Search Results (167)

Nosy Be in the northwestern Madagascar hosts one of the largest known seasonal feeding aggregations of whale sharks. However, the environmental drivers influencing whale shark surface feeding behaviour in this area remain poorly understood. This study investigates the relationship between environmental variability and surface feeding strategies of immature whale sharks at Nosy Be. Boat-based surveys were conducted in November 2018, 2019, 2022, and 2023, resulting in the photo-identification of 88 individuals and the recording of 85 surface feeding events. The influence of environmental factors on feeding behaviour was assessed using multicollinearity among the environmental covariates and three-level step approach: permanova, multinomial logistic regression, marginal effects, and Cochran’s Q, to evaluate whether environmental conditions discriminate feeding-behaviour categories and to quantify how individual covariates relate to behavioural composition under a multi-step framework. Results showed that there is not a strong enough predictive signal for behaviour based on environmental variables; however, thanks to the marginal effects, it is possible to better assess the probability of a certain type of eating behaviour in the presence of an increase in one of the environmental variables, for example, chlorophyll-a appears to be the most interesting, because its increase is associated with a greater probability of some behaviours instead the others. These preliminary observations provide the first insights to evaluate environmental influences on immature whale shark surface feeding behaviour in Nosy Be, highlighting that it is therefore necessary to deepen and increase data collection to have long and significant series of data, integrated also with data on the preys subject to feeding behaviour and to evaluate which other unobserved aspects, perhaps linked precisely to the consistency and quality of the prey, could allow us to predict feeding behaviour. Improving the understanding of these relationships is essential for predicting whale shark habitat use and for supporting conservation and management strategies in a region increasingly affected by climate variability and anthropogenic pressures. Full article

The Pacific sleeper shark (Somniosus pacificus) is a long-lived, deep-water, sub-polar species that exhibits flexible foraging strategies, likely combining scavenging with active predation on a broad range of prey, yet their role in marine food webs and impact on commercial species remain undetermined. Tracking the location of Pacific sleeper sharks in Alaskan coastal waters is extremely challenging given the predominantly aphotic depths that these sharks occupy, often in spatially constrained and critically under-sampled regions: deep, steep-flanked, convoluted fjords of Prince William Sound (PWS). From the first ever, year-long depth and temperature records recovered from archiving pop-up satellite-linked transmitters (n = 7), we characterized the residence distributions, depth, and thermal habitat for sharks within the PWS fjords and identified seasonal and temporal variation in habitat use. Depths recorded from the seven sharks ranged from 3 to 572 m, and pop-up tag locations suggested a high degree intra-annual residency within western PWS. Ambient water temperatures ranged from 2.65 to 11.1 °C, with little deviation from the median of 5.9 °C. Seasonal patterns emerged within and across individuals relative to the variation in vertical movements, ambient temperatures, and horizontal movements that could reflect resource-oriented strategies. The high degree of residency combined with extensive use of the water column facilitates the use of physically recoverable, high-resolution behavioral and environmental samplers on Pacific sleeper sharks. This adaptive sampling using Pacific sleeper sharks as platforms of opportunity may in turn enable the use of Pacific sleeper sharks as climate and ecosystem sentinels. Full article

Despite their ecological and conservation significance, morphometric relations remain scarce for elasmobranch species in the Mediterranean. This study examined morphometric parameters of the eight elasmobranch species (one shark and seven batoids) presented in the Amvrakikos Gulf that has been designated as a National Park. A total of 1247 specimens were sampled between 2022 and 2025, caught by small-scale fishing vessels using trammel nets, gillnets or bottom longlines and collected through onboard surveys or landing sites monitoring. Linear regressions were applied to describe relations between total length and other body measures (disc length, disc width, fork length), and length measurements and body weight. Results showed strong relations across morphometric traits, with R² values exceeding 0.655 for most relations. Growth patterns varied: four species (Aetomylaeus bovinus, Dasyatis pastinaca, D. tortonesei, Mustelus mustelus) exhibited positive allometry, one species (D. marmorata) displayed negative allometry and Gymnura altavela showed near-isometric growth. Sexual dimorphism was generally absent, although significant differences were found between sex in disc width slopes for D. marmorata, Myliobatis aquila and Torpedo torpedo, and in length–weight relations for M. mustelus. These findings substantially fill regional data gaps, offering new baseline estimates for rare and threatened elasmobranchs. Full article

Commercial smooth-hound sharks of the genus Mustelus are commonly landed and consumed in Mediterranean fisheries, raising concerns about potential human exposure to persistent contaminants. This study investigated the occurrence of organochlorine compounds (OCs), including hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDT) and its metabolites, and polychlorinated biphenyls (PCBs), together with per- and polyfluoroalkyl substances (PFAS), in muscle and liver tissues of Mustelus mustelus and Mustelus punctulatus collected in the waters of the Egadi Archipelago (central Mediterranean Sea). OCs were detected in all analyzed samples, with total PCB concentrations reaching higher values in liver compared to muscle tissues, reflecting tissue-specific accumulation and detoxification processes. PFAS were detected in all analyzed muscle samples (1.10–58.5 ng/g w.w.), with PFOS, PFOA and PFNA generally below current European regulatory thresholds, although isolated exceedances were observed. Stable isotope analysis (δ¹³C and δ¹⁵N) highlighted differences in trophic ecology between the two species and suggested that feeding habitat and trophic position may influence contaminant exposure patterns, particularly in M. punctulatus. The human health risk assessment, conducted as a screening-level evaluation, indicated potential concern associated with PCB concentrations in liver tissue, while risks associated with muscle consumption were generally lower. Overall, the integration of contaminant analysis and stable isotopes provides insights into organismal exposure pathways and supports the use of smooth-hound sharks as sentinels of contaminant presence in Mediterranean coastal ecosystems. Full article

The elasmobranch fauna was studied in the Khor Faridah region of Abu Dhabi, which is a mangrove-dominated inshore habitat historically reported to host a diversity of elasmobranch species. A stereo-baited remote underwater video system (Stereo-BRUVS) survey was conducted from September 2021 to August 2022 to assess the species diversity and relative abundance of elasmobranch fishes. A total of 12 elasmobranch taxa were encountered during the study, consisting of five rays (Myliobatiformes), four sharks (Selachii), two wedgefish and one guitarfish (Rhinopristiformes). The area was dominated by honeycomb-patterned rays in the genus Himantura and the Critically Endangered Arabic whipray Maculabatis arabica. Since Himantura uarnak and H. leoparda could not be reliably distinguished from footage, all sex- and size-based results are reported for a combined Himantura species complex and should be interpreted cautiously. Furthermore, the broad size range of individuals found in the area highlights its importance to all life stages of these taxa. This underlines the need for a conservation strategy to avoid detrimental changes to the elasmobranch fauna due to ongoing coastal development. Full article

Directional asymmetry (DA) is a widespread yet often overlooked feature of animal morphology. Here, we report a consistent right-biassed asymmetry in the crystalline lenses of the thornback ray Raja clavata. Across 71 individuals sampled from the Strait of Sicily, 24% exhibited lens asymmetry, and in all cases, the right lens was larger. This pattern, supported by binomial and distributional tests, represents the first evidence of ocular DA in this species. Body size and age emerged as the primary correlates of asymmetry: the odds of exhibiting DA increased significantly with body weight, whereas the effects of sex and sampling site were not significant. The prevalence of asymmetry thus appears to rise with age and ontogenetic growth. Two non-exclusive mechanisms may account for this pattern. First, the association with positive allometry (b = 3.33) suggests that right-lens enlargement could reflect a functional or developmental lateralisation, potentially conferring a visual or ecological advantage to larger individuals. Alternatively, the same right-lens bias could arise through an age-related pathological process, such as oxidative or osmotic lens swelling preceding cataract formation, consistent with asymmetric physiological wear. These findings reveal a novel case of morphological lateralisation in an elasmobranch and highlight the need for comparative, histological, and functional approaches to disentangle adaptive asymmetry from lateralised senescence in the visual system of R. clavata. Full article

The reproductive biology of the whale shark (Rhincodon typus), the world’s largest fish, remains poorly understood, in large part due to the rarity of observations of neonates and of breeding behaviours. Although several regions in Indonesia, including Saleh Bay (West Nusa Tenggara Province), have been identified as aggregation and sighting sites for juvenile whale sharks (2–7 m total length, TL), smaller individuals from these potential nursery areas have not been previously documented. In August 2024, fishermen operating lift-net fishing vessels (bagans) in eastern Saleh Bay reported five separate sightings of a small whale shark estimated at 1.2–1.5 m TL and approximately four months old. Subsequently, on 6 September 2024, a male neonate measuring approximately 135–145 cm TL, estimated to be around four months old, was incidentally caught inside a bagan lift-net. These observations represent the first records of neonatal whale sharks in Indonesia and among the smallest free-swimming individuals ever documented globally, and suggest that Saleh Bay may serve as a pupping and early nursery area for whale sharks. These findings highlight the ecological significance of Saleh Bay for the early life stages of whale sharks and underscore the importance of collaborative monitoring and citizen science involving bagan fishermen in advancing the research and conservation of this endangered species. Full article

The development of systems for the identification of elasmobranchs, including sharks and rays, is crucial for biodiversity conservation and fisheries management, as they represent one of the most threatened marine taxa. This challenge is constrained by data scarcity and the high morphological similarity among species, which limits the applicability of traditional supervised models trained on specific datasets. In this work, we propose an informed zero-shot learning approach that integrates external expert knowledge into the inference process, leveraging the multimodal CLIP framework. The methodology incorporates three main sources of knowledge: detailed text descriptions provided by specialists, schematic illustrations highlighting distinctive morphological traits, and the taxonomic hierarchy that organizes species at different levels. Based on these resources, we design a pipeline for prompt extraction and validation, taxonomy-aware classification strategies, and enriched embeddings through a prototype-guided attention mechanism. The results show significant improvements in CLIP’s discriminative capacity in a complex problem characterized by high inter-class similarity and the absence of annotated examples, demonstrating the value of integrating domain knowledge into methodology development and providing a framework adaptable to other problems with similar constraints. Full article

Whale sharks seasonally aggregate in Djibouti (East Africa), supporting ecotourism activities which benefit the local community. However, the environmental factors influencing whale shark relative abundance at this site are still not well understood. Environmental drivers of immature whale shark surface sightings have been analyzed across a five-year period (2017, 2020, 2022, 2024 and 2025) in the Gulf of Tadjoura (Djibouti) using a Generalized Additive Model (GAM) and Hurdle model. Across 111 surface sightings and 83 photo-identified whale sharks, both sea surface chlorophyll-a (SSC) concentrations and sea surface temperature (SST) have significantly affected their relative abundance (p < 0.001), while wind strength appeared to have a weaker and more complex effect (p < 0.05). Whale shark surface sightings in the Gulf of Tadjoura increased when SSC and SST exceeded thresholds of 0.5 mg/m⁻³ and 26 °C, respectively. In contrast, the positive effect of wind strength ≥ 7 knots was limited, indicating that wind-driven influences on whale shark surface detections are localized and transient. Since prey abundance and distribution are the main drivers of whale shark seasonal aggregations, understanding the environmental effects on food availability at coastal locations and, consequently, on whale shark surface sightings is crucial. The present study highlights temporal and seasonal trends in whale shark sighting data, contributing to broader initiatives aimed at improving conservation and management strategies for this endangered species. Full article

Catsharks are benthic elasmobranchs that share spatial niches with littoral and demersal bony fishes. The genus Schroederichthys includes five species, two of which, S. chilensis and S. bivius, occur in the waters of Chile. These species are morphologically similar and are often misidentified because of their overlapping external features and color patterns. To improve species discrimination, we analyzed the egg case morphology of both species based on 36 egg cases (12 S. chilensis, 24 S. bivius) collected from gravid females captured as bycatch in artisanal fisheries between Iquique and Puerto Montt (July–December 2021). Nine morphometric variables were measured and standardized using the total egg case length. Although the egg cases were similar in general appearance, multivariate analyses revealed significant interspecific differences, with egg case height and anterior border width emerging as the most diagnostic variables. Linear discriminant analysis achieved a 100% classification accuracy within this dataset. To confirm species identity, 24 tissue samples (12 per species) were sequenced for the mitochondrial cytochrome c oxidase subunit I (COI) gene. The haplotypes corresponded to previously published sequences from Chile (S. chilensis) and Argentina (S. bivius), with reciprocal monophyly and 100% bootstrap support. While COI barcoding provided robust confirmation, the core contribution of this study lies in the identification of species-specific egg case morphometrics. Together, these findings establish a dual-track toolkit, egg case morphology for primary discrimination and COI barcodes for confirmatory validation, that can be incorporated into bycatch monitoring and biodiversity assessments, supporting the conservation of poorly known catsharks in the Southeast Pacific. Full article

Galapagos sharks (Carcharhinus galapagensis) are a large species belonging to the family Carcharhinidae, with poorly understood biology, especially along continental coastlines, which limits effective management of their populations. Here, we report preliminary findings of their movements along the Pacific Coast of Panama and Colombia, derived from satellite telemetry. Sharks were tracked for as long as 1.5 years, during which they moved extensively (~400 km along the Panama/Colombia coastline), with areas with high concentrations of detections that spanned the Panama/Colombia border. Sharks appeared to alter their movements during the diel cycle (more detections at night) and in relation to the wet/dry seasons (more detections during the dry season). Locations of shark detections were most commonly within 20 km of shore and in water less than 100 m deep, although detections were recorded hundreds of kilometers from shore in water thousands of meters deep. Galapagos sharks appear to move more extensively along continental coastlines compared to island locations, where most previous information about their movements and biology has been obtained. Full article

The present study characterizes the complete mitochondrial genome of Potamotrygon leopoldi, commonly referred to as the “white blotched stingray”, a member of the Potamotrygonidae family that are a group of stingrays that live exclusively in freshwater environments. Potamotrygon leopoldi, endemic to the Xingu River in the Amazon region, are exploited by commercial fisheries for food and commonly exploited by the ornamental industry, and this has a significant impact on the populations. Here, newly assembled PacBio long-read sequencing assesses the complete mitogenome of P. leopoldi and performs a comparative investigation into the evolutionary connections to other extant taxa of elasmobranchs. The mitogenome has 17,504 bp, containing 13 protein-coding, 22 tRNA, and 2 rRNA genes. The mitogenome comprises A: 32.32%, T: 24.41%, C: 12.84%, and G: 30.42%, with an AT content of 56.73%. The values of AT and GC skewness were 0.13 and −0.40, respectively. Our phylogenetic analyses with mitogenome sequences of 40 elasmobranch species support the monophyly for the Potamotrygonidae family and indicate a close relationship to the Dasyatidae family and a sister relationship with Potamotrygon orbignyi and Potamotrygon falkneri. We also detected various amino acid sites in positive selection exclusively in P. leopoldi. This extensive comparative mitogenomic investigation offers novel and significant insights into the evolutionary lineage of neotropical freshwater stingrays and their closely related taxa. It is an indispensable resource for facilitating ongoing and prospective investigations into the molecular evolution of elasmobranchs. Full article

In the context of the progressive tendency to perceive a degraded environmental state as normal, due to the loss of memory of past ecological conditions (i.e., the Shifting Baseline Syndrome), natural history museum collections represent invaluable resources for studying long-term biodiversity shifts. This study deals with the taxonomic validation of the chondrichthyan species from the historical ichthyological collection assembled by Pietro Doderlein from 1863 to 1922 at the Museum of Zoology of the University of Palermo. The chondrichthyan specimens were digitally catalogued to meet current standards of museum documental identification. Biometric measurements were taken for each specimen, and an integrated analytical approach—combining morphology and ancient DNA analysis—was applied to assign species identities. The collection comprises 342 specimens associated with 76 valid codes. Of these, 288 specimens were identified to species level by morphology, revealing 58 discrepancies with the historical identifications. Sixteen specimens that could not be morphologically assigned were analyzed by DNA barcoding, resulting in eight additional species-level identifications. In total, 62 valid species belonging to 27 families were digitally catalogued according to ministerial guidelines. This taxonomic validation and cataloguing of the “P. Doderlein” chondrichthyan collection represent the first successful effort to bridge the gap in available data and tissue resources from Italian historical natural museums. Full article

Protecting threatened elasmobranch species despite limited data on their distribution and abundance is a critical challenge, particularly in the context of increasing human impacts on marine ecosystems. In the northeastern Atlantic, species such as the leafscale gulper shark, Portuguese dogfish, spurdog, and spotted ray are facing pressures from overfishing, bycatch, habitat degradation, and climate change. The OSPAR Commission has listed these species as threatened and/or declining and aims to protect them by reliably identifying suitable habitats and integrating these areas into Marine Protected Areas (MPAs). In this study, we present a spatial modelling framework using regression-based approaches to identify suitable habitats for these four species. Results show that suitable habitats of the spotted ray (25.8%) and spurdog (18.8%) are relatively well represented within existing MPAs, while those of the deep-water sharks are underrepresented (6.0% for leafscale gulper shark, and 6.8% for Portuguese dogfish). Our findings highlight the need for additional MPAs in deep-sea continental slope areas, particularly west and northwest of Scotland and Ireland. Such expansions would support OSPAR’s goal to protect 30% of its maritime area by 2030 and could benefit broader deep-sea biodiversity, including other vulnerable demersal species and benthic communities. Full article

The small-spotted catshark and the smooth-hound are cartilaginous, carnivorous fish with similar depth ranges in their habitats. These two species are among the most abundant elasmobranchs in the Adriatic Sea and are frequently caught by local fishermen using longline fishing. Despite their ecological similarities, little is known about the physiological differences in their digestive processes. The study of enzymatic digestion in these ecologically relevant species helps to fill the knowledge gap in the understanding of nutrient processing in cartilaginous fish. Therefore, the aim of this study was to determine, measure and compare the enzymatic activity of alkaline phosphatase, acid phosphatase, non-specific esterase and aminopeptidase. Fish were caught in the central part of the Adriatic Sea between 2021 and 2023. A total of 60 adult individuals were analyzed, with samples taken from six parts of the digestive tract. Histochemical analysis of 1440 slides revealed clear differences in enzyme activity between the two species. In the small-spotted catshark, cellular protein degradation was most pronounced in esophagus, posterior stomach and rectum, whereas in the smooth-hound, it was concentrated in posterior stomach and spiral intestine. Cellular digestion of lipids in the small-spotted catshark appears to occur primarily in the stomach. The results of this study provide new insights into the distribution of cellular digestive enzymes in cartilaginous fish and emphasize the importance of studying the entire digestive tract as an integrated system rather than focusing on individual parts. This study fills an important knowledge gap and contributes to a deeper understanding of digestive physiology, which in turn has implications for species conservation and biological variability. Full article