Clearnose Skate Raja Eglanteria Research Articles

High postural costs and anaerobic metabolism during swimming support the hypothesis of a U-shaped metabolism–speed curve in fishes

Swimming performance is considered a key trait determining the ability of fish to survive. Hydrodynamic theory predicts that the energetic costs required for fishes to swim should vary with speed according to a U-shaped curve, with an expected energetic minimum at intermediate cruising speeds and increasing expenditure at low and high speeds. However, to date no complete datasets have shown an energetic minimum for swimming fish at intermediate speeds rather than low speeds. To address this knowledge gap, we used a negatively buoyant fish, the clearnose skate Raja eglanteria, and took two approaches: a classic critical swimming speed protocol and a single-speed exercise and recovery procedure. We found an anaerobic component at each velocity tested. The two approaches showed U-shaped, though significantly different, speed-metabolic relationships. These results suggest that (i) postural costs, especially at low speeds, may result in J- or U-shaped metabolism-speed curves; (ii) anaerobic metabolism is involved at all swimming speeds in the clearnose skate; and (iii) critical swimming protocols might misrepresent the true costs of locomotion across speeds, at least in negatively buoyant fish.

Fishery Management Complexes: An Impediment or Aid to Sustainable Harvest? A Discussion Based on the Northeast Skate Complex

AbstractThere are seven species of skates (family Rajidae) found along the East Coast of the USA. All seven species are currently managed by the New England Fisheries Management Council as a single management complex extending from the Gulf of Maine to Cape Hatteras. The objective of the management plan is to ensure the long‐term sustainability of fishing for each species via a trip limit approach. Two species are harvested in two distinct commercial fisheries. Northeast Fisheries Science Center trawl survey data and published literature were examined to investigate differences between the individual species in the skate complex. Each species exhibited a unique thermal and geographic range in addition to vital life history traits (e.g., age at maturity, longevity, and maximum size). Thorny Skate Amblyraja radiata and Smooth Skate Malacoraja senta have narrow thermal ranges and maintain a more northern distribution. Barndoor Skate Dipturus laevis have a moderate thermal habitat. Little Skate Leucoraja erinacea and Winter Skate L. ocellata have broad thermal ranges and are distributed throughout the management area. Limited inferences can be made about the thermal preferences of Clearnose Skate Raja eglanteria without data from south of Cape Hatteras, but they appear to have a broad thermal range within the management area. Rosette Skate L. garmani have a narrow thermal range and tend to be found in the deep offshore mid‐Atlantic region. The validity of managing multiple distinct species in a complex is questioned. This example shows that a mixed‐stock management strategy may be inadequate to meet the sustainability needs of each species and the associated fisheries. A management strategy focused on individual species may lead to a more efficient harvest of targeted species while allowing for the rebuilding of overfished species.Received November 8, 2011; accepted December 27, 2012

Do Alternative Leaders Affect Fish Catch in Pound Nets at the Mouth of Chesapeake Bay?

AbstractMore than 60 stranded bottlenose dolphins Tursiops truncatus exhibiting marks consistent with interactions with pound nets have been documented in the lower Chesapeake Bay since 1997. Given this high rate of bycatch, action is needed to mitigate bottlenose dolphin mortality. However, for any such modification to the pound net to be acceptable to fishers, a clear demonstration of no reduction in the targeted finfish catch will be required. Working with fishers, we evaluated the effect of an experimental pound net leader on finfish catch. The experimental leader entailed similar or greater catches of finfish than did the traditional leader. Further, catches of Batoidea (cownose ray Rhinoptera bonasus, bullnose ray Myliobatis freminvillei, southern stingray Dasyatis americana and Atlantic stingray D. sabina) and clearnose skates Raja eglanteria were reduced with the experimental leader. This example demonstrates that working with fishers to devise alternative harvest strategies can result in the implementation of conservation measures. Although finfish catch was maintained with the experimental leader design, we must continue to monitor bottlenose dolphin bycatch to confirm that the use of the experimental leader reduces the bottlenose dolphin mortality associated with pound nets.Received December 11, 2009; accepted December 10, 2010

Trophic Resource Overlap Between Small Elasmobranchs and Sympatric Teleosts in Mid-Atlantic Bight Nearshore Habitats

Small, abundant elasmobranchs use shallow marine areas (<20 m depth) of the US Middle Atlantic coast as nurseries and adult foraging habitat, an area also used by a diverse assemblage of economically important juvenile and adult teleost species. Specimens of three small elasmobranch species (smooth dogfish Mustelus canis, clearnose skate Raja eglanteria, and bullnose ray Myliobatis freminvillii) were collected in August 2007 and 2008 from a study area of ∼150 km2, extending 22 km south from Ocean City, Maryland, USA (38° 19′ N) and offshore from 5- to 20-m depth. Stomach contents indicated that fish were part of the diets of smooth dogfish and clearnose skate at a level comparable with sympatric piscivorous teleosts. However, stable isotope data suggest that piscivory is likely an opportunistic foraging behavior in this habitat. Studied elasmobranchs were secondary-tertiary consumers with diets composed primarily of decapod crustaceans, fish, and mollusks. There was significant overlap in diet composition, spatial distribution, and diel stomach fullness patterns between clearnose skate, southern kingfish Menticirrhus americanus (teleost) and, to a lesser extent, smooth dogfish. Despite this evidence for piscivory, their relatively low densities suggest that predation by these elasmobranchs is unlikely to affect teleost populations in shallow coastal ocean habitats. If shared prey were to become scarce, then competitive interactions are possible.

Assemblage Structure in Shoal and Flat‐Bottom Habitats on the Inner Continental Shelf of the Middle Atlantic Bight, USA

AbstractSand shoals provide both a potentially unique habitat resource for marine organisms and a source of sand for the replenishment of eroded beaches. Sand removal may negatively influence marine communities, so understanding how marine fauna utilize habitats at and around shoals would provide much‐needed guidance in selecting sites for sand harvest. A 2‐year study was conducted on the inner continental shelf of the Middle Atlantic Bight, U.S.A., to compare finfish and invertebrate assemblages at sand shoal and nearby flat‐bottom habitats. Multiple sampling modalities were used to sample organisms across a range of sizes, living habits, and life history stages. There was a trend toward greater abundance, species richness, and species diversity in flat‐bottom habitats than in shoal habitats, and all of these community measures were generally lower during winter than in spring, summer, or fall. Moreover, species groups, including pelagic finfish, pelagic invertebrates, benthic finfish, and benthic invertebrates, were all more abundant in the flat‐bottom habitats. Particular species characterized each type of habitat and these associations varied with season. Sampling with a large commercial trawl indicated that shoal finfish assemblages were characterized by striped bass Morone saxatilis and little skate Leucoraja erinacea in the fall, by scup Stenostomus chrysops in the spring, and by American sand lance Ammodytes americanus, scup, and clearnose skate Raja eglanteria in the summer. Experimental trawl sampling, which targeted primarily smaller organisms, found that communities on shoals were characterized by gastropods in the winter, squid (class Cephalopoda), and right‐handed hermit crabs (family Paguridae) in the spring, and right‐handed hermit crabs and scup in the summer. Winter was the period of lowest finfish and invertebrate use of shoal habitat and thus would be the best time of year for dredging sand to minimize acute impacts.

Gastro-intestinal handling of water and solutes in three species of elasmobranch fish, the white-spotted bamboo shark, Chiloscyllium plagiosum, little skate, Leucoraja erinacea and the clear nose skate Raja eglanteria

The present study reports aspects of GI tract physiology in the white-spotted bamboo shark, Chiloscyllium plagiosum, little skate, Leucoraja erinacea and the clear nose skate, Raja eglanteria. Plasma and stomach fluid osmolality and solute values were comparable between species, and stomach pH was low in all species (2.2 to 3.4) suggesting these elasmobranchs may maintain a consistently low stomach pH. Intestinal osmolality, pH and ion values were comparable between species, however, some differences in ion values were observed. In particular Ca 2+ (19.67 ± 3.65 mM) and Mg 2+ (43.99 ± 5.11 mM) were high in L. erinacea and Mg 2+ was high (130.0 ± 39.8 mM) in C. palgiosum which may be an indication of drinking. Furthermore, intestinal fluid HCO 3 − values were low (8.19 ± 2.42 and 8.63 ± 1.48 mM) in both skates but very high in C. plagiosum (73.3 ± 16.3 mM) suggesting ingested seawater may be processed by species-specific mechanisms. Urea values from the intestine to the colon dropped precipitously in all species, with the greatest decrease seen in C. plagiosum (426.0 ± 8.1 to 0 mM). This led to the examination of the molecular expression of both a urea transporter and a Rhesus like ammonia transporter in the intestine, rectal gland and kidney in L. erinacea. Both these transporters were expressed in all tissues; however, expression levels of the Rhesus like ammonia transporter were orders of magnitude higher than the urea transporter in the same tissue. Intestinal flux rates of solutes in L. erinacea were, for the most part, in an inward direction with the notable exception of urea. Colon flux rates of solutes in L. erinacea were all in an outward direction, although absolute rates were considerably lower than the intestine, suggestive of a much tighter epithelia. Results are discussed in the context of the potential role of the GI tract in salt and water, and nitrogen, homeostasis in elasmobranchs.

Using Species Composition Data from a Trawl Survey to Determine Potential Bycatch of the Commercial Trawl Fishery for Horseshoe Crab Limulus polyphemus in the Middle Atlantic Bight

AbstractThe National Marine Fisheries Service has encouraged regional management councils to develop fisheries ecosystem plans that, among other things, identify the bycatch species within each fishery and describe each species' spatial and temporal distribution. Horseshoe crabs Limulus polyphemus have been harvested along the eastern coast of the USA since the 1800s; however, a fishery management plan was only recently created for this species, and bycatch in the horseshoe crab trawl fishery has not been quantified. We identified species likely to be caught as bycatch in the commercial fishery by using species composition data from a fishery‐independent horseshoe crab trawl survey conducted in 2005 and 2006. Seventy‐six taxa were caught, including 47 finfish species from 33 families. Skates (family Rajidae) comprised more than one‐half of the total biomass caught, and horseshoe crabs comprised one‐third of the total biomass. Catch per unit effort (kg/km towed) was greatest for little skate Leucoraja erinacea, winter skate Leucoraja ocellata, horseshoe crab, and clearnose skate Raja eglanteria. We also caught species that are currently on the World Conservation Union's Red List (e.g., Atlantic sturgeon Acipenser oxyrinchus) and unmanaged within U.S. waters (e.g., cownose ray Rhinoptera bonasus and smooth dogfish Mustelus canis). Nonmetric multidimensional scaling was used to cluster sites based on species abundance. The sites strongly clustered into two distinct groups, suggesting that species composition differs between northern and southern sites. Bottom water temperature was a significant factor in determining species composition, explaining almost 70% of the variation in species composition among sites. The results of this study provide managers with a better understanding of which nontarget populations may be affected by the horseshoe crab trawl fishery, a first step in understanding the broader ecosystem‐level effects of this fishery.

Ultrastructure of the male genital ducts of the clearnose skateRaja eglanteria

Tissues from the male genital ducts of six specimens of the clearnose skate Raja eglanteria, comprising the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle, were fixed and embedded for ultrastructural examination. In the Leydig gland, two types of columnar cells were identified, one bearing microvilli, a basal nucleus and evidence of active secretion with plentiful endoplasmic reticulum and numerous secretory droplets, and the other pyriform with cilia, and swathes of cytofilaments emanating from prominent desmosomes. Occasional crystalloid intramitochondrial inclusions were seen in the first type, with a periodicity of 24 nm. The upper epididymis was composed of cuboidal cells with microvilli and cilia and irregular electron dense granules, some of which were basally situated and extremely large, often within cells resembling intraepithelial leucocytes; such cells were also seen in the stroma underlying the epithelium. The lower epididymis cells also bore microvilli and cilia and were heavily vacuolated with fatty inclusions as well as the granule-laden leucocytes seen previously. In the ductus deferens, cells had masses of long cilia with occasional microvilli; endoplasmic reticulum was well developed, forming complex arrays with sparse secretory droplets and basal mitochondria. In the seminal vesicle there were two cell types, the most common having long cilia and short microvilli and an occasional, paler cell with supranuclear accumulations of small, round mitochondria. These ultrastructural appearances have been related to cell glycosylation and functions including protein secretion, water absorption and waste removal, and illustrate how structure and function vary down the length of the genital tract.

Glycosylation of the male genital ducts and spermatozeugmata formation in the clearnose skate Raja eglanteria.

Genital ducts of three male Raja eglanteria were fixed and embedded in epoxy and methacrylate resin. Epoxy resin sections from the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle were stained with 20 labelled lectins to examine their glycosylation. The Leydig gland consisted of columnar epithelial cells expressing N-linked glycans, N-acetyl galactosamine, glucosamine and lactosamine residues and sialic acid. Interspersed were ciliated cells of a different glycotype. The upper epididymis of cuboidal epithelium had a strongly glycosylated, ciliated apical surface and cytoplasmic granules that stained heavily with many lectins, with increased glycosylation compared to the Leydig gland. In the lower epididymis, tall, vacuolated cells showed some differences and a slight reduction in lectin staining. The ductus deferens contained two cell types and showed increased terminal N-acetyl galactosamine. The ciliated cuboidal epithelium of the seminal vesicle had marked differences from the ductus epithelium, with decreased N-acetyl galactosamine and lactosamine expression but increased subterminal N-acetyl lactosamine and galactosamine expression and sialylation. Spermatozoa were suspended in a glycosylated matrix and, in the seminal vesicle, were embedded in solid masses of matrix forming spermatozeugmata. These data show changes in glycan expression along the male genital tract, probably related to the nurture and maturation of the spermatozoa as they travel towards the seminal vesicle.

Ascending lateral line pathways to the midbrain of the clearnose skate, Raja eglanteria.

Efferent projections of the electroreceptive dorsal and the mechanoreceptive intermediate octavolateralis nuclei were revealed, in the brain of the clearnose skate Raja eglanteria, by means of silver degeneration and autoradiographic methods. Efferents from each nucleus give rise to ipsilateral and contralateral lemnisci and commissural components. Commissural fibers terminate within their respective nuclei of the opposite side. The lemnisci from each nucleus parallel each other as they course from medullary to mesencephalic levels; those from the dorsal nucleus assume a lateral position and terminate within the lateral part of the nucleus of the lateral line lemniscus and the lateral nucleus of the midbrain; those within the medial part of the lateral line lemniscal nucleus and the dorsomedial mesencephalic nucleus. A substantial number of fibers of each class course through the mesencephalic nuclei and terminate bilaterally within the central tectal zone. The segregation of electroreceptive and mechanoreceptive information is maintained from medullary to mesencephalic levels although there probably is convergence within the central tectal zone and the magnocellular nucleus, the only octaval center to receive terminals from secondary lateral line fibers.