Skate Species Research Articles

A taxonomic revision of Northeast Pacific softnose skates (Rajiformes: Arhynchobatidae: Bathyraja Ishiyama)

Softnose skates (Rajiformes: Arhynchobatidae: Bathyraja Ishiyama) are the most diverse skate genus, with 54 species, and are readily distinguishable from other genera in the family by their poorly calcified, flexible rostral cartilages. Six Bathyraja species are considered valid in the eastern North Pacific, including: B. abyssicola, B. aleutica, B. kincaidii, microtrachys, B. spinosissima, and B. trachura. Similar to other skate genera, eastern North Pacific Bathyraja lack a robust species-specific identification, which leads to issues with setting catch limits and creating management plans. This study identifies and formally redescribes the eastern North Pacific softnose skate species based on morphometric and meristic measurements and includes an Alaskan species, Bering Skate, B. interrupta due to its close morphological relationship to B. kincaidii. A lectotype for B. interrupta is designated. Multivariate tests determined that significant differences existed between the study species. Parsimonious phylogenetic trees showed that B kincaidii represents the basal condition, with B. abyssicola and B. aleutica being the most derived species in the study. The formerly synonymized species B. interrupta and B. kincaidii were shown to be separate, as were the previously synonymized species B. microtrachys and B. trachura. Improved Bathyraja species identification will hopefully assist fisheries managers in developing conservation policies easing the impacts of deep-sea fishing expansion.

The complete mitochondrial genomes of the flapper skate Dipturus intermedius and the longnose skate Dipturus oxyrinchus

We describe the complete mitochondrial genomes of the flapper skate Dipturus intermedius (Parnell 1837) and the longnose skate Dipturus oxyrinchus (Linnaeus 1758), which have been obtained by Sanger sequencing. We report the length of the sequences to be 16,906 and 16,911 bp, respectively. The length and structure of gene regions, containing 13 protein-coding regions, 22 tRNA genes, two rRNA genes, and two non-coding areas, resemble those of related skate species. Despite D. intermedius being considered a cryptic species with D. batis, the full mitogenomes confirm that D. intermedius and D. oxyrinchus are more genetically similar. In comparison to other Dipturus species, D. intermedius is missing a whole codon in its cytochrome oxidase subunit 2 gene. These mitogenomes will be a useful resource furthering investigation of the population genetic differences and evolutionary history of skate species.

Pelvic fin musculature in skates: Morphological variation, phylogeny, and locomotor implications (Chondrichthyes: Batoidea: Rajiformes).

Skates of the order Rajiformes are known to perform benthic locomotion by using their specialized bilobed pelvic fins and thus walking or punting on the substrate. Several studies have investigated the pelvic fin and girdle skeleton but little is known about the muscles responsible for walking performance. The goals of this study, therefore, are to investigate, describe, and illustrate the morphological variation in pelvic musculature among skate species and to discuss the phylogenetic and locomotor implications of the conditions observed. Specimens belonging to 19 of the 36 skate genera currently valid for the order Rajiformes were manually dissected and specimens of Narcine, Tetronarce, and Zapteryx were also examined as comparative material. The pelvic musculature varies among skates, mainly regarding the extension of ventral and dorsal muscles related to the anterior lobe of pelvic fin. All rafijorms examined and the torpediniforms Narcine and Tetronarce possess a complex pelvic musculature associated to the anterior pelvic lobe, which is herein suggested as a synapomorphy of Batoidea with independent losses in guitarfishes and stingrays. Nevertheless, the examination of additional taxa is required to better understand the evolution of pelvic musculature within batoids. The morphological variation observed could indicate different walking abilities as discussed herein but further studies on the locomotion behavior in skates are needed to confirm these assumptions.

Morphological abnormalities in seven American round ray specimens: A review of America's batomorph anomalies.

Although morphological abnormalities in several rays and skate species around the American continents have frequently mentioned, their numbers are unknown. The present work record morphological abnormalities in four Urotrygonidae species. Two anophthalmic specimens were detected (Urotrygon microphthalmum and Urobatis halleri). Two individuals lacked caudal fins (Urobatis maculatus and Urotrygon chilensis). Two round rays showed incomplete fusion of the pectoral fin to the head (U.microphthalmum and U.chilensis). Vertebral compression and fusion were found in a 6-year-old female Urotrygon rogersi. In addition, 118 abnormal batomorph specimens were gathered from the available bibliography, spanning the last six decades (1959-2021). Amblyraja doellojuradoi was the species with the highest number of abnormalities (18). The most common anomaly was an incomplete fusion of the pectoral fin with the head. Since 2010, at least 30 anomalous batomorphs have been recorded every 5years. Sixty-nine abnormal specimens occurred in the Northern Hemisphere (1.00-60.00N). The Cortezian (Pacific) and Southeastern Brazil (Atlantic) marine ecoregions stood out with the highest number of these specimens. Mexico recorded 58 anomalous specimens, followed by Brazil (n=36). Biological, abiotic and anthropogenic factors are probably the leading causes. However, additional studies are necessary to elucidate these speculations.

Movement patterns of a Critically Endangered elasmobranch (Dipturus intermedius) in a Marine Protected Area

Abstract Marine Protected Areas (MPAs) are widely used in marine management, but for mobile species understanding the spatio‐temporal scale of management measures that is required to deliver conservation benefits depends on a detailed knowledge of species’ movements that is often lacking. This is especially the case for species of skate (Rajidae) for which relatively few movement studies have been conducted. In Scotland, the Loch Sunart to the Sound of Jura MPA covering 741 km2 has been designated for the conservation of the Critically Endangered flapper skate (Dipturus intermedius), but fine‐scale movements within this area remain poorly understood. A passive acoustic telemetry study which coupled acoustic tagging of 42 individuals and a static array of 58 receivers was conducted from March 2016 to June 2017. Using acoustic detection time series, angler capture–recapture data and depth time series from archival tags, fine‐scale movements of individuals were investigated. Overall, 33 of the 42 tagged individuals were detected. Residency, site fidelity and transiency were documented. Residency around receivers, lasting from 3 to more than 12 months, was documented in 16 acoustically detected individuals (48%) and all life‐history categories, but was most noticeable among females. Acoustic detections were associated with depth, salinity and season, but there was no evidence that individuals formed close‐knit groups in the areas in which they were detected. Taken together with historical occurrence records of flapper skate, the prevalence and scale of residency documented here suggest that the MPA is sufficiently large to benefit a notable percentage (38 [24–52]%) of skate found in the study area over monthly and seasonal timescales. This result strengthens the case for the use of MPAs to support the conservation of flapper skate and other skate species that display similar movement patterns in areas of high local abundance.

Historical trophic ecology of some divergent shark and skate species in the Dutch coastal North Sea zone

Over the last century the fish community of the Dutch coastal North Sea zone has lost most its shark and skate species. Whether their disappearance has changed the trophic structure of these shallow waters has not been properly investigated. In this study historical dietary data of sharks and skates, being in the past (near)-residents, juvenile marine migrants and marine seasonal visitors of the Dutch coastal North Sea zone were analyzed for the period 1946–1954. Near-resident and juvenile marine migrant species were demersal while all marine seasonal visitors species were pelagic. Based on stomach content composition, the trophic position of four of the various shark and skate species could be reconstructed. The (near)-resident species, the lesser spotted dogfish, the marine juvenile migrant, the starry smooth hound, and the benthopelagic marine seasonal visitor, the thornback ray, had a benthic/demersal diet (polychaetes, molluscs and crustaceans), while the pelagic marine seasonal visitor, the tope shark, fed dominantly on cephalopods and fishes. Diet overlap occurred for fish (tope shark and lesser spotted dogfish), for hermit crabs (lesser spotted dogfish and starry smooth hound) and for shrimps (thornback ray and starry smooth hound). Trophic position ranged from 3.2 for thornback ray preying exclusively on crustaceans to 4.6 for the tope shark consuming higher trophic prey (crustaceans and fish). The analysis indicates that most of the shark and skate species were generalist predators. The calculated trophic positions of shark and skate species indicate that those species were not necessarily at the top of the marine ecosystem food web, but they might have been the top predators of their particular ecological assemblage.

Age, growth, maturity and extinction risk of an exploited and endangered skate, Atlantoraja castelnaui, from off Uruguay and northern Argentina.

The spotback skate Atlantoraja castelnaui (Arhynchobatidae) is a large and threatened skate species subjected to fishing pressure, endemic to the Southwest Atlantic that occurs from Rio de Janeiro, Brazil, to San Jorge Gulf, Argentina. The age, growth, age at maturity and the maximum intrinsic rate of population increase rmax of A. castelnaui were studied using 152 specimens collected from off Uruguay and north Argentina (35°-42° S), between June 2013 and February 2020. Vertebrae from 143 individuals were used for ageing (females: n= 83, size range 404-1300 mm total length, TL; males: n= 60, size range 400-1270 mm TL). Maximum ages determined for females and males were 30 and 28 years, respectively. To fit growth models, non-linear and Bayesian estimation approaches were considered. For the first approach, a set of four candidate growth (size-at-age) models were fitted: three-parameter von Bertalanffy, two-parameter von Bertalanffy with fixed L0 , Gompertz and Logistic. In the second approach, von Bertalanffy, Gompertz and Logistic were fitted. For non-linear estimation, model selection indicated that the entire set of candidate growth models were supported by the data. The von Bertalanffy was selected as the best model for Bayesian estimation. There were no differences in growth between sexes. For the sexes combined, the von Bertalanffy growth model by Bayesian method was considered the most adequate to describe the growth of A. castelnaui (growth mean parameters ± S.D.: L∞ = 1210.29 ± 40.68 mm; k=0.12 ± 0.01 years-1 ; L0 = 179.20 ± 11.62 mm). The age at maturity was estimated at 16.21 and 14.04 years for females and males, respectively. The maximum intrinsic rate of population increase rmax was estimated as 0.252 years-1 . Life-history traits and rmax provided in the present study suggest that this species has a relatively low productivity and may be vulnerable to an intense fishing pressure.

Management Implications for Skates and Rays Based on Analysis of Life History Parameters

The life history (age and growth and reproduction) parameters of 35 species (41 stocks) of skates and rays were analyzed using multivariate analyses. Three groups were categorized by cluster analysis (CA) based on principal component scores. Empirical equation was developed for each group to describe the relationships between the predicted a finite rate of population increase (λ′) and the life history parameters: growth coefficient (k), asymptotic length (L∞), age at maturity (Tm), annual fecundity (f/Rc), ratio between size at birth (Lb), and L∞ (Lb/L∞), and ratio between size at maturity (Lm) and L∞ (Lm/L∞). Group 1 included species with slow growth rates (k < 0.011 year–1), early maturity (Lm/L∞ < 0.62), and extended longevity (Tmax > 25 years); Group 2 included species with intermediate growth rates (0.080 year–1 < k < 0.190 year–1), intermediate longevity (17 years < Tmax < 35 years), and late maturity (Lm/L∞ > 0.60); Group 3 included species with a fast growth rate (k > 0.160 year–1), short longevity (Tmax < 23 years), and large size at birth (Lb/L∞ > 0.18). The λ′ values estimated by these empirical equations showed good agreement with those calculated using conventional demographic analysis, suggesting that this approach can be applied in the implementation of management measures for data-limited skates and rays in a precautionary manner.

Remarkable multicuspid teeth in a new elusive skate (Chondrichthyes, Rajiformes) from the Mediterranean Pliocene

Here we report on four highly peculiar skate teeth from Arcille and Certaldo, two Pliocene localities of Tuscany (central Italy). While being attributable to Rajiformes and somewhat reminiscent of Dipturus and Rostroraja, these specimens display an unusual multicuspid tooth design that does not match any extinct or extant skate taxon known to date. The studied teeth are thus referred to a new genus and species of Rajiformes, Nebriimimus wardi gen. et sp. nov., which is here tentatively assigned to the family Rajidae. Based on pronounced morphological similarities between the rather large-sized teeth of the latter and those of extant nurse sharks, we hypothesise that N. wardi might have been capable of actively foraging upon relatively large food items compared to other rays. This extinct skate species was likely not a common component of the Pliocene Tuscan marine vertebrate assemblages. The palaeoenvironmental scenarios that N. wardi inhabited were marginal-marine and open shelf settings characterised by tropical climate conditions. As N. wardi is currently known only from lower to mid-Pliocene deposits of the Mediterranean Basin, it is tempting to speculate that its speciation dates back to an earliest Pliocene phase of diversification that also contributed to the emergence of the Mediterranean endemic stock of extant skate species.

Genetic evidence from embryos suggests a new species of skate related to Bathyraja parmifera (Rajiformes: Arhynchobatidae) in the Bering Sea

Several new species of oviparous skates of the genus Bathyraja have been identified over the past 2 decades, yet it is possible that a complete understanding of species diversity among this group has not been achieved. We used genetics and morphology to screen for the presence of species-level differentiation among embryos from nursery areas in the eastern Bering Sea that were initially identified as the Alaska skate B. parmifera. A sample of 57 embryos from Pervenets Canyon differed significantly at single nucleotide polymorphism loci from 297 other B. parmifera samples, and differences were on the order of those observed between B. parmifera and the leopard skate B. panthera. This sample of embryos was similar to B. parmifera at the cytochrome c oxidase subunit I (COI) mitochondrial locus. We discuss whether this group of embryos may represent an undescribed cryptic species. Although morphological description of adults is required for a complete species description, only embryo samples were available. Even so, results presented here are a step toward understanding, managing, and conserving the diversity of skate species in the North Pacific Ocean.

Distributions of threatened skates and commercial fisheries inform conservation hotspots

Bycatch in commercial fisheries is a pressing conservation concern and has spurred global interest in adopting ecosystem-based management practices. To address such concerns, a thorough understanding of spatiotemporal relationships among bycatch species, their environment and fisheries is required. Here we used a generalized linear mixed model framework incorporating spatiotemporal random effects to model abundance patterns for 3 skate species caught as bycatch in commercial fisheries (thorny skate Amblyraja radiata, winter skate Leucoraja ocellata and smooth skate Malacoraja senta), as well as 10 target species on the Scotian Shelf, NW Atlantic. Spatiotemporal estimates of relative abundance for at-risk skates within the years 2005-2015 were modelled from research trawl survey data and overlaid with those for target species to identify hotspots of bycatch risk. In addition, abundance estimates for at-risk skates within the years 1975-1985, a period of higher stock abundance, were used to identify areas of previously important habitat. Historically, skate species densely occupied areas near Sable Island and Banquereau Banks, Georges Bank and the Bay of Fundy. Bycatch hotspots between at-risk skates and commercial targets were identified in regions across the Scotian Shelf. These hotspots were independently validated by predicting species presence from at-sea observer data that monitor skate bycatch directly. We discuss spatial relationships between target and bycatch species, highlighting limitations of at-sea observer programmes that this method helps to address. This framework can be applied more broadly to inform ecosystem management and priority areas for conservation or fisheries regulation.

Skate egg nursery areas support genetic diversity of Alaska and Aleutian skates in the Bering Sea

Skate egg case nursery sites are specific locations on the ocean floor where some species of skates deposit egg cases to incubate for up to 5 yr until hatching. We examined genetic diversity within and among skate egg nursery sites of the Alaska skate Bathyraja parmifera and the Aleutian skate B. aleutica in the eastern Bering Sea to gain a better understanding of how skates utilize these areas. Restriction-site associated DNA (RAD) sequencing libraries were used to obtain single nucleotide polymorphism (SNP) datasets for B. parmifera (5285 SNPs) and B. aleutica (3309 SNPs). We found evidence for significant genetic differentiation among all B. parmifera and B. aleutica nursery areas, with 1 exception. B. parmifera from the spatially proximate Pribilof and Bering Canyons were genetically similar, suggesting that this may represent a large contiguous nursery area. Genetic differences between embryos at distinct developmental stages within nursery areas were not significant. Adult B. parmifera taken from the Bering Sea and Aleutian Islands were genetically distinct from embryo collections, implying that additional genetic types of B. parmifera may exist that were not represented by the nursery areas sampled in this study. Our data also showed evidence for low effective population sizes and low Ne/N ratios. Results indicate that nursery areas support genetically distinct components of each species, which underscores the importance of skate egg nursery areas for conservation of genetic diversity.

Right on target: using data from targeted stocks to reconstruct removals of bycatch species, a case study of longnose skate from Northeast Pacific Ocean

Fisheries stock assessments rely heavily on historical catch information to understand how a stock responds to exploitation and make meaningful forecasts under alternative management and environmental scenarios. However, for many bycatch species historical removals are virtually unknown, as a large portion of the catch was discarded at sea. For example, historical discard of elasmobranch species such as skates and sharks have been reported as 95 percent of the total catch based on available data. The longnose skate is one the most abundant groundfishes on the continental slope of the U.S. Pacific Coast by biomass, and the most abundant skate species in the Northeast Pacific Ocean. We developed a method to estimate catch of longnose skate on the U.S. West Coast based on the catch of Dover sole, a co-occurring targeted species with which longnose skate is caught. This method allowed us to reconstruct historical longnose skate catches back to the beginning of the bottom trawl fisheries and improve stock assessment for this species. We also examined the impact of using our method versus other common methods of catch reconstruction to inform stock assessment models, and found that the target-based predictive method produced results that more accurately reflected the life history and typical stock dynamics of elasmobranch taxa. Our method is not limited to the specific case of longnose skate and could be easily adapted for other species and areas.

Growth and Age Validation of the Thornback Ray (Raja clavata Linnaeus, 1758) in the South Adriatic Sea (Central Mediterranean)

Raja clavata is the most widespread and landed skate species in the Mediterranean Basin. Despite its diffusion and economic importance, several aspects of its life history, such as age and growth, are poorly understood. This study evaluated the species’ growth in the South Adriatic Sea (Central Mediterranean Sea) and for the first time attempted an age validation through a tagging experiment. Thin sectioning of vertebral centra proved to be a more accurate preparation method in terms of age estimation precision and reproducibility than whole vertebral centrum staining (cobalt nitrate and ammonium sulfide technique). Marginal analysis showed a clear seasonal pattern, confirming the hypothesis of a single annulus deposition per year. A total of 291 vertebral centra were sampled and used for age estimation purposes. The oldest female was estimated to be 12 years old [total length (TL) = 89 cm], while the oldest male was aged 8 years (TL = 79.9 cm). Females were also found to be characterized by a slightly wider longevity range (ωL = 11.5, ωU = 16.8 years) than males (ωL = 7.8, ωU = 11.2 years). The von Bertalanffy growth curve fit the age and length data more accurately than the Gompertz and logistic models. Eighty-three thornback rays were tagged and released, of which two were recaptured. In both recaptured specimens, oxytetracycline marks were clearly visible. The band deposition after oxytetracycline injection and growth during the freedom period (about 1 year) were consistent with the age estimation method and criteria used and with the obtained growth results. Thus, the analysis of the vertebral centra extracted from the two recaptured specimens confirmed the hypothesis of the deposition of a single annulus per year and in general the age estimation criteria used in this study.

Glycan expression as a tool for a deeper understanding of a reproductive gland in a skate of economic importance.

Atlantoraja platana is an oviparous skate endemic to the south-west Atlantic Ocean, and is one of the skate species most exploited by local industrial bottom trawl fisheries. Oviparous elasmobranchs encapsulate their eggs in complex egg cases produced by the oviductal gland (OG). This organ is exclusively present in these fishes and comprises four distinct zones: club, baffle, papillary and terminal. The relative size and structural complexity of these zones correlate with mode of reproduction. Glycans are known to play major roles in reproduction so their distribution in each zone of the OG could explain the functional multiplicity of the gland in skates, but this topic has not been previously investigated. In this study, morphological, histochemical and lectin-histochemical analysis revealed various novel aspects of A. platana's OG. The club, papillary and terminal zones positively stained for periodic acid Schiff's reagent (PAS) and Alcian Blue (AB), indicating the presence of neutral and acid mucopolysaccharides. However, the buffle zone was negative for PAS and AB stains, but was positive for all the lectins used. Each zone of the OG had a characteristic pattern of glycan expression. Finally, we confirmed the presence of sperm but not sperm storage. This is the first lectin-histochemical study of the OG in chondrichtyan fish and it has proven to be an important tool to understand some of the mechanisms of fertility and reproductive success in economic important species such as A. platana.

Bathyraja (Arctoraja) sexoculata sp. nov., a new softnose skate (Rajiformes: Arhynchobatidae) from Simushir Island, Kuril Islands (western North Pacific), with special reference to geographic variations in Bathyraja (Arctoraja) smirnovi.

A new species of softnose skate (Arhynchobatidae), Bathyraja sexoculata Misawa, Orlov, Orlova, Gordeev and Ishihara is described on the basis of five specimens collected from off the east coast of Simushir Island, Kuril Islands, located in the western North Pacific. The specimens conformed to the genus Bathyraja by having the anteriormost pectoral-fin skeleton almost reaching the snout tip, and a slender unsegmented rostral cartilage. Within Bathyraja, the new species belongs to the subgenus Arctoraja (currently with four valid species) due to the relatively short tail (79-86% of disc width), high count of predorsal caudal vertebrae (more than 86), and large strong nuchal and scapular thorns. It is most similar to Bathyraja (Arctoraja) smirnovi, distributed in the Seas of Japan and Okhotsk, in having tail thorns not extending to the nuchal area, median thorns discontinuous from the nape to the tail, and no mid-dorsal thorns. However, B. sexoculata can be distinguished from B. smirnovi by the following characters: three pairs of white blotches on the dorsal disc surface (vs. blotches absent, or a pair of white or dark blotches in B. smirnovi), dark blotch around cloaca, dark bands along mid ventral line of tail (vs. dark blotch and band usually absent ventral disc surface in B. smirnovi), 86-93 predorsal caudal vertebrae (vs. 80-87 in B. smirnovi), and a unique mitochondrial DNA cytochrome c oxidase subunit I sequence. Proportional measurements, including disc width, disc length, head length, preoral length, prenarial length, internarial distance, eye diameter, and tail length, also differ between the two species. For the referential purpose, geographical variations of B. smirnovi distributed in the Seas of Japan and Okhotsk are analyzed and clarified based on morphological and genetic data. Significant morphological and genetic differences were found between local populations in the Seas of Japan and Okhotsk.

Genetic methods reveal high diversity and no evidence of stock structure among cuckoo rays (Leucoraja naevus) in the northern part of Northeast Atlantic

The high level of exploitation and slow population growth rates of many elasmobranch species are causing worldwide concern about their status. Genetic methods offer the potential to gather information on the stock structure and conservation status of species with implications in the delineation of appropriate management units. However, despite several calls from the scientific community, they are still largely ignored in fisheries management. Here, we use genetic approaches to assess the population structure of the cuckoo ray (Leucoraja naevus), a species of skate that was recently designated as vulnerable in Irish waters by the International Union for Conservation of Nature. By genotyping individuals (n = 188) originating from a vast geographic area with nine microsatellite markers, we do not find evidence of population structuring but cuckoo rays in Irish waters seem to belong to a single genetically diverse panmictic population despite the restricted movements observed in closely related species. Cuckoo rays should therefore continue to be monitored as a single stock in this area.

Trophic niche partitioning of five skate species of genus Bathyraja in northern and central Patagonia, Argentina.

Overexploitation of marine communities can lead to modifications in the structure of the food web and can force organisms like elasmobranchs to change their feeding habits. To evaluate the impact that fisheries have on food webs and on the interactions between species, it is necessary to describe and quantify the diet of the species involved and follow it through time. This study compares the diet of five skate species using the data obtained from the by-catch of the Argentine hake (Merluccius hubbsi) fishery in north and central Patagonia, Argentina. Diet composition was assessed by analysing the digestive tract contents and trophic overlapping between species of the genus Bathyraja: Bathyraja albomaculata, Bathyraja brachyurops, Bathyraja macloviana, Bathyraja magellanica and Bathyraja multispinis. A total of 184 stomachs were analysed. The diets of B. albomaculata and B. macloviana mainly comprised annelids, whereas that of B. brachyurops primarily comprised fish, including hake heads discarded by the fishery. The diets of B. magellanica and B. multispinis were largely based on crustaceans. Despite the morphological similarities and their shared preference for benthic habitats, no complete diet overlaps were found between the different species. These results suggest that these skate species have undergone a process of diet specialisation. This is a common feeding strategy that occurs to successfully eliminate competition when resources are limited, which corresponds to the conditions found in an environment being affected by the pressures of overfishing.

Comparative embryonic development patterns in three deep-water skates from the southwest Atlantic

Embryonic development is a vulnerable and key period during the life cycle of an oviparous elasmobranch. Captivity studies are difficult to carry out on non-coastal species; hence, embryonic development can be analysed from samples collected from the seabed. Here, embryonic development of three shelf and deep-water skates from the southwest Atlantic Ocean, Bathyraja brachyurops, B. macloviana and Amblyraja doellojuradoi was studied. Egg cases containing embryos in different stages were collected from 84 to 1006 m depth in the northern part of the Argentinean continental shelf and continental slope (36°S-41°S), including the Mar del Plata Canyon (38° S). Common development patterns were observed among the three species and also with other skate species previously studied. Anatomical structures and embryo features were similar among species in initial, early and middle stages. Advanced and pre-hatching embryos showed species-specific differences regarding pigmentation and spinulation. This study sheds light on the identification of neonates of shelf and deep-water species, which could be useful to recognize specific nursery areas in the deep ocean. In addition, this embryological comparative study expands the existing database on the different chondrichthyan lineages, thus making a step forward towards understanding of their phylogenetic relationships.

Distribution and thermal niche of the common skate species complex in the north-east Atlantic

Temperature is one of the most significant variables affecting the geographic distribution and physiology of elasmobranchs. Differing thermal gradients across a species’ range can lead to adaptive divergence and differing developmental times, an important consideration for recruitment rates of exploited species. The Critically Endangered common skate (formerlyDipturus batis) has been divided into 2 species, the flapper skateD.intermediusand blue skateD.batis, both of which have undergone dramatic population declines. Here we examine the environmental thermal and geographic distribution of these species, using observations from scientific trawling surveys and recreational angling around the British Isles. As similar-sized specimens of the 2 species can be confused, we validated species identity using molecular genetic techniques. Both species had more extensive geographic ranges than previously reported and different spatial patterns of abundance. The distribution of the blue skate appears to reflect its partiality to thermally less variable and warmer waters, while flapper skate were found in more variable and notably colder areas. The thermal range and current geographic distribution of these species indicate that future projected climate change could have a differential impact on distribution of flapper and blue skate in the north-east Atlantic.