Skate Dipturus Research Articles

A ray of hope? The re-appearance of Irish Sea skate decades after local extinction.

A 2019 photograph of a blue skate Dipturus batis caught in Welsh waters is the first species-specific evidence of the "common skate" complex occurring in its former range in the main body of the Irish Sea over four decades after it was considered to have been extirpated there. This possible precursor to recolonisation of former range adds to growing evidence for the recovery of skate species in the North Atlantic and highlights the valuable role of anglers and social media as a complement to important, yet costly, scientific surveys when monitoring rare fishes.

Mitochondrial haplotypes reveal low diversity and restricted connectivity of the critically endangered batoid population in a Marine Protected Area

Stability and long-term persistence of a species rely heavily on its genetic diversity, which is closely allied to its capacity for adaptation. In threatened species, population connectivity can play a major role in maintaining that diversity, and genetic assessments of their populations can be crucial for the design of effective spatial conservation management. Not only is it worth evaluating the amount of diversity in a candidate population for protection, but the magnitude of outgoing gene flow can provide insight into its potential to replenish others via emigrants. The critically endangered flapper skate Dipturus intermedius receives protection in the Loch Sunart to the Sound of Jura Marine Protected Area (MPA) in Scotland. However, there is insufficient knowledge of genetic diversity and connectivity across its range. Recent tagging studies in the MPA suggest the presence of animals with high levels of site fidelity and residency, as well as transient individuals, raising concerns of limited connectivity to populations beyond the MPA. In this study, a newly developed mitochondrial haplotype marker allowed use of DNA sourced from fin clips, mucus and egg cases to investigate population structure and mitochondrial variability across several sites around the British Isles, including the MPA. Unfortunately, results characterized the MPA as having particularly low haplotype diversity and significant population differentiation from other sample sites. More than a quarter of its individuals carry a haplotype rarely observed elsewhere, leaving outgoing gene flow questionable. The MPA appears unlikely to sustain the species’ existing mtDNA genetic diversity or act as an effective source population.

Preliminary insight into the reproductive traits of the flapper skate Dipturus intermedius using in-field ultrasonography and circulating hormone concentrations

Due to global population declines, there is a pressing need for data on the life history traits of many elasmobranch species to support the development of species-specific management plans. A lack of information on the reproductive cycle of the Critically Endangered flapper skate Dipturus intermedius was recently identified as a hindrance to its conservation. To address this data gap, we combined non-lethal ultrasound and hormone analysis to investigate the size at maturity and reproductive cycle of the flapper skate in the Loch Sunart to the Sound of Jura Marine Protected Area off the west coast of Scotland. In-field ultrasound imagery revealed encapsulated eggs in utero and was used to determine the presence and size of ovarian follicles. Combining these images with levels of plasma testosterone, progesterone and oestradiol provided valuable insights into the timing of the reproductive cycle and maturity state of the flapper skate. This preliminary study suggests that male skate start to mature at 165 cm and females at 203 cm total length. Oestradiol appears to be the primary hormone controlling the female reproductive cycle and, along with ultrasound images, indicates that females lay pairs of eggs throughout a winter egg-laying season. Our study further highlights how non-lethal methods can be used to investigate the life history of oviparous elasmobranchs in the field. This information will support the identification of important life history groups and their associated habitats and contribute to the development of management strategies for these species.

First report of an egg nursery for the Critically Endangered flapper skate Dipturus intermedius (Rajiformes: Rajidae)

Abstract This study provides the first report of an egg nursery for the Critically Endangered flapper skate Dipturus intermedius and a description of the habitat, thus contributing towards our understanding of essential habitats for the species. In total, 1,395 flapper skate egg cases were recorded (accounting for overlapping surveys) in two egg case collection dives (n = 67 egg cases), one photogrammetry dive (n = 10 egg cases), 509 drop‐down video (DDV) camera drifts (n = 510 egg cases) and 18 remote operated vehicle (ROV) flights (n = 1,031 egg cases), carried out in the Inner Sound on the west coast of Scotland from 2018 to 2021. All of the egg cases were found on a shallow bedrock plateau between the Isle of Scalpay and a deep (>100 m) water channel between the Isle of Longay and the Crowlin Islands. Egg cases were observed on a cobble/boulder reef between 25 and 58 m depth, with a modelled annual temperature range of 9–12 °C, modelled current speeds up to 0.2 m−1, a rugosity index of 1.7 and low levels of sedimentation. Flapper skate egg cases are large and the incubation period is protracted (18 months), making them potentially vulnerable to anthropogenic disturbance. A description of the habitat where egg cases were observed in this study will help inform the search for egg nurseries for this Critically Endangered species elsewhere. Targeted DDV, ROV and scuba diving surveys will support this search in areas where suitable bathymetric and hydrodynamic conditions are identified. Safeguarding egg nurseries is essential for successful conservation. Protection should involve the designation of egg nurseries as Marine Protected Areas where activities that are likely to damage or alter seabed habitats are managed.

Morphological study of members of the genus Echeneibothrium (Cestoda: Rhinebothriidea: Echeneibothriidae) from rajiform skates of the Argentine Sea and analysis of the phylogenetic relationships within the family Echeneibothriidae

A new cestode species of the genus Echeneibothrium van Beneden, 1850 was collected from a rajiform skate in the Argentine Sea: Echeneibothrium cristinae sp. nov. parasitizing Bathyraja cousseauae Díaz de Astarloa & Mabragaña, 2004, from coastal waters off Tierra del Fuego Province. This species differs from congeners by a particular combination of the following features: total length, number of proglottids, number of loculi and presence of a medial longitudinal septum on the distal bothridial surface, length of bothridial stalks, size of the cirrus sac, position of the ovarian isthmus and course of the vagina. In addition, two known Echeneibothrium species, originally reported in Dipturus chilensis (Guichenot, 1848) from the Chilean Sea, were collected from a new host in the Argentine Sea: Echeneibothrium multiloculatum Carvajal & Dailey, 1975 and Echeneibothrium williamsi Carvajal & Dailey, 1975, parasitizing the rajiform skate Dipturus brevicaudatus (Marini, 1933) from coastal waters off Buenos Aires Province (E. multiloculatum) and Patagonia (both species). Emendations were made to the descriptions and redescriptions of these species, including the modification of measurement ranges for several characters and the addition of important morphological information omitted in previous studies. Finally, a phylogenetic analysis based on morphological characters was performed to determine the inter-relationships within the family Echeneibothriidae, this study being the first to consider representatives of all known genera belonging to that family.

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.

Morphological Traits and Capture Depth of the Norwegian Skate (Dipturus nidarosiensis (Storm, 1881)) from Two Mediterranean Populations

The Norwegian skate Dipturus nidarosiensis (Storm, 1881) has only recently been recorded in the western-central Mediterranean Sea. It was hypothesized a more ancient presence of the species, which has not been detected due to a misidentification with other species of the same genus. This situation could lead to underestimate the risk of a dramatic decline of the spawning stock. In the IUCN Red List, the species is listed as near threatened and considered rare in both the northeast Atlantic and the Mediterranean areas. In the Mediterranean Sea, Norwegian skates were repeatedly caught mostly in two areas between 2005 and 2020: Sardinia Seas and Adriatic-Ionian Seas. In total, 58 specimens were caught, and 28 morphological length measurements were taken on all specimens. The Canonical Discriminant Analysis proved the presence of significant differences only for assemblages made on the basis of the specimen’s area of capture, but not on the basis of sex or ontogenetic development. This analysis could be the first step to highlight the differences between the populations of Norwegian skate in the Mediterranean basin. Moreover, a preliminary analysis of depth of capture was performed as a first step to study this species vertical distribution.

Resolving the spatial distributions of Dipturus intermedius and Dipturus batis\u2014the two taxa formerly known as the \u2018common skate\u2019

Batoid fishes are among the most endangered marine vertebrates, yet conservation efforts have been confounded by incomplete taxonomy. Evidence suggest that the critically endangered ‘common skate’ actually represents two species: the flapper skate (Dipturus intermedius) and the blue skate (Dipturus batis). However, knowledge of the geographic range of these two nominal species is limited. Here, DNA sequencing is used to distinguish these species, allowing their spatial distributions to be clarified. These records were also used as the basis for species distribution modelling, providing the first broad scale models for each species across the Northeast Atlantic. Samples were obtained from Iceland, the UK (specifically Shetland), the North Sea and the Azores. Results suggest that D. batis was commonly distributed in the Western Approaches and Celtic Sea, extending out to Rockall and Iceland. D. intermedius generally appears to be less abundant, but was most frequent around northern Scotland and Ireland, including the northern North Sea, and was also present in Portugal. Two individuals were also identified from seamounts in remote areas of the Atlantic around the Azores, the furthest south and west the species has been found. This supports reports that the flapper skate historically had a much wider distribution (which was also highlighted in the distribution model), emphasising the large scale over which fisheries may have led to extirpations. Furthermore, these Azorean samples shared a unique control region haplotype, highlighting the importance of seamounts in preserving genetic diversity.

First confirmed complete incubation of a flapper skate (Dipturus intermedius) egg in captivity.

An egg of the critically endangered flapper skate Dipturus intermedius was successfully incubated to hatching in captivity in what is believed to be a first for the species. Water conditions (temperature, salinity, flow rate) were recorded, with mean water temperatures ranging from a monthly mean of 8.3 ± 1.2 to 13.2 ± 0.3°C and salinity from a monthly mean of 30.5 ± 1.2 to 36.6 ± 2.3 ppt. Hatching occurred after 534 days, suggesting that flapper skate eggs take c. 5700 growing degree‐days to incubate to hatching. The egg's prolonged embryonic development raises concerns about flapper skate eggs' vulnerability to anthropogenic disturbance.

Additional records of Norwegian skate Dipturus nidarosiensis (Storm, 1881) (Pisces: Rajidae) in the Adriatic Sea

Additional records of the Norwegian skate Dipturus nidarosiensis (Storm, 1881)from the Adri-atic Sea are reported. Two specimens were collected in the southern Adriatic Pit in 2008 and 2010, respectively. The records reported herein improve our knowledge on the distribution of a rarely encountered, deep water species D. nidarosiensis, whose presence has only recently been confirmed in the Mediterranean Sea.

Trophic ecology and ontogenetic diet shift of the blue skate (Dipturus cf. flossada)

Knowledge of skate ecology must be improved to ensure their effective protection. This study represents the first description of diet composition for one of the largest European rajid, the blue skate Dipturus cf. flossada. A total of 346 specimens collected in the Celtic Sea from 2012 to 2015 were analysed for their gut content, with respect to individual total length, maturity stage and sex. Overall, the blue skate diet mainly consisted of shrimps and prawns, crabs and teleost fishes. Nonmetric multidimensional scaling and ANOSIM analyses revealed major ontogenetic shifts in the feeding strategy that were related to size and maturity, but not to sex. Shrimps and prawns, mostly composed of Crangon allmanni, dominated the diet of small and immature individuals, while mod-size skate primarily preyed on crabs. The prevalence of crustaceans decreased with size and maturity, and was gradually replaced by teleost fishes in large mature individuals. A concomitant increase of the trophic level with size revealed that large blue skate become a top predator within the ecosystem. These results highlight the need to include ontogenetic changes in the diet description. As individuals grow and mature, blue skates can play a fundamental role in the structure of the Celtic Sea food web.

The complete mitochondrial genome of the blue skate Dipturus batis

The complete mitochondrial genome of the blue skate Dipturus batis is described from shotgun sequencing on an Illumina next-generation sequencing platform. We report a 16,911 bp long sequence similar in size to other members of the genus, containing 13 protein-coding regions, 22 tRNA genes, 2 rRNA genes, and 2 non-coding areas. Phylogenetic analysis was performed using the complete mitochondrial genomes of 17 related species, placing D. batis within the Rajini tribe of the Rajidae family, consistent with current taxonomy. The new resource adds to a growing database of rajid mitogenomes which will help resolve phylogenetic relationships within the family.

On the presence of Dipturus nidarosiensis (Storm, 1881) in the Central Mediterranean area.

The Norwegian skate Dipturus nidarosiensis (Storm, 1881) has only recently been recorded in the western Mediterranean Sea along the coast of southern Sardinia, off Algeria and the Alboran Sea. The present study confirmed the presence of the species in the Central Mediterranean Sea by identifying morphometric, morphological features and molecular markers. Biological sampling was conducted from 2010 to 2016 on eight specimens collected through commercial landings, offshore observer programmes and scientific surveys in Adriatic and Ionian waters at depths between 320 and 720 m. The total lengths of the specimens (juveniles and adults) ranged from 268 to 1,422 mm, and their body weights ranged from 44.5 to 12,540.0 g. They showed morphometric features that corresponded to those of Norwegian skates in the Northeast Atlantic and the Western Mediterranean. In previous analyses, molecular data were obtained by mitochondrial COI sequences. The haplotype network showed the occurrence of a common haplotype (Hap_1) shared by the individuals from areas in the North Atlantic, Sardinian, Algerian and Spanish Mediterranean Sea areas but not South Africa. The occurrence of individuals in different stages of life (i.e., juveniles, sub-adults and adults) and sexual development (immature and mature) suggested the presence of a species with a permanent reproductive allocation in the deep waters of the Mediterranean, which was exposed to a low level of fishing exploitation. Indeed, the deep depth distribution of the species could be the reason for the absence of information about this species in onshore or offshore fishery data collection programmes and scientific surveys.

Chondrichthyes Chitinase: Molecular Cloning, Distribution, and Phylogenetic Analysis

We have previously reported the presence of three types of chitinase (acidic fish chitinase-1: AFCase-1, acidic fish chitinase-2: AFCase-2, fish chitinase-3: FCase-3) in Actinopterygii. In the present research, we report the identification of the novel chitinase genes HjChi (ORF: 1380 bp) and DkChi (ORF: 1440 bp) from the stomach of Chondrichthyes, Japanese bullhead shark (Heterodontus japonicas) and Kwangtung skate (Dipturus kwangtungensis), respectively. Organ-specific expression analysis identified the stomach-specific expression of HjChi, whereas DkChi was expressed widely in all organs. Chitinase activity was measured using pNP-(GlcNAc)n (n = 2, 3) as a substrate and β-N-acetylhexosaminidase (Hex) activity was measured using pNPGlcNAc. Relatively high values of chitinase activity were observed in the stomach, spleen, and gonads of the Japanese bullhead shark, H. japonicas , compared with that observed in the stomach of the Kwangtung skate D. kwangtungensis . However, Hex activity was detected throughout the body of both species. The optimal pH of chitinase in both the Japanese bullhead shark, H. japonicas, and the Kwangtung skate, D. kwangtungensis, were 3.5 - 5.5 and 3.5 - 4.0, respectively, and 4.0 for Hex in both species. Phylogenetic analysis revealed that Chondrichthyes chitinase forms a unique group (Chondrichthyes chitinase). These results suggested that the possibility of the formation of chitinase groups for each class in the phylogenetic analysis based on the observation of class-specific chitinase.

Age, growth, and age at sexual maturity of the commercially landed skate species,Dipturus chinensis(Basilewsky, 1855), in the northern East China Sea

Age, growth, and age at sexual maturity of the polkadot skate Dipturus chinensis, in the northern East China Sea were determined for a total of 614 specimens collected from April 2009 to December 2014. Vertebral centrum analysis was used to calculate the age of the skates. Annual band deposition was determined by marginal increment analysis. The von Bertalanffy growth model was fitted to the observed length‐at‐age data for each sex (males, L∞ = 76.8, k = 0.109, t₀ = −1.28; females, L∞ = 83.1, k = 0.103, t₀ = −1.20). Growth patterns of females and males were similar until the age of 6; thereafter, females grew larger than males. Maximum age recorded was 13 years for males and 15 years for females. Age at 50% sexual maturity was 8.22 years for males and 9.39 years for females. These results indicate that D. chinensis is slow growing, relatively long‐lived, and late maturing, and therefore vulnerable to exploitation.

Evidence for the decline and possible extinction of a marine parasite species caused by intensive fishing

Abstract The first good evidence of the role of intensive commercial fishing in the decline and possible extinction of a marine parasite is presented. The larva of Stichocotyle nephropis Cunningham, 1887 (Aspidogastrea: Digenea) was described in 1887 from the outside of the intestine of the decapod crustacean Nephrops norvegicus (L., 1758) in the North Sea. Similar larvae were found outside the intestine of the American lobster Homarus americanus (Milne-Edwards, 1837) caught off New England in 1895. The adult form was first found in 1898 in the bile ducts of the thornback ray Raja clavata (L., 1758) from the North Sea, then in the barndoor skate Dipturus laevis (Mitchill, 1818) caught off Nantucket. There have been few records of its occurrence since then, with the last records from the northeast and northwest Atlantic published in 1972 and 1986 respectively, despite the recent attempts reported herein to find larvae and adult parasites. Both the elasmobranch final hosts are listed as “near threatened” or “ endangered” and the latest evidence on their population sizes suggests that they may have been fished to below the thresholds required for successful transmission of the parasite.

New morphological and molecular evidence confirm the presence of the Norwegian skate Dipturus nidarosiensis (Storm, 1881) in the Mediterranean Sea and extend its distribution to the western basin

The present study confirms the presence of the Norwegian skate Dipturus nidarosiensis (Storm, 1881) in the Mediterranean Sea, by means of morphological traits and molecular markers providing the first record of this species in the Alboran Sea. Cannas et al. (2010) reported D. nidarosiensis for the first time in the Mediterranean from specimens captured in the central western basin, but Ebert & Stehmann (2013) and Stehmann et al. (2015) considered these records "likely refer to the smaller morphotype, Dipturus sp.", a species not yet described. Eight specimens of the Dipturus genus (Rajiformes: Rajidae) were caught off the Alboran Island (western Mediterranean) in 2012, 2013 and 2016, between 620 and 819 m depth. These specimens showed morphometric diagnostic features corresponding to those of Norwegian skates from the Northeast Atlantic and the central western Mediterranean Sea. Moreover, the Alboran individuals were genetically compared to Northeast Atlantic specimens available in GenBank by means of two mitochondrial DNA fragments: cytochrome c oxidase subunit I (COI) and cytochrome b (Cytb). Analyses showed that the Northeast Atlantic Norwegian skate specimens and the Alboran Sea ones were genetically similar and shared haplotypes, corroborating the identification of the Alboran individuals as Dipturus nidarosiensis. However, they were different from other Dipturus species distributed throughout the Northeast Atlantic and the Mediterranean Sea. Our results confirm the occurrence of this deep-sea large skate species in the Mediterranean, although the IUCN Red List of Threatened Species does not consider it possible (Stehmann et al., 2015).

First evidence of persistent organic contaminants as potential anthropogenic stressors in the Barndoor Skate Dipturus laevis

Although exploited populations of elasmobranchs may be able to recover from fishing pressure, there is little information regarding the Barndoor Skate's ability to cope with other anthropogenic stressors such as organic contaminants (OCs). Legacy OCs were measured in liver, muscle and ova from fourteen Barndoor Skates with mature skates having significantly greater mean concentrations of OCs than immature skates, demonstrating bioaccumulation with age. Using Toxic Equivalency Factors, skates were found to have levels of PCBs that have been shown to elicit negative physiological responses in other fishes and these results highlight the need for future studies to investigate the potential impacts that bioaccumulated organic contaminants have on the recovery and conservation of this vulnerable species.

Life-history traits of the long-nosed skate Dipturus oxyrinchus.

This work investigates life-history traits of the long-nosed skate Dipturus oxyrinchus, which is a common by-catch in Sardinian waters. The reproductive variables were analysed from 979 specimens sampled during scientific and commercial hauls. Females (10·4-117·5 cm total length, LT ) attained larger sizes than males (14·5-99·5 cm LT ). To evaluate age and growth, a sub-sample of 130 individuals (76 females and 54 males) were used. The age was estimated by annuli counts of sectioned vertebral centra. Four models were used for the length-at-age data: the von Bertalanffy, the exponential, the Gompertz and the logistic functions. According to the Akaike's information criterion, the Gompertz model seemed to provide the best fitting curve (L∞ mean ± s.e.: 127·55 ± 4·90 cm, k: 0·14 ± 0·09, IP: 3·97 ± 0·90 years). The oldest female and male were aged 17 (115·5 cm LT ) and 15 years (96·0 cm LT ), respectively. Lengths at maturity were 103·5 cm for females and 91·0 cm for males, corresponding to 90% of the maximum observed length in both sexes. The monthly distribution of maturity stages highlighted an extended reproductive cycle, with spawning females and active males being present almost throughout the year, as confirmed by the gonado-somatic index. Ovarian fecundity reached a maximum of 26 yolked follicles with a mean ± s.e. size of 19·7 ± 6·5 mm.

Seamount egg-laying grounds of the deep-water skate Bathyraja richardsoni.

Highly localized concentrations of elasmobranch egg capsules of the deep-water skate Bathyraja richardsoni were discovered during the first remotely operated vehicle (ROV) survey of the Hebrides Terrace Seamount in the Rockall Trough, north-east Atlantic Ocean. Conductivity-temperature-depth profiling indicated that the eggs were bathed in a specific environmental niche of well-oxygenated waters between 4·20 and 4·55° C, and salinity 34·95-35·06, on a coarse to fine-grained sandy seabed on the seamount's eastern flank, whereas a second type of egg capsule (possibly belonging to the skate Dipturus sp.) was recorded exclusively amongst the reef-building stony coral Solenosmilia variabilis. The depths of both egg-laying habitats (1489-1580 m) provide a de facto refuge from fisheries mortality for younger life stages of these skates.