Mother–embryo isotope fractionation in the pygmy devilray Mobula kuhlii cf. eregoodootenkee

We determined stable-isotope ratios for replicate muscle tissues in 13 gravid Mobula kuhlii cf. eregoodootenkee (110.4–120.4 cm disc width; W_D) and their embryos (7.0–42.3 cm W_D) and also yolks and histrotroph, to assess the potential implications for juvenile nutrition and habitat use. Irrespective of their development in the uterus, embryos had similar δ¹³C values in their muscle tissue as the mothers and both had greater values than in the histotroph. During gestation, δ¹³C values increased across all sample types. However, while embryo muscle tissue and the histotroph were associated with increasing ¹⁵N levels during embryonic development, this was depleted in the mothers’ muscle tissue and yolk. Although speculative, the observed variation in stable-isotope ratios might imply a dietary shift among gravid females during their early gestation. Irrespective of the underlying mechanisms, the results indicate neonates will have relatively greater δ¹⁵N values than post-partum females, which would probably confound juvenile foraging-ecology estimates.     

Length – length relationships of 16 Caribbean elasmobranchs

Elasmobranchs are commonly landed without fins and heads; however, the knowledge of total length is necessary for fishery management. We provide useful equations to extrapolate partial from elasmobranch carcasses lengths, to total lengths. Length‐Length Relationships (LLR) are described for 16 commercial elasmobranch species all included in the IUCN Red List: Hexanchus nakamurai, Heptranchias perlo, Squalus cubensis, Squatina dumeril, Scyliorhinus boa, Mustelus minicanis, M. higmani, M. norrisi, Carcharhinus acronotus, C. falciformis, C. limbatus, Galeocerdo cuvier, Rhizoprionodon lalandii, R. porosus, Sphyrna lewini and Pseudobatos percellens. This study provide new LLR equations and rsepresent an improvement in the knowledge of these species.     

Sexual, ontogenetic, temporal and spatial effects on the diet of Urotrygon rogersi (Elasmobranchii:Myliobatiformes)

The food habits and trophic ecology of Urotrygon rogersi were analysed to ascertain sexual, ontogenetic, temporal and spatial effects on the diet. These effects were examined with contingency tables, simple correspondence analysis and MANOVA. The trophic relationships of the species were determined with Levin's niche amplitude index and Pianka's diet overlap index. Urotrygon rogersi is a predator of benthic organisms, especially shrimp and polychaetes. Analysis of sex and time showed no significant differences, but differences among class sizes were found. A strong trend towards diet specialization and a strong overlap between sexes and between class sizes were found. Finally, fishing activity on shrimps in the study area could force U. rogersi to change its trophic spectrum over time as an adaptation to the considerable reduction in its principal prey.     

Updated taxonomic list and conservation status of chondrichthyans from the exclusive economic zone of Venezuela,with first generic and specific records

Using the last taxonomic review of chondrichthyans of the world, we selected the species distributed in the north-western Atlantic Ocean (NWA) and compared it with the available published literature related to the class Chondrichthyes in the Venezuelan exclusive economic zone. We also revised information from worldwide databases such as: FAO (NWA-31 area), GBIF, iSpecies, IUCN and OBIS, as well as available museum collection databases. The taxonomic validity was checked using the Catalogue of Fishes of the California Academy of Sciences and recent references. The past published Venezuelan lists of chondrichthyans combined included nine orders, 30 families and 108 species. The updated list with 12 orders, 36 families and 122 species increased by three new orders, six families, three shark genera, nine shark species (one replacing another species), one chimaera genus, two chimaera species, three batoid genera and six batoid species (two replacing other species). Four holotype specimens (two sharks and two rays) are deposited in Venezuelan Museums. Most of the species have an IUCN conservation status, including four species catalogued as Critically Endangered, six as Endangered and 18 species as Vulnerable. Deep-sea fisheries, scientific exploration and taxonomic/genetic revisions might add future increments to the Venezuelan chondrichthyan list.     

Age and growth of three endemic threatened guitarfishes Pseudobatos horkelii,P. percellens and Zapteryx brevirostris in the western South Atlantic Ocean

The age and growth of three endemic threatened guitarfish species were analysed using vertebrae of Pseudobatos horkelii, P. percellens and Zapteryx brevirostris. Edge and marginal-increment analyses were used to evaluate the periodicity of the formation of the band-pairs, suggesting deposition of one band-pair per year, from late winter to late spring. The von Bertalanffy growth model was used to describe the growth of these species with the following parameters, for pooled sexes: P. horkelii L_∞ = 126.93, k = 0.19 and t₀ = −1.51; P. percellens L_∞ = 109.31, k = 0.16 and t₀ = −1.78; Z. brevirostris L_∞ = 60.37, k = 0.24 and t₀ = −1.42. Our results are essential to understanding the resilience and vulnerability of these species to harvest, which can contribute to management and conservation actions of these species.     

Best practices for non-lethal blood sampling of fish via the caudal vasculature

Mature male and female little skates Leucoraja erinacea were injected with oxytetracycline and maintained in captivity for 13 months to assess centrum growth and the frequency of band-pair deposition. Sixty per cent of the individuals analysed did not deposit a full band pair over the 13 month period. Thus, a majority of captive skates did not exhibit annual band-pair deposition. Previous research confirms annual band-pair deposition in all juvenile and most adult L. erinacea, therefore sexual maturation may lead to decreased frequency of band-pair formation. Age underestimation of larger, older elasmobranchs is being identified in an increasing number of elasmobranch species including L. erinacea as demonstrated in this study. The effect of age underestimation from band-pair counts on studies that use age-based characteristics needs to be addressed.     

Lesser devil rays Mobula cf. hypostoma from Venezuela are almost twice their previously reported maximum size and may be a new sub‐species

Three rays opportunistically obtained near Margarita Island, Venezuela, were identified as lesser devil rays Mobula cf. hypostoma, but their disc widths were between 207 and 230 cm, which is almost double the reported maximum disc width of 120 cm for this species. These morphometric data suggest that lesser devil rays are either larger than previously recognized or that these specimens belong to an unknown sub‐species of Mobula in the Caribbean Sea. Better data are needed to describe the distribution, phenotypic variation and population structure of this poorly known species.     

Habitat use and nursery evaluation for the longnose stingray Hypanus guttatus (Bloch & Schneider, 1801) using vertebral microchemistry

Here, we use vertebral microchemistry to investigate the habitat use patterns of the longnose stingray Hypanus guttatus in the northern and northeastern regions of Brazil, and to evaluate the existence of potential nurseries of the species. Samples were collected in Maranhão, Rio Grande do Norte and Pernambuco states between 2008 and 2019. Trace element concentrations of Ba:Ca, Mg:Ca, Mn:Ca and Sr:Ca in vertebrae were determined through laser ablation inductively coupled plasma mass spectrometry. We used one-step permutational ANOVAs to evaluate multi and single-element differences in element:Ca ratios among life stages and sexes across and within sites. Element signatures differed among all sites for both single and multi-element settings. However, there were few differences among life stages within sites, except for Mn:Ca between life stages for Maranhão state, and between sexes for Sr:Ca for both Pernambuco and Rio Grande do Norte, and Ba:Ca for Rio Grande do Norte. Furthermore, all multi and single-element analyses across locations were significant and highlight the strong differences by sites. We also performed a nonmetric multidimensional scaling analysis, which demonstrated the strong differences for samples from Rio Grande do Norte and the other two sites. By analysing the lifetime transects of each sample, we observed that there was little variation in all element:Ca ratios within sites. Therefore, we argue that the longnose stingray likely does not display extensive habitat use shifts and fulfils much of its life cycle within each area. Finally, we obtained no evidence of nurseries for any site and suggest this is likely a pattern across the species distribution. Future studies investigating habitat use with samples from larger specimens (disc width > 90 cm, >15 years) are needed to better understand the species habitat use patterns across all size ranges.     

Comparative anatomy of the superfamily Myliobatoidea (Chondrichthyes) with some comments on phylogeny

The anatomy of species belonging to the superfamily Myliobatoidea was examined with the aim of better determining their phylogenetic relationships. A wide variation among genera was observed in skeletal anatomy, despite the fact that they all share a common morphological pattern. However, variation among species of the same genus was low, excepting Mobula. Dorsal musculature showed a substantial consistency, except for the epiaxialis muscle, which was larger in rhinopterids and mobulids. Variation in the ventral muscles was low among species of the same genus, but considerable among different genera. Mobulids have a reduction in ventral muscles, while rhinopterids and myliobatoids show an increase in muscular mass. A consensus tree shows a basal split into two groups. The first includes the family Gymnuridae with the genera Gymnura and Aetoplatea; this group is supported by seven synapomorphies, including: 27(1) ceratobranchialis fused proximally, 36(1) anterior lateral processes present in the synarcual, 52(0) quadratomandibularis internal muscle present. The second group is composed of the family Myliobatidae (Myliobatis, Aetomylaeus, Aetobatus, Rhinoptera, Mobula, and Manta), this group is supported by 11 synapomorphies, including: 5(1) first postorbital process fused with the second, 21(1) fused mandibular symphysis, 24(1) first hypobranchial cartilage absent, 48(2) epiaxialis muscle inserted in the cranial orbital region, 73(1) pectoral fins joined behind the orbital region. This study concluded that myliobatoids (Myliobatis, Aetomylaeus, and Aetobatus) integrate a monophyletic group which, unlike other phylogenies previously obtained, is the sister group of rhinopterids (Rhinoptera). Mobulids (Mobula and Manta) are the sister group of myliobatoids-rhinopterids.     

A review of capture and post‐release mortality of elasmobranchs

There is a need to better understand the survivorship of discarded fishes, both for commercial stocks and species of conservation concern. Within European waters, the landing obligations that are currently being phased in as part of the European Union's reformed common fisheries policy means that an increasing number of fish stocks, with certain exceptions, should not be discarded unless it can be demonstrated that there is a high probability of survival. This study reviews the various approaches that have been used to examine the discard survival of elasmobranchs, both in terms of at‐vessel mortality (AVM) and post‐release mortality (PRM), with relevant findings summarized for both the main types of fishing gear used and by taxonomic group. Discard survival varies with a range of biological attributes (species, size, sex and mode of gill ventilation) as well as the range of factors associated with capture (e.g. gear type, soak time, catch mass and composition, handling practices and the degree of exposure to air and any associated change in ambient temperature). In general, demersal species with buccal‐pump ventilation have a higher survival than obligate ram ventilators. Several studies have indicated that females may have a higher survival than males. Certain taxa (including hammerhead sharks Sphyrna spp. and thresher sharks Alopias spp.) may be particularly prone to higher rates of mortality when caught.