Regulatory peptides in the pancreas of two species of elasmobranchs and in the Brockmann bodies of four teleost species

Summary Immunohistochemistry was used to localize regulatory peptides in endocrine cells and nerve fibres in the pancreas of two species of elasmobranchs (starry ray,Raja radiata and spiny dogfish,Squalus acanthias), and in the Brockmann bodies of four teleost species (goldfish,Carassius auratus, brown troutSalmo trutta, rainbow trout,Oncorhynchus mykiss and cod,Gadus morhua). In the elasmobranchs, the classical pancreatic hormones somatostatin, glucagon and insulin were present in endocrine cells of the islets. In addition, endocrine cells were labelled with antisera to enkephalins, FMRF-amide, gastrin/cholecystokinin-(CCK)/caerulein, neurotensin, neuropeptide Y (NPY), and peptide YY (PYY). Nerve fibres were demonstrated with antisera against bombesin, galanin and vasoactive intestinal polypeptide (VIP). These nerve fibres innervated the walls of blood vessels, in the exocrine as well as the endocrine tissue. In the four teleost species immunoreactivity to somatostatin, insulin and glucagon was intense in the Brockmann bodies. Cells were labelled with antisera to enkephalin, neurotensin, FMRFamide, gastrin/CCK/ caerulein, NPY, PYY and VIP. Only a few nerve fibres were found with antisera against dopamine--hydroxylase (DBH, cod), enkephalin (met-enkephalin-Arg-Phe, cod), bombesin (cod), gastrin/CCK/caerulein (cod) and VIP. Galanin-like-immunoreactive fibres were numerous in the Brockmann bodies of all teleosts examined. Immunoreactivity to calcitonin gene-related peptide (CGRP), substance P, tyrosine hydroxylase (TH), and phenyl-N-methyl transferase (PNMT) could not be found in any of the species studied.     

The arrangement of neurosecretory and catecholamine fibres in relation to the pituitary intermedia cells of the skate,Raja radiata

Summary The innervation pattern of the intermediate lobe of the skate (Raja radiata) was studied with histological and fluorescence histochemical methods. Neurosecretory fibres, stained with i.a. pseudo-iso-cyanine, were found running in bundles in the central parts of the cell cords. They terminated partly around the perinuclear parts of the intermedia cells, partly around the apices of the cells close to the vascular walls.A catecholamine innervation of the intermedia was also established. Catecholaminecontaining fibres with the appearance of nerve terminals were found around the intermedia cell apices close to the vessels. In some specimens, catecholamine fibres also seemed to terminate at the perinuclear parts of the cells.Thus it is possible, judging solely from structural relations, that both the cell body (the synthesis pole) and the cell apex (the release pole) receive a dual innervation. Recent experimental evidence indicates that the release of MSH from the pars intermedia is controlled by catecholamine fibres, but as yet there is only structural evidence for a special control of hormone synthesis.This study was supported by grants from the Swedish Natural Science Research Council (No. 99-35 and 2126-2) and was carried out within a research organization sponsored by the Swedish Medical Research Council (Projects No. B70-14X-712-05 and B70-14X-56-06).     

Barrier membranes at the outer surface of the brain of an elasmobranch,Raja erinacea

Summary This report gives the results of the first electron-microscopic examination of the cell layers covering the outer brain surface and the inner surface of the cartilaginous skull in the skate, Raja erinacea. The perivascular glial blood-brain barrier — a characteristic of elasmobranchs — extends to the outer surface of the brain. This outer barrier layer is surrounded, in turn, by a subarachnoid compartment (depth: 30–40 m), containing loose connective tissue and blood vessels; by an arachnoid-like epithelium (10–15 cell layers), impermeable to horseradish peroxidase; and, by perimeningeal fluid, a fluid with a slow turnover rate and a protein composition different from plasma. The inside of the skull, facing the perimeningeal fluid, is covered by a multilayered (10–15 layers) cuboidal epithelium, also impermeable to horseradish peroxidase. Closely apposed cells in the luminal layer of this epithelium have apical microvilli and numerous vesicular profiles, containing material of moderate electron density. These observations may explain, in terms of structure, the regulated protein content of perimeningeal fluid and the restricted exchange of solutes between brain and perimeningeal fluid in elasmobranchs.     

Food habits of the longnose skate, <Emphasis Type="Italic">Raja rhina</Emphasis> (Jordan and Gilbert, 1880), in central California waters

Feeding studies can provide researchers with important insights towards understanding potential fishery impacts on marine systems. Raja rhina is one of the most common elasmobranch species landed in central and northern California demersal fisheries, yet life history information is extremely limited for this species and aspects of its diet are unknown. Specimens of R. rhina were collected between September, 2002 and August, 2003 from fisheries-independent trawl surveys. Percent Index of Relative Importance values indicated that the five most important prey items in 618 stomachs of R. rhina were unidentified teleosts (31.6% IRI), unidentified shrimps (19.6% IRI), unidentified euphausiids (10.9% IRI), Crangonidae (7.4% IRI), and Neocrangon resima (6.0% IRI). There were significant dietary shifts with increasing skate total length and with increasing depths. Smaller skates ate small crustaceans and larger skates ate larger fishes and cephalopods. With increasing depths, diet included bentho-pelagic teleosts and more cephalopods and euphausiids. The findings of this study are consistent with previous researchers that report similar diet shifts in skate species with size and depth.     

Granulated peripolar epithelial cells in the renal corpuscle of marine elasmobranch fish

Summary Granulated epithelial cells at the vascular pole of the renal corpuscle, peripolar cells, have been found in the kidneys of five species of elasmobranchs, the little skate (Raja erinaced), the smooth dogfish shark (Mustelus canis), the Atlantic sharpnose shark (Rhizoprionodon terraenovae), the scalloped hammerhead shark (Sphryna lewini), and the cow-nosed ray (Rhinoptera bonasus). In a sixth elasmobranch, the spiny dogfish shark (Squalus acanthias), the peripolar cells could not be identified among numerous other granulated epithelial cells. The peripolar cells are located at the transition between the parietal epithelium of Bowman's capsule and the visceral epithelium (podocytes) of the glomerulus, thus forming a cuff-like arrangement surrounding the hilar vessels of the renal corpuscle. These cells may have granules and/or vacuoles. Electron microscopy shows that the granules are membrane-bounded, and contain either a homogeneous material or a paracrystalline structure with a repeating period of about 18 nm. The vacuoles are electron lucent or may contain remnants of a granule. These epithelial cells lie close to the granulated cells of the glomerular afferent arteriole. They correspond to the granular peripolar cells of the mammalian, avian and amphibian kidney. The present study is the first reported occurrence of peripolar cells in a marine organism or in either bony or cartilagenous fish.     

The occurrence and distribution of peptide- or 5-HT-containing nerves in the swimbladder of four different species of teleosts (Gadus morhua,Ctenolabrus rupestris,Anguilla anguilla,Salmo gairdneri)

Summary The innervation of the swimbladder in four different teleost species has been studied by the use of immunohistochemical methods. The teleosts examined belong to two different groups regarding their swimbladder morphology: physoclists (the cod, Gadus morhua and the goldsinny wrasse, Ctenolabrus rupestris) and physostomes (the eel, Anguilla anguilla and the rainbow trout, Salmo gairdneri). Vasoactive intestinal polypeptide-like immunoreactivity was demonstrated in nerves of the swimbladder walls of all four species, and in the gas glands of the cod and the goldsinny wrasse. Substance P-like immunoreactivity was shown in swimbladders of the cod, eel and rainbow trout but not the goldsinny wrasse. Immunoreactivity to met-enkephalin antiserum was revealed in the swimbladder walls of the eel and the goldsinny wrasse, while neurotensin-like immunoreactivity was present in the goldsinny wrasse and rainbow trout swimbladders. Neurotensin-like immunoreactivity was also seen in the gas gland of the goldsinny wrasse. 5-Hydroxytryptamine immunoreactivity was found in endocrine cells in the pneumatic duct of the eel and in the swimbladder walls of the goldsinny wrasse and the rainbow trout. In conclusion, all teleosts examined showed a very close resemblance in the peptidergic/tryptaminergic innervation of the swimbladder to that of the gut, inasmuch as the immunoreactivity present in the swimbladders always occurred in the gut of the same species.     

Distribution of PACAP (pituitary adenylate cyclase-activating polypeptide)-like and helospectin-like peptides in the teleost gut

Pituitary adenylate cyclase-activating polypeptide (PACAP) and helospectin are two vasoactive intestinal polypeptide (VIP)-related neuropeptides that have recently been demonstrated in the mammalian gut; the aim of this study was to reveal their occurrence and localisation in the gastrointestinal tract, swimbladder, urinary bladder and the vagal innervation of the gut of teleosts, using immunohistochemical methods on whole-mounts and sections of these tissues from the Atlantic cod, Gadus morhua and the rainbow trout, Oncorhynchus mykiss. Both PACAP-like and helospectin-like peptides were present in the gut wall of the two species. Immunoreactive nerve fibres were found in all layers but were most frequent in the myenteric plexus and along the circular muscle fibres. Immunoreactivity was also demonstrated in nerves innervating the swimbladder wall, the urinary bladder and blood vessels to the gut. Immunoreactive nerve cell bodies were found in the myenteric plexus of the gut and in the muscularis mucosae of the swimbladder. In the vagus nerve, non-immunoreactive nerve cells were surrounded by PACAP-immunoreactive fibres. Double staining revealed the coexistence of PACAP-like and helospectin-like peptides with VIP in all visualized nerve fibres and in some endocrine cells. It is concluded that PACAP-like and helospectin-like peptides coexist with VIP in nerves innervating the gut of two teleost species. The distribution suggests that both PACAP and helospectin, like VIP, are involved in the control of gut motility and secretion.     

Raja koreana, a new species of skate (Elasmobranchii, Rajoidei) from Korea

A new rajid species,Raja koreana, is described from a single adult female specimen, 735 mm in total length, collected off the southwestern coast of the Korean Peninsula. AlthoughR. koreana is included in the group of species characterized by the scapulocoracoid lacking an anterior bridge and having the postventral fenestra expanded, it is unique among the latter in possessing: pectoral girdle propterygium not extending to snout tip; rostral shaft of neurocranium narrow and thick, unsegmented base with filamentous cartilage; snout fleshy; pores of ampullae of Lorenzini densely distributed over much of ventral surface to behind cloaca; most thorns on tail directed anteriorly; tail short; a pair of longitudinally elongated black blotches on middle of dorsal surface of disc when fresh; a pair of black blotches (grayish at center) posteriorly on pectoral fins; ventral surface of dise uniformly blackish-brown, except for areas around pores.     

Immunohistochemical study of 5-HT-containing neurons in the teleost intestine: relationship to the presence of enterochromaffin cells

Summary The formaldehyde-induced fluorescence technique had shown 5-hydroxytryptamine-containing enteric neurons in the intestine of the teleost Platycephalus bassensis, but did not reveal such neurons in the intestine of Tetractenos glaber or Anguilla australis. Re-examination of these animals with 5-hydroxytryptamine immunohistochemistry showed immunoreactive enteric neurons in the intestine of all three teleost species. The 5-hydroxytryptamine-containing enteric neurons showed essentially the same morphology in all species examined: the somata were situated in the myenteric plexus, extending down into the circular muscle layer, but none were found in the submucosa; processes were found in the myenteric plexus, the circular muscle layer and the lamina propria. It was concluded that the neurons may innervate the muscle layers or the mucosal epithelium, but were unlikely to be interneurons. In a range of teleosts, enterochromaffin cells were found in the intestine of only those species in which the formaldehyde technique did not visualize neuronal 5-hydroxytryptamine. Available evidence suggests that, in vertebrates, 5-HT-containing enterochromaffin cells are lacking only where there is an innervation of the gut mucosa by nerve fibres containing high concentrations of 5-HT.     

Observations on the electric organ discharge of two skate species (Chondrichthyes: Rajidae) and its relationship to behaviour

Synopsis The electric organ discharge (EOD) of the little skate,Raja erinacea and winter skate,R. ocellata was recorded both from isolated individuals and from small groups using methods that allowed for the identification of individuals producing EODs. Pulse duration, train lengh, frequency, and pulse patterns are characterized and correlated with behaviour. The two species,R. erinacea andR. ocellata, were found to have characteristically different EOD pulse durations of 70 ms and 217 ms respectively. Isolated skates rarely discharged whereas groups of skates were found to discharge regularly. The EOD was evoked by tactile prodding, physical contact with other skates and electrical stimulation. Skates also discharged reflexively in response to an artificially induced head-positive DC stimulus, sine wave and monopolar square pulses. During approach and contact, skates responded to each other with interacting EOD displays. EOD interaction and pulse duration differences between other species suggest a possible intra-specific communication function of the EOD inRaja.     

The paraventricular and posterior recess organs of elasmobranchs: A system of cerebrospinal fluid-contacting neurons containing immunoreactive serotonin and somatostatin

Summary The paraventricular organ (PVO) and the posterior recess organ (PRO) of two elasmobranch species, the spiny dogfish,Squalus acanthias, and the skate,Raja radiata, were investigated by use of scanning and transmission electron microscopy and immunocytochemistry employing a series of primary antisera. The PVO and PRO contained four types of cerebrospinal fluid (CSF)-contacting neurons. One type was free of secretory granules and projected a dendrite-like process into the ventricle. The other three types were distinguished according to the size of their secretory granules. The ventricular extensions of these cells were filled with secretory granules. By means of immunocytochemistry three types of CSF-contacting neurons were observed in the PVO and PRO. Type I contained only serotonin; type 2 displayed only somatostatin; type 3 was endowed with both serotonin and somatostatin. Type I dominated in the PRO, whereas type 3 was the most frequent in the PVO. The latter cells appear to be the site of origin of a loose tract formed by serotonin- and somatostatinimmunoreactive fibers projecting from the PVO into the neuropil of the PRO. Compact bundles formed exclusively by serotonin fibers were also shown to extend between the PVO and PRO. The basal processes of the CSF-contacting neurons of the PRO penetrated into the underlying neuropil. This neuropil is rich in synapses and can be regarded as an integrative area to which the basal processes of the local CSF-contacting neurons, serotonin and somatostatin fibers from the PVO, and fibers containing immunoreactive thyrotropin-releasing hormone of unknown origin, support a conspicuous input. The present findings indicate that the PVO and PRO of elasmobranchs are functionally integrated structures.Dedicated to Professor Erik Dahl on the occasion of his 75th birthday.     

Immunocytochemical localization of S-100 protein in the hypophysis and saccus vasculosus of the elasmobranchs Mustelus manazo and Scyliorhinus torazame

Summary S-100 protein-immunoreactive cells were demonstrated by immunocytochemical procedures in the hypophysis and saccus vasculosus of two species of elasmobranchs (Mustelus manazo and Scyliorhinus torazame). In the saccus vasculosus of M. manazo, immunoreactivity was detectable exclusively in the fibrous portions interposed between the epithelial layer and the blood vessels. In the neurohypophysis, tanycytes and astrocytes of the median eminence were immunostained, but only a few labeled cells were found in the neurointermediate lobe. In S. torazame, the neurohypophysis displayed a similar distribution of immunoreactivity, but there were no labeled cells in the saccus vasculosus. In both species, none of the glandular cells of the hypophysis displayed immunoreactivity. Electron-microscopic examination showed that the immunostained cells in the saccus vasculosus correspond to astrocytes.     

Comparative study of gill neuroepithelial cells and their innervation in teleosts and Xenopus tadpoles

Peripheral O₂ chemoreceptors initiate adaptive cardiorespiratory responses to hypoxia in vertebrates. Morphological and physiological evidence suggests that, in fish, neuroepithelial cells (NECs) of the gill perform this role. We conducted a comparative examination in three species of teleosts (zebrafish, goldfish and trout) and larvae of the amphibian Xenopus laevis, using whole-mount gill preparations and confocal immunofluorescence, to elucidate the distribution, morphology and innervation of gill NECs. Nerve fibres were immunolabelled with the neuronal marker zn-12 and were associated with serotonin-immunoreactive NECs in the gills of all species tested. With the exception of trout, innervated NECs were present on all gill arches in the filaments and respiratory lamellae in fish and on homologous structures in Xenopus (i.e. gill “tufts”, including respiratory terminal branches). Thus, the distribution and innervation of NECs of the internal gills of amphibians and teleosts are relatively well conserved, suggesting an important role for gill NECs as O₂ chemoreceptors in aquatic vertebrates. Furthermore, the size and density of gill NECs is variable among teleosts and developmental stages of Xenopus larvae and may be dependent on general gill dimensions or environmental conditions. This report constitutes the first comparative study of gill NECs in fish and amphibians and highlights the significance of gill NECs as an evolutionary model for studying O₂ sensing in vertebrates. We acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding through an operating grant to C.A.N., and the NSERC and the Ontario Graduate Scholarship (OGS) program for postgraduate scholarships to M.G.J.     

Diet composition of the Thornback Ray,Raja clavata Linnaeus, 1758 (Elasmobranchii: Rajidae) in the Turkish Aegean Sea

The diet of Raja clavata (Thornback Ray) was studied in 2008–2009 in S??ac?k Bay in the eastern Mediterranean, based on 187 stomach contents obtained by commercial trawl operations. 40 different prey species were found. Mysidacea, Parapenaeus longirostris and Plesionika sp. were the most abundant prey items. The results indicate that the main prey varies considerably depending on the ecological characteristics of the habitat.     

Immunoreactive atrial natriuretic peptide in the fish heart and blood plasma examined by electron microscopy,immunohistochemistry and radioimmunoassay

Summary The immunoreactivity of atrial natriuretic peptide and ultrastructure of cardiocytes were examined in 5 species each of freshwater and seawater teleosts, as well as in 2 species each of elasmobranchs and cyclostomes. Immunoreactivity was strong in the atria of Cyprinus carpio, Anguilla japonica and Conger myriaster, rather weak in atria of Channa maculata, Lepomis macrochirus, Salmo gairdneri, Oplegnathus fasciatus and Eptatretus burgeri, very weak in atria of Pagrus major, Trachurus japonicus and Triakis scyllia, and not detectable in atria of Hexagrammos otakii, Narke japonica and Lampetra japonica. The immunoreactivity of the atrial cardiocytes was generally stronger in freshwater than seawater fish. Ventricular immunoreactivity was detected only in 7 species, always being weaker than that observed in the atrium. Ultrastructurally, however, secretory granules were found in atria and ventricles of all species examined, being more frequent in the former than the latter. By radioimmunoassay, immunoreactive ANP was detected in the extracts of blood plasma and both atrial and ventricular tissues of all species examined. There were no statistically significant differences in the values between freshwater and seawater species.     

The distribution of GABA-like-immunoreactive neurons in the brain of the newt,Triturus cristatus carnifex,and the green frog,Rana esculenta

Summary The distribution of -aminobutyric acid (GABA) immunoreactivity was studied in the brain of two amphibian species (Triturus cristatus carnifex, Urodela; Rana esculenta, Anura) by employing a specific GABA antiserum. A noteworthy immunoreactive neuronal system was found in the telencephalic dorsal and medial pallium (primordium pallii dorsalis and primordium hippocampi) and in the olfactory bulbs. In the diencephalic habenular nuclei there was a rich GABAergic innervation, and immunoreactive neurons were observed in the dorsal thalamus. In the hypothalamus the GABA immunoreactivity was found in the preoptic area, the paraventricular organ and in the hypothalamo-hypophysial complex. In the preoptic area of the frog some GABA-immunoreactive CSF-contacting cells were shown. In the optic tectum immunolabeled neurons were present in all the cellular layers. A rich GABAergic innervation characterized both the fibrous layers of the tectum and the neuropil of the tegmentum and interpeduncular nucleus. In the cerebellum, in addition to the Purkinje cells showing a variable immunopositivity, some immunoreactive cell bodies appeared in the central grey. Abundant immunolabeled nerve fibers in the acoustico-lateral area and some immunopositive neurons in the region of the raphe nucleus were observed. In conclusion, the GABAergic central systems, well-developed in the amphibian species studied, were generally characterized by close similarities to the pattern described in mammals.Dedicated to Professor Valdo Mazzi (Dipartimento di Biologia Animale, Università di Torino), in honor of his 70th birthday     

Functional morphology of the neuropeptidergic light-yellow-cell system in pulmonate snails

The light yellow neuropeptidergic cell system of the basommatophoran snail Lymnaea stagnalis is homologous to the R3-R14 system of the opisthobranch Aplysia californica, and produces three different neuropeptides. Systems homologous to the light yellow cells of Lymnaea stagnalis have been investigated morphologically in two Basommatophora (Lymnaea ovata, Bulinus truncatus) and three Stylommatophora (Helix aspersa, Cepaea nemoralis, Deroceras reticulatum). To this end, an antibody to synthetic light-yellow-cell peptide-II and oligonucleotides to mRNAs encoding parts of peptide-I and peptide-III, were used. The in situ hybridization probes gave negative results. On the other hand, neuronal cell clusters were observed in the central nervous system of all specias studied by immunocytochemistry. These clusters were located in the ganglia of the visceral complex. The neurons project axons into all nerves of these ganglia, especially into the pallial nerves, into the connective tissue of the central nervous system, and into the neuropile of various ganglia. The morphology of the systems is similar to that of the light-yellow-cell system of Lymnaea stagnalis. In all species, the wall of the aorta was innervated by immunoreactive axons. Peripheral innervation by the light-yellow-cell system was investigated in Helix aspersa and Deroceras reticulatum. Serial and alternate sections of whole snails were studied. Reconstructions were made of the heart-kidney-lung complex of these animals. In both species, the muscular vessels of the pulmonary system at the right side of the body were strongly innervated by immunoreactive axons. Furthermore, immunopositive innervation was observed to muscles in the secondary ureter-pneumostome area. The light-yellow-cell system of pulmonates is thus probably involved in the regulation of blood pressure and urine release.     

The pineal organ of Raja clavata: Opsin immunoreactivity and ultrastructure

Summary The pineal organ of Raja clavata was studied by light and electron microscopy, including the immunocytochemical antiopsin reaction. The pineal organ of the ray consists of three portions: (i) a large proximal pineal, (ii) a long tube-like connecting stalk, and (iii) a short distal terminal enlargement. This latter end-vesicle lies in the deep connective tissue layers of the braincase. All portions of the pineal are composed of pinealocytes, intrinsic neurons, ependymal/glial cells, and bundles of nerve fibers embedded in thin neuropil formations. The inner segments of the pinealocytes protrude into the lumen in all parts of the organ and usually contain basal bodies and numerous mitochondria. Often, two outer segments were found to arise from the basal bodies of a single inner segment. By means of light-microscopic immunocytochemistry the outer segments showed a strong antiopsin reaction.The axons of the pinealocytes form ribbon-containing synapses on dendritelike profiles, which appear to belong to the intrinsic pineal neurons. There are other axo-dendritic synapses established by presynaptic terminals lacking ribbons and containing granular and synaptic vesicles. Pineal neurons may contain granular vesicles approximately 60–100 nm in diameter; their processes contribute to the bundles of unmyelinated axons.The fine structural organization of the pineal organ and the opsin immunoreactivity of the outer segments of the pinealocytes indicate a photoreceptive capacity of the organ. The double outer segments represent a peculiar multiplication of the photoreceptor structures.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft to A. Oksche (Ok 1/24; 1/25: Mechanismen biologischer Uhren)On leave from the 2nd Department of Anatomy, Semmelweis OTE, Budapest, Hungary     

Dietary comparisons of six skate species (Rajidae) in south-eastern Australian waters

The diet of six skate species caught as bycatch in south-eastern Australian waters was examined over a 2-year period. The skates were segregated into two regions (continental shelf and continental slope) based on prey species and depth of capture. The shelf group consisted of four species, Dipturus sp. A, D. cerva, D. lemprieri and D. whitleyi, while the slope group comprised two species, Dipturus sp. B and D. gudgeri. The two groups varied in feeding strategies with the shelf species generally occupying a broader feeding niche and preying on a larger diversity of prey including a variety of crustaceans (brachyurans, anomurans, achelates, carideans and dendobranchiates), cephalopods, elasmobranchs and teleosts. Within the slope group, Dipturus sp. B and D. gudgeri were more specialised. Dipturus sp. B preyed primarily on anomurans (galatheids) and bachyurans (homolids), whereas D. gudgeri preyed primarily on teleosts. A size related change in diet was evident for all species with the exception of D. gudgeri in which all sizes preyed predominantly on teleosts. Smaller representatives of the four shelf species all preyed on numerous amounts of caridean shrimps, in particular Leptochela sydniensis. In contrast, the continental slope species, Dipturus sp. B consumed anomurans when small, shifting to brachyurans with increasing size. Of the six skate species examined in this study, three were secondary consumers (trophic level <3) and the remaining three tertiary consumers (trophic level >4). Although ANOSIM found significant differences in dietary composition between species within groups, there was some overlap in prey species amongst co-existing skates, which suggests that there is some degree of resource partitioning amongst them.     

Characterization and isolation of DNA microsatellite primers in Raja clavata L. (thornback ray,Rajidae)

The thornback ray, Raja clavata, is an elasmobranch (cartilaginous fish). Since the 1950s, its stock has severely declined. In order to investigate the genetic population structure of the species, we developed microsatellite loci. The five loci reported here have eight to 48 alleles per locus and display an observed heterozygosity ranging from 0.32 to 0.98 with no deviation from the Hardy–Weinberg equilibrium. In the test panel of 122 individuals from three populations, no null alleles, band‐stuttering, large allele dropouts or linkage disequilibrium was detected.