawn are consistent with surface-dwelling prey being both more dispersed and silhouetted by ambient light conditions, making individual prey more visible. The application of this analysis to other species of sharks may provide further insights on knifing behaviour.
The oral denticles of some elasmobranchs are found on the surface of the oral cavity and are homologous to those on the body surface, being well developed, independent and non-growing, with varying morphology and distribution depending on the species. The structural and three-dimensional characteristics of oral denticles from the rostro-ventral surface of the sharpnose shark Rhizoprionodon lalandii were described following imaging by both light and scanning electron microscopy. The light microscopy results showed that the triangular shape of the denticles consisted of a base and an apex. Picrosirius staining showed the arrangement of collagen fibres and oral denticles, and a predominance of type-I collagen was found in both structures under polarized light. There was a broad homogeneous distribution of denticles on the ventral surface, forming a leaf-like shape with the cusp facing the caudal region. Interlocking, hexagonal, geometric structures on its rostral side and ridges on the rostral side of the oral denticles were observed under increased magnification. We concluded that the denticle morphology found in R. lalandii differ of others analysed species, and the descriptions of these structures therefore provide important information for the classification of the species. In this species, the main functions can be assigned to help reduce hydrodynamic drag, particularly by this being a species that uses ram ventilation, and to protect the epithelium of the oropharynx of abrasion and parasites.
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Spatial structuring and segregation by sex and size is considered to be an intrinsic attribute of shark populations. These spatial patterns remain poorly understood, particularly for oceanic species such as blue shark (Prionace glauca), despite its importance for the management and conservation of this highly migratory species. This study presents the results of a long-term electronic tagging experiment to investigate the migratory patterns of blue shark, to elucidate how these patterns change across its life history and to assess the existence of a nursery area in the central North Atlantic. Blue sharks belonging to different life stages (n = 34) were tracked for periods up to 952 days during which they moved extensively (up to an estimated 28.139 km), occupying large parts of the oceanic basin. Notwithstanding a large individual variability, there were pronounced differences in movements and space use across the species' life history. The study provides strong evidence for the existence of a discrete central North Atlantic nursery, where juveniles can reside for up to at least 2 years. In contrast with previously described nurseries of coastal and semi-pelagic sharks, this oceanic nursery is comparatively vast and open suggesting that shelter from predators is not its main function. Subsequently, male and female blue sharks spatially segregate. Females engage in seasonal latitudinal migrations until approaching maturity, when they undergo an ontogenic habitat shift towards tropical latitudes. In contrast, juvenile males generally expanded their range southward and apparently displayed a higher degree of behavioural polymorphism. These results provide important insights into the spatial ecology of pelagic sharks, with implications for the sustainable management of this heavily exploited shark, especially in the central North Atlantic where the presence of a nursery and the seasonal overlap and alternation of different life stages coincides with a high fishing mortality., et al. (2014) Movements of Blue Sharks (Prionace glauca) across Their Life History. PLoS ONE 9(8): e103538. doi:10.1371/journal.pone.0103538 Copyright: ß 2014 Vandeperre et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: This work was conducted in the framework of the EU FP7 project MADE, Mitigating adverse ecological impacts of open ocean fisheries (FP7 KBBE/2007/ 1/210496). FCT/MCTES-MEC provided individual funding to FV, JF and PA (SFRH/BD/46891/2008, SFRH/BPD/66532/2009, and Ciência 2008/POPH/QREN). IMAR-DOP/UAz is Research and Development Unit no. 531 and LARSyS-Associated Laboratory no. 9 funded by the Portuguese Foundation for Science and Technology (FCT) through FCT -Pest/OE/EEI/LA0009/2011–2014 and by the Azores Fund for Science and Technology (FRCT), funded by OE, COMPETE, QREN and ProConvergencia. The Open Access of this paper is funded by FCT -Pest/OE/EEI/LA0009/2014 (OE, COMPETE and QREN). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist.
Bycatch (the unintentional catch of non-target species or sizes) is consistently ranked as one of the greatest threats to marine fish populations; yet species-specific rates of bycatch survival are rarely considered in risk assessments. Regulations often require that bycatch of threatened species be released; but, if animals are already dead, their release serves no conservation purpose. We examined the survival of 12 shark species caught as bycatch in the US Atlantic pelagic longline fishery. Shark survival was evaluated in relation to fishery target (swordfish versus tuna) and four operational, environmental, and biological variables to evaluate the underlying mechanisms affecting mortality. Survival estimates ranged from 33% (night shark) to 97% (tiger shark) with seven of the 12 species being significantly affected by at least one variable. We placed our survival results within a framework that assessed each species' relative vulnerability by integrating survival estimates with reproductive potential and found that the bigeye thresher, dusky, night, and scalloped hammerhead shark exhibited the highest vulnerabilities to bycatch. We suggest that considering ecological and biological traits of species shows promise for designing effective conservation measures, whereas techniques that reduce fisheries interactions in the first place may be the best strategy for highly vulnerable species. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
Inertial suction feeding is known to occur in some sharks, but the sequence and temporal kinematics of head and jaw movements have not been defined. We inves- tigated the feeding kinematics of a suction feeding shark, the nurse shark Gingly- mostoma cirratum, to test for differences in the timing and magnitude of feeding components with other shark taxa when sharks were fed pieces of bony fish. Thir- teen kinematic variables were measured from high-speed video recordings. Food capture in this species consists of expansive, compressive, and recovery phases, as in most other sharks, but there is little or no cranial elevation. Mean time to maxi- mum gape (32 msec) is the fastest recorded for an elasmobranch fish. Other rela- tively rapid events include mandibular depression (26 msec), elevation (66 msec), and total bite time (100 msec). Buccal valves assist the unidirectional flow of water into the mouth and out of the gill chambers. Food capture under these experimental conditions appears to be a stereotyped modal action pattern but with significant interindividual variability in timing of kinematic events. Ginglymostoma cirratum ex- hibits a suite of specializations for inertial suction feeding that include (1) the for- mation of a small, anteriorly directed mouth that is approximately round and lat- erally enclosed by modified labial cartilages; (2) small teeth; (3) buccal valves to prevent the backflow of water; and (4) extremely rapid buccal expansion. Sharks that capture food by inertial suction have faster and more stereotyped capture be- havior than sharks that primarily ram feed. Inertial suction feeding, which has evolved multiple times in sharks, represents an example of functional convergence with inertial suction feeding bony fishes.
Stomachs from 222 blue sharks collected along the Brazilian coast were analyzed - 116 from the northeastern region and 106 from the southern region. A total of 51 prey taxa were identified. The most important prey items in the southern region were Mysticeti whales, teleosteans, the gempylid Ruvettus pretiosus and the nomeid Arioma bondi. Cephalopods were more diverse, with dominance of vertical migrants Histioteuthis spp., Cranchiidae and the epipelagic octopus Ocythoe tuberculata. In the northeastern region, blue sharks consumed mainly teleosteans, including the alepisaurid Alepisaurus ferox and the gempylid Gempylus serpens. Among cephalopods, Histioteuthis spp. and the epipelagic octopus Tremoctopus violaceus were the dominant items. Predation upon schooling prey was occasional, as observed on Arioma bondi. Birds also were consumed in both regions; Puffinus gravis was the fifth most frequent item in the northeastern region. During the reproductive migration cycle, blue sharks likely prey in the thermocline, which is deeper in the northeastern region and closer to the surface in the southern region.
Of 159 blue sharks Prionace glauca examined (59% female, 41% male) from oceanic waters SW of Britain and Ireland, all but two males were immature. Size-at-age and growth rate were similar to previous studies in the North Atlantic, while stomach contents included cephalopods, fish, cetaceans, and to a lesser degree, birds and crustaceans.
The fish gustatory system provides the final sensory evaluation in the feeding process. Unlike other vertebrates, the gustatory system in fish may be divided into two distinct subsystems, oral and extraoral, both of them mediating behavioural responses to food items brought in contact with the fish. The abundance of taste buds is another peculiarity of the fish gustatory system. For many years, morphological and electrophysiological techniques dominated the studies of the fish gustatory system, and systematic investigations of fish taste preferences have only been performed during the last 10 years. In the present review, basic principles in the taste preferences of fish are formulated. Categories or types of taste substances are defined in accordance with their effects on fish feeding behaviour and further mediation by the oral or extraoral taste systems (incitants, suppressants, stimulants, deterrents, enhancers and indifferent substances). Information on taste preferences to different types of substances including classical taste substances, free amino acids, betaine, nucleotides, nucleosides, amines, sugars and other hydrocarbons, organic acids, alcohols and aldehydes, and their mixtures, is summarised. The threshold concentrations for taste substances are discussed, and the relationship between fish taste preferences with fish systematic position and fish ecology is evaluated. Fish taste preferences are highly species-specific, and the differences among fish species are apparent when comparing the width and composition of spectra for both the stimulants and the deterrents. What is evident is that there is a strong similarity in the taste preferences between geographically isolated fish populations of the same species, and that taste preferences are similar in males and females, although at the individual level, it may vary dramatically among conspecifics. What is noteworthy is that taste responses are more stable and invariable for highly palatable substances than for substances with a low level of palatability. Taste preferences as a function of pH is analysed. There is a good correspondence between development of the gustatory system in fish ontogeny and its ability to discriminate taste properties of food items. There is also a correspondence between oral and extraoral taste preferences for a given species; however, there is no correlation between smell and taste preferences. Taste preferences in fish show low plasticity (in relation to the diet), appear to be determined genetically and seem to be patroclinous. Fish feeding motivation and various environmental factors like water temperature and pollutants such as heavy metals and low pH water may shift fish taste preferences. Comparisons between bioassay and electrophysiological data show that palatability is not synonymous with excitability in the gustatory system. The chemical nature of stimulants and deterrents in various hydrobionts is outlined. The significance of basic knowledge in fish taste preferences for aquaculture and fisheries is emphasised.
We conducted an ultrasonographic experiment on a pregnant manta ray, Manta alfredi (Chondrichthyes, Batoidea). This study showed how the embryo of the live-bearing elasmobranchs respires in the body of the female. In the embryonic stage, the manta ray embryo takes in uterine fluid by buccal-pumping. After birth, the manta ray shifts its respiratory mode from buccal-pumping to ram-ventilation. The rapid reduction of the spiracle size in the young manta ray may reflect this shift of respiratory mode.
Unlike mammals or some carcharhinid sharks that acquire oxygen through a placenta and umbilical cord, the manta ray embryo does not have a direct connection with the mother. Thus, the manta ray embryo obtains oxygen by buccal-pumping of the uterine fluid, in the same way that the embryos of egg-laying species obtain oxygen from the water in the egg case. This finding extends our understanding of the diversity of embryonic respiratory systems in live-bearing vertebrates.
It has long been suspected that the denticles on shark skin reduce hydrodynamic drag during locomotion, and a number of man-made materials have been produced that purport to use shark-skin-like surface roughness to reduce drag during swimming. But no studies to date have tested these claims of drag reduction under dynamic and controlled conditions in which the swimming speed and hydrodynamics of shark skin and skin-like materials can be quantitatively compared with those of controls lacking surface ornamentation or with surfaces in different orientations. We use a flapping foil robotic device that allows accurate determination of the self-propelled swimming (SPS) speed of both rigid and flexible membrane-like foils made of shark skin and two biomimetic models of shark skin to measure locomotor performance. We studied the SPS speed of real shark skin, a silicone riblet material with evenly spaced ridges and a Speedo® 'shark skin-like' swimsuit fabric attached to rigid flat-plate foils and when made into flexible membrane-like foils. We found no consistent increase in swimming speed with Speedo® fabric, a 7.2% increase with riblet material, whereas shark skin membranes (but not rigid shark skin plates) showed a mean 12.3% increase in swimming speed compared with the same skin foils after removing the denticles. Deformation of the shark skin membrane is thus crucial to the drag-reducing effect of surface denticles. Digital particle image velocimetry (DPIV) of the flow field surrounding moving shark skin foils shows that skin denticles promote enhanced leading-edge suction, which might have contributed to the observed increase in swimming speed. Shark skin denticles might thus enhance thrust, as well as reduce drag.
The blue shark Prionace glauca is the most abundant large pelagic shark in the Atlantic Ocean. Although recaptures of tagged sharks have shown that the species is highly migratory, migration pathways towards the overwintering grounds remain poorly understood. We used archival satellite pop-up tags to track 23 blue sharks over a mean period of 88 days as they departed the coastal waters of North America in the autumn. Within 1-2 days of entering the Gulf Stream (median date of 21 Oct), all sharks initiated a striking diel vertical migration, taking them from a mean nighttime depth of 74 m to a mean depth of 412 m during the day as they appeared to pursue vertically migrating squid and fish prey. Although functionally blind at depth, calculations suggest that there would be a ~2.5-fold thermoregulatory advantage to swimming and feeding in the markedly cooler deep waters, even if there was any reduced foraging success associated with the extreme depth. Noting that the Gulf Stream current speeds are reduced at depth, we used a detailed circulation model of the North Atlantic to examine the influence of the diving behaviour on the advection experienced by the sharks. However, there was no indication that the shark diving resulted in a significant modification of their net migratory pathway. The relative abundance of deep-diving sharks, swordfish, and sperm whales in the Gulf Stream and adjacent waters suggests that it may serve as a key winter feeding ground for large pelagic predators in the North Atlantic.
The total number, distribution and peak density of presumed retinal ganglion cells was assessed in 10 species of elasmobranch (nine species of shark and one species of batoid) using counts of Nissl-stained cells in retinal wholemounts. The species sampled include a number of active, predatory benthopelagic and pelagic sharks that are found in a variety of coastal and oceanic habitats and represent elasmobranch groups for which information of this nature is currently lacking. The topographic distribution of cells was heterogeneous in all species. Two benthic species, the shark Chiloscyllium punctatum and the batoid Taeniura lymma, have a dorsal or dorso-central horizontal streak of increased cell density, whereas the majority of the benthopelagic and pelagic sharks examined exhibit a more concentric pattern of increasing cell density, culminating in a central area centralis of higher cell density located close to the optic nerve head. The exception is the shark Alopias superciliosus, which possesses a ventral horizontal streak. Variation in retinal ganglion cell topography appears to be related to the visual demands of different habitats and lifestyles, as well as the positioning of the eyes in the head. The upper limits of spatial resolving power were calculated for all 10 species, using peak ganglion cell densities and estimates of focal length taken from cryo-sectioned eyes in combination with information from the literature. Spatial resolving power ranged from 2.02 to 10.56 cycles deg(-1), which is in accordance with previous studies. Species with a lower spatial resolving power tend to be benthic and/or coastal species that feed on benthic invertebrates and fishes. Active, benthopelagic and pelagic species from more oceanic habitats which feed on larger, more active prey, possess a higher resolving power. Additionally, ganglion cells in a juvenile of C. punctatum, were retrogradely-labeled from the optic nerve with biotinylated dextran amine (BDA). A comparison of the BDA- labeled material and tissue stained for Nissl substance indicates that 76% of the cells in the retinal ganglion cell and inner plexiform layers of the central retina in this species are non-ganglion cells.
The widespread variation in brain size and complexity that is evident in sharks and holocephalans is related to both phylogeny and ecology. Relative brain size (expressed as encephalization quotients) and the relative development of the five major brain areas (the telencephalon, diencephalon, mesencephalon, cerebellum, and medulla) was assessed for over 40 species from 20 families that represent a range of different lifestyles and occupy a number of habitats. In addition, an index (1-5) quantifying structural complexity of the cerebellum was created based on length, number, and depth of folds. Although the variation in brain size, morphology, and complexity is due in part to phylogeny, as basal groups have smaller brains, less structural hypertrophy, and lower foliation indices, there is also substantial variation within and across clades that does not reflect phylogenetic relationships. Ecological correlations, with the relative development of different brain areas as well as the complexity of the cerebellar corpus, are supported by cluster analysis and are suggestive of a range of 'cerebrotypes'. These correlations suggest that relative brain development reflects the dimensionality of the environment and/or agile prey capture in addition to phylogeny.
In the male blue shark Prionace glauca, paired testes produce spermatozoa year-round.
The placoid scales, or denticles, of the external epidermis of elasmobranchs are well known as a hard protective coat over the skin to reduce abrasion or as elements to reduce hydrodynamic drag. However, the structure and function of denticles within the oral cavity is uncertain. Using stereological and scanning electron microscopy, this study examines the structure and distribution of oral denticles in a range of elasmobranchs. Of the batoids analyzed, only members of the Rhinobatidae possessed oral denticles, with no denticles found in the members sampled in the Gymnuridae or Dasyatidae. In contrast, oral denticles were located in all the selachians examined, except for members of the Orectolobidae. Within the selachians, the denticles of the Carcharhinidae have a grooved surface and a central spine, which is angled toward the posterior of the mouth. These denticular adaptations are beneficial to reduce hydrodynamic drag, an advantage for these free-swimming species with ram ventilation. Alternatively, members of the Hemiscyllidae have broad bulbous denticles that often overlap, providing a hard surface to protect the epithelium from abrasion during the consumption of hard-bodied prey. The distribution and high number of oral denticles appears to spatially compromise the capacity for oral (taste) papillae to populate the oropharyngeal cavity but provides increased friction and grip on prey items as they are manipulated within the mouth.
This important and exciting title represents the first authoritative volume focussed on pelagic (open ocean) sharks as a group. Virtually every pelagic shark expert in the world has contributed to this landmark publication which includes the latest data and knowledge on pelagic shark biology, fisheries, management, and conservation. Pelagic sharks face unprecedented levels of exploitation in all the world's oceans through both direct fisheries and by-catch, and effective management for these species is contingent upon solid science and data, which this book brings together for the first time. All those involved in shark biology will need to have a copy of this book.
As an undergraduate at the University of Bangor studying marine biology and oceanography, Carla Atkinson developed a love for the ocean and fascination with sharks. During her master’s in marine biology, she carried out a study of the electroreceptive systems of the black mouth catshark, Galeus melastomatus, and velvet belly dogfish, Etmopterus spinax, at the University of Genova where she became more interested in elasmobranch sensory systems. It was during this research that she found a considerable lack of knowledge of their gustatory sense, something she found extraordinary since many people are afraid of sharks for fear of being eaten by them. She was determined to do her part to change this and was fortunate enough to gain a Northcote Graduate Scholarship and was accepted for her current Ph.D. looking at the gustatory systems of elasmobranchs in Prof. Shaun Collin’s lab at the University of Queensland. She has since learnt that relatively little is understood of the gustatory systems of vertebrates in general and hopes to continue her research in this field.
Mello, W.C., de Carvalho, J.J., Brito, P.M.M. 2011. Microstructural morphology in early dermal denticles of hammerhead sharks (Elasmobranchii: Sphyrnidae) and related taxa. —Acta Zoologica (Stockholm) 00: 1–7.
This study uses scanning electron microscopies to investigate and describe the microstructural diversity of dermal denticles in the family Sphyrnidae, which comprises all living hammerhead shark species, comparing them to other related taxa (i.e. Carcharhinus dussumieri, Carcharhinus plumbeus, Carcharhinus acronotus, Rhizoprionodon acutus, Negaprion brevirostris and Hemigaleus microstoma). The results reveal that sphyrnids present noticeable microstructures in the dermal denticles, distinguishing them from the other related species investigated. Additionally, scale patterns are the same in three distinct body regions (i.e. cephalic, branchial and dorsal fin). Species of Sphyrnidae that reach bigger total lengths and that are widely distributed (i.e. Sphyrna lewini and Sphyrna mokarran) presented more, smaller and nearly hexagonal microstructures that do not cover the entire scale surface, unlike species reaching smaller sizes and restricted to coastal habits (i.e. Sphyrna tiburo, Sphyrna tudes, Sphyrna media and Eusphyra blochii). The sphyrnid scales are similar to R. acutus and C. dussumieri rather than to the other species, but it is not possible to identify the sphyrnid species only by scale features. It is clear that a similar morphology of scales is not necessarily related to similar life habits, and that they are candidates to provide new characters in phylogenetical studies among sphyrnids.
A set of hypotheses are developed for the origin of living sharks and rays and the interrelationships of their major groups, using some methods of cladistic analysis to relate groups with shared derived characters. Comparative studies on living sharks and rays combined with new data on fossil sharks suggests that the living groups ultimately stem from a common ancestral group of “neoselachian” sharks with many modern characters. Reinterpretations of “amphistyly” in modern sharks is presented on new data.
The anatomy of the feeding apparatus of the lemon shark, Negaprion brevirostris, is investigated by gross dissection, computer axial tomography, and histological staining. The muscles and ligaments of the head associated with feeding are described. The upper and lower jaws are suspended by the hyoid arch, which in turn is braced against the chondrocranium by a complex series of ligaments. In addition, various muscles and the integument contribute to the suspension and stability of the jaws. The dual jaw joint is comprised of lateral and medial quadratomandibular joints that resist lateral movement of the upper and lower jaws on one another. This is important during feeding involving vigorous head shaking. An elastic ethmoplatine ligament that unites the anterior portion of the upper jaw to the neurocranium is involved with upper jaw retraction. The quadratomandibularis muscle is divided into four divisions with a bipinnate fiber arrangement of the two large superficial divisions. This arrangement would permit a relatively greater force per unit volume and reduce muscle bulging of the jaw adductor muscle in the spatially confined cheek region. Regions of relatively diffuse integumental ligaments overlying the adductor mandibulae complex and the levator palatoquadrati muscle, interspersed with localized regions of longer tendonlike attachments between the skin and the underlying muscle, permit greater musculoskeletal movement relative to the skin. The nomenclature of the hypobranchial muscles is discussed. In this shark they are comprised of the unsegmented coracomandibularis and coracohyoideus, and the segmented coracoarcualis. © 1995 Wiley-Liss, Inc.
The epidermal layer of the skin of the cartilaginous fish Raja clavata taken from a suitable place on the ventral side of the head was examined by light and electron microscopy. Cells of the basal layer divide and differentiate as epithelial cells of the mid‐epidermis, characterized by numerous electron‐dense, membrane‐bound vesicles; their content is assumed to be mucus. Cells at the superficial layer of the epidermis lose these vesicles but have a palisade of mucous vesicles below the apical membrane. A modified glycocalyx coats the outer leaflet of the apical membrane.
In addition to epithelial cells, there are epidermal cells differentiated as two kinds of mucous goblet cells, sacciform cells similar to those found in teleost fish epidermis, holocrine cells with some resemblance to the club cells of teleosts but differing in their cytology, and chemosensory cells similar to those found in the oral epithelium of Raja . Merkel cells were not found. Cells that might be interpreted as ionocytes were seen but their nature was not confirmed. The epidermis is penetrated by numerous nerve fibres; swellings contain mitochondria and glycogen granules. The nerve fibres also contain 50–70 nm vesicles with an indistinct core. Cells intrusive to the epidermis include melanocytes, lymphocytes, macrophages, and several types of granular leucocyte. The cytology of the various epidermal elements is generally similar to that known in teleost fishes.
As apex predators, chondrichthyans, or cartilaginous fishes, hold an important position within a range of aquatic ecosystems and influence the balance between species' abundance and biodiversity. Having been in existence for over 400 million years and representing the earliest stages of the evolution of jawed vertebrates, this group also covers a diverse range of eco-morphotypes, occupying both marine and freshwater habitats. The class Chondrichthyes is divided into two subclasses: the Elasmobranchii (sharks, skates, and rays) and the Holocephali (elephant sharks and chimaeras). However, many of their life history traits, such as low fecundity, the production of small numbers of highly precocious young, slow growth rates, and late maturity, make them highly susceptible to human exploitation. To mitigate the negative effects of human impacts, it is important that we understand the sensory strategies that elasmobranchs use for navigating within their environment, forming reproductive aggregations, feeding, and even communicating. One approach to investigate the sensory bases of their behavior is to examine the peripheral sense organs mediating vision, olfaction, gustation, lateral line, electroreception, and audition in a large range of species in order to identify specific adaptations, the range of sensitivity thresholds, and the compromise between sensory spatial resolution and sensitivity. In addition, we can quantitatively assess the convergence of sensory input to the central nervous system and the relative importance of different sensory modalities. Using a comparative approach and often a combination of anatomical, electrophysiological, and molecular techniques, significant variation has been identified in the spatial and chromatic sampling of the photoreceptors in the eye, the surface area and the number of olfactory lamellae within the nasal cavity, the level of gustatory sampling within the oral cavity, the type and innervation of neuromasts of the lateral line system, the distribution of electroreceptive pores over the head, and the morphology of the inner ear. These results are presented in the context of predictions of sensory capabilities for species living in a range of ecological niches, what further research is needed, and how this sensory input may be a predictor of behavior.
The blue shark (Prionace glauca) is widely distributed in the world's oceans. For an elasmobranch, it is relatively productive, giving birth to an average of 30 pups after a 9- to 12-month gestation. Annual fecundity is uncertain, but individual females may breed every year. The young are usually born in spring and summer, and the pupping and nursery areas seem to be located in transition zones where there is a large prey biomass for the juveniles. Growth is relatively rapid, with males maturing at 4-6 years and females at 5-7 years. The blue shark diet consists mainly of small pelagic fish and cephalopods, particularly squid. Relative abundance is generally lowest in equatorial waters and increases with latitude. Distinct sex and size segregation is evident, with size generally decreasing with increasing latitude. Blue sharks are highly migratory, with complex movement patterns related to reproduction and to the distribution of prey. Tagging studies have shown extensive movements with numerous transoceanic migrations; distance moved increases with age. Blue sharks are a major bycatch of longline and gill-net fleets, but because of poor reporting, the magnitude of the catch and mortality is not reflected by official statistics, and the limited population assessments carried out to date do not indicate major impacts on their populations. A number of blue shark populations are thought to be stable despite heavy fishing pressure.
Examination by scanning and transmission electron microscopy (SEM and TEM) has found no actual taste buds in the mouth of Raja clavata. Prominences of the epithelium on the roof and floor of the mouth, and on the oral valves, contain large numbers of innervated bipolar cells, not associated in the form of taste buds, with a cytology intimating that they have a chemosensory function. The apices of these sensory cells, each bearing a group of microvilli, protrude between the superficial epithelial cells. Neurite profiles are associated with the sensory cells; synaptic specializations are marked by a cluster of vesicles with inconspicuous dense cores and some densities on the cell membrane. Shrunken, electron-dense, cell profiles are interpreted as apoptotic. Shrunken sensory cell profiles are commoner than similar epithelial cells, especially in young individuals, indicating a relatively rapid turnover of sensory cells. The epithelium contains a variety of granulocytic leucocytes, some of which contain large phagosomes.
The fine structure of the intra-uterine epithelium of the pregnant blue shark,Prionace glauca, was examined. The intra-uterine epithelium was bilaminar and the underlying epithelial cell was extremely reduced in cytoplasm.
Two cytological characteristics were shown in the outer epithelial cell; open inter-cellular spaces closed in the apical portion
by a junction complex and, numerous mitochondria distributed in the basal and lateral portions of the cytoplasm. Secretive
characteristics were not recognized in the outer epithelial cell, although few regions composed only of mucous cells were
seen. The flattened endothelium of the capillary lay closely beneath the epithelium. These structures are thought to facilitate
the water-solute transport and gaseous exchange. It suggests that the intra-uterine epithelium is involved in the osmoregulation
of the uterine fluid and the exchange of respiratory gases between mother and fetus.
The mucosa covering the tongue of the Chimaera monstrosa has been investigated with histological and immunohistochemical methods allowing to describe, for the first time, gustatory structures (taste buds) in this subclass of cartilaginous fish. G-protein-alpha-subunit-inhibitory-like (Gαi-like) immunoreactivity has been detected in the taste buds of C. monstrosa, as described in other vertebrates. In order to gain confidence on the antiserum used, able to recognize three Gαi proteins in mammals, alignments of the antigenic sequence in mammals and other vertebrates were performed. The data were used for a research of putative genes in the genome of the holocephalan Callorhinchus milii, to date the only cartilaginous fish with a sequenced genome; the highlighted sequences could suggest the presence of all three genes (gnai1, gnai2 and gnai3) in holocephalans. The sequences of the predicted proteins present a high identity with the mammalian proteins.
The placoid scales, or denticles, of the external epidermis of elasmobranchs are well known as a hard protective coat over the skin to reduce abrasion or as elements to reduce hydrodynamic drag. However, the structure and function of denticles within the oral cavity is uncertain. Using stereological and scanning electron microscopy, this study examines the structure and distribution of oral denticles in a range of elasmobranchs. Of the batoids analyzed, only members of the Rhinobatidae possessed oral denticles, with no denticles found in the members sampled in the Gymnuridae or Dasyatidae. In contrast, oral denticles were located in all the selachians examined, except for members of the Orectolobidae. Within the selachians, the denticles of the Carcharhinidae have a grooved surface and a central spine, which is angled toward the posterior of the mouth. These denticular adaptations are beneficial to reduce hydrodynamic drag, an advantage for these free-swimming species with ram ventilation. Alternatively, members of the Hemiscyllidae have broad bulbous denticles that often overlap, providing a hard surface to protect the epithelium from abrasion during the consumption of hard-bodied prey. The distribution and high number of oral denticles appears to spatially compromise the capacity for oral (taste) papillae to populate the oropharyngeal cavity but provides increased friction and grip on prey items as they are manipulated within the mouth.
The gustatory system in vertebrates comprises peripheral receptors (taste buds), innervated by three cranial nerves (VII, IX, and X), and a series of central neural centers and pathways. All vertebrates, with the exception of hagfishes, have taste buds. These receptors vary morphologically in different vertebrates but usually consist of at least four types of cells (dark, light, basal, and stem cells). An out-group analysis indicates that taste buds were restricted to the oropharynx, primitively, and that external taste buds, distributed over the head and, in some cases, even the trunk, evolved a number of times independently. The sensory neurons of the cranial nerves that innervate taste buds are believed to arise from epibranchial placodes, which are induced by pharyngeal endoderm, but it has never been demonstrated experimentally that these sensory neurons do, in fact, arise from these placodes. Although many details of the development of the innervation of taste buds are still unknown, it is now clear that taste buds are induced from either ecto- or endodermal epithelia, rather than arising from either placodes or neural crest. At present, there are two developmental models of taste bud induction: The neural induction model claims that peripheral nerve fibers induce taste buds, whereas the early specification model claims that oropharyngeal epithelium is specified by or during gastrulation and that taste buds arise from cell-cell interactions within the specified epithelium. There is now substantial evidence that the early specification model best describes the induction of taste buds.
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