Shark Species Research Articles

Global risk assessment of sharks to climate change

In what has been referred to as a ‘perfect storm’, it is now clear that we will be concurrently facing both a biodiversity and climate crisis over the incoming decades. In this context, we propose a broadly applicable framework to evaluate the climate-associated risk for marine life at the species-level, based on the ecosystem-level assessment developed by the Intergovernmental Panel on Climate Change (IPCC). We apply this framework to all extant marine shark species — given their major ecological and socioeconomic importance, alongside their precarious conservation status —at the global scale. Through the integration of expert-assessed information on each risk dimension, we consider the ecosystem dependencies of the targeted species, alongside with their vulnerability to human pressures. More specifically, we estimate the threat (exposure * hazard) level imposed by different climate change scenarios [Shared Socioeconomic Pathway (SSP) 1, SSP2, SSP3 and SSP5] across meaningful timeframes (2021–2040, 2041–2060 and 2081–2100) and contrast the normalized threat, vulnerability, and risk scores of each species across regions and attributes (order, habitat use, climate preference, lifestyle, trophic position, reproductive mode, and extinction risk category). Our analysis showcases how all shark species should be affected by climate change regardless of the emission scenario. With effects widely expected over the short-term, discrepancies between emission scenarios escalate considerably over time, with associated changes in the level and type of ecological implications. Moreover, with distinct lineages and functional attributes expected to be differently affected and with distinct consequences expected across scenarios, this analysis highlights how climate change is poised to exacerbate the already disproportional risk of functional and phylogenetic loss documented for this key group of marine predators.

Sharks and Rays of Northern Australia’s Roper River, with a Range Extension for the Threatened Speartooth Shark Glyphis glyphis

Northern Australia is considered a ‘lifeboat’ region for globally threatened shark and ray species (elasmobranchs), although much of the region is understudied. The Roper River in the Northern Territory’s Gulf of Carpentaria has been inadequately surveyed, with most elasmobranch data gained opportunistically through freshwater fish surveys. This study aimed to report the occurrence of elasmobranch species in the Roper River through targeted field surveys conducted between 2016 and 2024 and to review data from other sources. Four euryhaline species were recorded, comprising two sharks, a sawfish, and a stingray. Records of the globally Vulnerable speartooth shark (Glyphis glyphis) represented a significant range extension and the documentation of a new reproductive population. Records of the Critically Endangered largetooth sawfish (Pristis pristis) extended almost 400 km upstream, highlighting extensive use of the river system. A predominantly marine species, the Critically Endangered giant guitarfish (Glaucostegus typus) was observed in brackish estuarine waters, approximately 15 km upstream from the river mouth. Further research is required to develop a more thorough understanding of abundance, life history, and population connectivity for these species in this river system. Given the documentation of threatened species, the potential impacts of fisheries, water allocations, and climate change require assessment.

Aerobic and anaerobic poise of white swimming muscles of the deep-diving scalloped hammerhead shark: comparison to sympatric coastal and deep-water species

Scalloped hammerhead sharks (Sphyrna lewini) routinely perform rapid dives to forage on mesopelagic prey. These deep dives consist of intensive swimming followed by recovery periods in the surface mixed layer. Swimming muscle temperature profiles suggest that S. lewini suppresses gill function as a means to reduce convective heat loss during dives into cool water. Such intensive swimming behavior coupled with reduced respiration prompted us to test whether the aerobic and anaerobic metabolic capacities of the white swimming muscle tissue of this species are greater than those of other shark species from the same region. The activities of key enzymes used in aerobic and anaerobic metabolism provide an indirect indicator of the metabolic potential (“poise”) of a tissue. Here we measured the maximal activities [international units (µmol substrate converted to product per min, U) per gram of wet tissue mass at 10°C] of the citric acid cycle enzymes citrate synthase (CS) and malate dehydrogenase (MDH) and glycolytic enzymes pyruvate kinase (PK) and lactate dehydrogenase (LDH) from white swimming muscle of S. lewini. Enzyme activities, and ratios of these enzyme activities that indicate relative indexes of aerobic to anaerobic capacity, were compared to those measured in three sympatric coastal carcharhinid sharks and two deep-dwelling species, Echinorhinus cookei and Hexanchus griseus. This is the first report of swimming-muscle enzyme activity for these deep-dwelling species. In comparison to the other species, S. lewini had significantly higher activities of both LDH and MDH in the white muscle, and a higher MDH/CS ratio. The high LDH activities suggest that the white muscle of S. lewini relies on relatively high rates of anaerobic ATP production, with would result in build up of high lactate levels, during deep foraging dives. High MDH activity in S. lewini white muscle suggests the potential for lactate levels to be rapidly reduced when aerobic conditions are restored while in the surface mixed layer between dives. These biochemical characteristics may enable S. lewini to swim rapidly while suppressing gill function during deep dives and thereby exploit a very different ecological niche from sympatric shark species (e.g., coastal carcharhinids) and hunt more rapidly via faster swimming for deep-water prey compared to species that permanently inhabit deep depths.

Essential and non-essential elements in the endemic Peppered catshark Galeus piperatus of deep-ocean waters from northern Gulf of California: Baseline levels and Se ratios

In this study the levels of As, Cd, Cu, Hg, Pb, Se, Zn and Se:Hg molar ratios, in muscle and liver of the endemic catshark Galeus piperatus from the northern Gulf of California (NGC) and relationships with biological parameters, were determined. Arsenic presented the highest levels for both tissues and Pb the lowest. The Se:Hg molar ratios were >1. Sex was not a determining factor in accumulation of elements. The differences in the levels of the studied elements between tissues might be due to their physiological functions and the availability of the metal(loid)s. The diet was considered as an important contribution pathway of these elements despite the fact that G. piperatus feeding habits are still unknown. More research is necessary to clearly understand the interactions between the biology of G. piperatus and the bioavailability of the studied elements and use this endemic shark species as a potential deep-sea biomonitor in NGC.

Shifts in the habitat suitability for large hammerhead sharks under climate change

One of the chief consequences of climate change is the worldwide redistribution of species in the pursuit of physiologically and ecologically favourable conditions. As part of one of the most threatened groups of vertebrates, assessing how climate change may affect the distribution patterns of key shark species is essential for their long-term conservation and management. The present study aims to assess how climate change projections (based on the representative concentration pathways 2.6, 4.5, 6.0, and 8.5) for the middle and end of the century may affect the global patterns of habitat suitability (HS) for large hammerhead sharks – a unique and particularly threatened group of elasmobranchs. Here, a decline in the global average HS is projected for both Sphyrna lewini and Sphyrna zygaena, with the scale of HS decline generally increasing with time and RCP severity, while a global increase in HS is generally projected for Sphyrna mokarran, albeit reduced over the long-term and overturned under long-term RCP8.5. Meanwhile, poleward shifts are projected for all three species, with considerable tropical declines for both S. lewini and S. zygaena. Additionally, regional shifts in suitability have the potential to affect population connectivity, the access to key habitats, and levels of exposure to other anthropogenic pressures, along with the effectiveness of conservation and management efforts. Taken together, these results showcase the importance of climate change for species’ extinction risk assessments and the urgency of the consideration of climate change within the design and implementation of conservation measures.

Vulnerability assessment of elasmobranch species to fisheries in coastal Kenya: Implications for conservation and management policies

Ecological Risk Assessment (ERA) of species is useful for making informed management decisions based on relative vulnerability of species to fisheries. Understanding the vulnerability of species to fishing gears is important for targeted management measures especially for species known to have delicate life-history strategies such as the elasmobranchs. Productivity and Susceptibility Analysis (PSA), an ERA approach, was applied to 61 elasmobranch species (32 shark and 29 ray species) to assess their relative vulnerability (low, medium and high) to fishing by artisanal, prawn trawl and tuna longline fisheries in Kenya’s coastal waters. Overall, results showed 28 % of rays and 31 % of sharks were highly vulnerable to the fisheries, with the pelagic longline tuna fishery having the largest proportions of rays (50 %) and sharks (54 %) being highly vulnerable. Most of the species were in the medium vulnerability category. Among the artisanal gears, only gillnets had species that were highly vulnerable to the fishery, while relatively fewer species (n = 9) of both sharks and rays were highly vulnerable to the prawn trawlers compared to those in the medium vulnerability category (n = 25). Seventeen species classified as High Vulnerability Species Assemblage (HVSA), were assessed to be at high risk of overfishing by different gears, and are recommended for management as a complex. Of the HVSA group, two species; Pseudoginglymostoma brevicaudatum and Rhina ancylostoma are classified by the IUCN Red List of Threatened Species as Critically Endangered (CR), while another four; Rhinoptera jayakari, Mobula birostris, Stegostoma tigrinum and Carcharhinus plumbeus are Endangered (EN), with the rest being either Vulnerable (Gymnura poecilura, Pastinachus ater, Pateobatis jenkinsii, Taeniurops meyeni, Triaenodon obsesus, Carcharhinus falciformis, and Carcharhinus carcharias), Near Threatened (Maculabatis ambigua, Mustelus mosis, Prionace glauca) or classified as Data Deficient (Pliotrema warreni). Although majority of the species have been placed in the medium vulnerability category, some of them are those that are threatened with extinction globally, calling for a more precautionary management approach. A validation approach has been used to mitigate the problems of PSAs associated with data quality and uncertainty around estimated vulnerability indices. More robust application of PSA will require continuous updating of life-history attributes to improve on sensitivity. A precautionary Shark and Ray Management Plan (SRMP) that takes into account the outputs of the PSA is recommended for management and conservation of the elasmobranch stocks in Kenya’s marine waters.

Temporal niche partitioning as a potential mechanism for coexistence in two sympatric mesopredator sharks

Global marine biodiversity declines require bold and ambitious plans to safeguard ecosystem services. Overfishing, habitat loss and projected climate impacts have yielded deleterious effects on marine predators, in particular, driving increasing threat of extinction for many shark species with implications for ecosystem health. Identifying and protecting critical habitat areas for sharks is fundamental to their protection, and may allow for species recovery. Here we use baited remote underwater video stations to investigate spatial and temporal patterns of habitat use by sharks in a Special Area of Conservation (SAC) that is centrally important to the regional blue economy in the UK, the Cardigan Bay SAC. Our results show heterogeneous spatial habitat use and temporal trends in habitat sharing between smallspotted catsharks Scyliorhinus canicula and nursehounds S. stellaris. Nearshore reef habitats are substantially more important than non-reef habitats for both species. The relative abundance of each species, however, is roughly inversely seasonally proportional, with S. canicula and S. stellaris relative abundance highest in March and September, respectively. Temporal niche partitioning may be an important mechanism in marine systems, but has not been widely investigated in sympatric shark communities. These findings are directly relevant for ongoing development of the Welsh Marine Evidence Strategy (2019-2025) and the Welsh National Marine Plan, particularly to inform spatial planning to strengthen the role of SAC management design in protecting important shark areas.

Small sharks, big problems: DNA analysis of small fins reveals trade regulation gaps and burgeoning trade in juvenile sharks.

Many shark species have been overexploited for international markets, including fins for shark fin soup in Southeast Asia. Previous studies highlighted the value of large, threatened shark species, regulated under CITES Appendix II. However, sampling biases may have overlooked small shark species. Here, we address this by identifying species from ~4000 small shark fins in Hong Kong. These fins included species not recorded in previous surveys, raising the market's species diversity to 106. Nearly 75% of the small fins came from small shark species and 58.1% of small species were threatened with extinction. We identified an important CITES listing gap: Trade in 19 small, threatened species, especially from the family Triakidae, is unregulated. In addition, a quarter of small fins come from large sharks, indicating that substantial exploitation of juveniles is occurring and may be affecting fisheries sustainability. Enhanced surveillance of small shark fin trade is essential to ensure effective conservation under emerging trade regulations.

Correction to: Novel deep-sea observations reveal twilight zone occurrence for two species of pelagic sharks: the bignose shark Carcharhinus altimus and the silky shark Carcharhinus falciformis

Correction to: Novel deep-sea observations reveal twilight zone occurrence for two species of pelagic sharks: the bignose shark Carcharhinus altimus and the silky shark Carcharhinus falciformis

From wild to captive: Understanding the main nutritional diseases of sharks in public aquariums.

The establishment of a well-adapted and accurate feeding regimen is a fundamental determining factor in the maintenance of shark species in captivity. Replicating the natural diet in captivity is commonly accepted and recommended. Given the diverse nutritional composition of prey, supplementation may become necessary. The captive environment poses unique challenges, making sharks susceptible to an array of health issues, including numerous diseases. Mitigating these risks demands detailed husbandry practices, an appropriate physical environment and a balanced diet. Supplementation, encompassing vitamins and minerals, becomes imperative for the provision of essential nutrients. This complexity has rendered the formulation of an adapted feeding plan for aquarists exceptionally challenging. The scarcity of information in these species adds to the issue, mandating extrapolation from various shark groups and to species with analogous characteristics. This literature review concentrates predominantly on benthic and pelagic shark species prevalent in contemporary aquariums. The central argument posits that dietary choices in captivity rely on factors such as availability, quality and consistency of supply. The advocated approach highlights the importance of a balanced, diverse feeding that closely mirrors natural diets. It is then crucial to emphasize that these are general guidelines, and the specific dietary requisites may diverge between shark species. Collaborating with experts in marine biology, shark husbandry and veterinary care is imperative for the formulation and perpetuation of an adapted diet for captive sharks.

Mitochondrial DNA patterns describe the evolutionary history of the bonnethead shark Sphyrna tiburo (Linneus 1758) complex in the western Atlantic Ocean.

The apparent lack of physical barriers in the marine realm has created the conception that many groups have a constant gene flow. However, changes in ocean circulation patterns, glacial cycles, temperature, and salinity gradients are responsible for vicariant events in many fish species, including sharks. The bonnethead shark, Sphyrna tiburo, is an endangered small coastal shark species. Although considerable efforts have recently been undertaken, little remains known about the possible biogeographic scenario that can explain its actual distribution within the western Atlantic (WA). Here, we used 599 mitochondrial sequences to assess the phylogeographic structure and implement Bayesian phylogenetic analyses to obtain divergence times and reconstruct the ancestral geographic range. This allowed us to infer processes responsible for the diversification of S. tiburo into major divergent lineages. Our results indicated that S. tiburo in the WA represents three independent lineages, with Brazilian samples differentiated into a distinct genetic cluster. The posterior probability of ancestral range analysis indicated that the species likely originated in the northern region (Carolina Province and the southern Gulf of Mexico), where it colonized southward through the uplifting of the Central American Isthmus (CAI). The Northern and Caribbean genetic clusters appear to have arisen from the intensification of the Loop Current, which currently flows northward passing the Yucatan Peninsula, Gulf of Mexico, and east Florida. Following initial colonization, the Northeastern Brazil group differentiated from the Caribbean region due to the sediment and freshwater discharge of the Amazon-Orinoco Plume. Thus, the evolutionary history of the S. tiburo complex can be explained by a combination of dispersal and vicariance events that occurred over the last ~5 million years (MY). We established and confirmed the species and population limits, demonstrating that the Amazon-Orinoco Plume constitutes a significant dispersal barrier for coastal sharks. Finally, we discuss some recommendations for the conservation of the bonnethead shark.

PLAbDab-nano: a database of camelid and shark nanobodies from patents and literature.

Nanobodies are essential proteins of the adaptive immune systems of camelid and shark species, complementing conventional antibodies. Properties such as their relatively small size, solubility and high thermostability makeVHH (variable heavy domain of theheavy chain) and VNAR (variable new antigen receptor) modalities a promising therapeutic format and a valuable resource for a wide range of biological applications. The volume of academic literature and patents related to nanobodies has risen significantly over the past decade. Here, we present PLAbDab-nano, a nanobody complement to the Patent and Literature Antibody Database (PLAbDab). PLAbDab-nano is a self-updating, searchable repository containing ∼5000 annotated VHH and VNAR sequences. We describe the methods used to curate the entries in PLAbDab-nano, and highlight how PLAbDab-nano could be used to design diverse libraries, as well as find sequences similar to known patented or therapeutic entries. PLAbDab-nano is freely available as a searchable web server (https://opig.stats.ox.ac.uk/webapps/plabdab-nano/).

First Record of Bramble Sharks, Echinorhinus brucus (Echinorhiniformes, Echinorhinidae), in the United Arab Emirates

The first record of bramble sharks (Echinorhinus brucus) in the United Arab Emirates is presented. In situ observations of multiple bramble sharks were made at depths between 460 and 720 m from two piloted submersibles and a remotely operated vessel, representing the first known observations of this species in its native deep-water habitat in Arabia and the Indian Ocean. Notably, this research expands on the documented regional distribution of E. brucus for the Gulf of Oman/Arabian Sea and extends this species’ regional records to deeper mesophotic zones. These findings underscore the need for further research to understand the ecology and distribution of this cryptic shark species, particularly given its global endangered status and the limited knowledge of its regional population dynamics.

Vulnerability of elasmobranchs caught by artisanal fishery in the Southeastern Caribbean

The Caribbean is recognized as one of the regions with significant elasmobranch diversity; however, the lack of scientific data has made it difficult to conduct traditional fish stock assessments. A Productivity Susceptibility Analysis (PSA) was used to evaluate the vulnerability of the most important elasmobranch species caught by artisanal fishery carried out in the northeastern region of Venezuela. The fishing database created by the INIA and CIT was used as a starting point to obtain catch composition, and biological and fishing information. Of the 37 elasmobranch species captured in the study area, 12 were selected for the PSA. Analysis indicated that the selected species, most of them small-sized sharks, had medium and high productivity; while all species evaluated showed high susceptibility to artisanal fishing. Among the elasmobranch species with high vulnerability, seven of them comprised five sharks (C. limbatus, R. lalandii, R. porosus, C. brevipinna and C. acronotus) and two batoids (H. guttatus and A. narinari). Elasmobranchs species with medium vulnerability included two sharks (M. higmani and M. minicanis) and one batoid (H. americanus); while those with low vulnerability comprised two shark species (M canis and M. norrisi). In the study area, elasmobranch fishing has been carried out for decades without effective management strategies, and the artisanal fishery poses a potential threat to the populations of this group of fish. The species identified in this study as having high vulnerability or risk should be immediately prioritised for serious management by the national agencies responsible for fishing administration.

Reef fish biodiversity and occurrence of endangered sharks within a small marine protected area off Sint Maarten, Dutch Caribbean

AbstractMarine protected areas (MPAs) are common conservation tools supporting the protection of threatened marine fishes, such as sharks. However, the creation of shark MPAs has been less common in the Greater Caribbean region despite a growing need and opportunity. In this study, we evaluated the occurrence of shark and reef fish biodiversity off Sint Maarten, Dutch Caribbean, with a particular emphasis on endangered shark presence within the Man of War Shoal Marine Protected Area (MPA). We utilized baited remote underwater video systems (BRUVs) to gather non-invasive data on the abundance and diversity of reef fish and shark species inside and outside the local MPA. Generalized linear models (GLMs) revealed no significant effect of protection status on the presence of the endangered Caribbean reef shark (Carcharhinus perezi). However, we found a significant influence of depth on shark occurrence, as well as an effect of habitat type on shark and reef fish biodiversity, with reef habitats showing the greatest significance. These results suggest that the effect of small coastal MPAs on bolstering local endangered species conservation in the Dutch Caribbean is likely to vary according to habitat features and how those species utilize those habitats across various life stages. These findings have implications for adaptive MPA management, which should be informed by the ecology and habitat preferences of target species to achieve maximum benefits for biodiversity conservation.

UT-1 Transporter Expression in the Spiny Dogfish (Squalus acanthias): UT-1 Protein Shows a Different Localization in Comparison to That of Other Sharks.

The original UT-1 transporter gene was initially identified in the spiny dogfish (Squalus acanthias), but localization of the UT-1 protein was not determined. Subsequent UT-1 expression was shown to localize to the collecting tubule (CT) of the shark nephron in other shark species, with expression in a closely related chimaera species also located additionally at a lower level in the intermediate-I segment (IS-I) of the nephron. In spiny dogfish, two UT-1 splice variants are known (UT-1 long and short), and there was also a second UT-1 gene described (here termed Brain UT). In this study, a second splice variant of the second Brain UT gene was discovered. Expression profiles (mRNA) of UT-1 long and short and Brain UT were determined in a number of spiny dogfish tissues. Quantitative PCR in kidney samples showed that the level of the short variant of UT-1 was around 100 times higher than the long variant, which was itself expressed around 10 times higher than Brain UT cDNA/mRNA (in kidney). For the long variant, there was a significantly higher level of mRNA abundance in fish acclimatized to 75% seawater. Ultimately, three UT-1 antibodies were made that could bind to both the UT-1 short and long variant proteins. The first two of these showed bands of appropriate sizes on Western blots of around 52.5 and 46 kDa. The second antibody had some additional lower molecular weight bands. The third antibody was mainly bound to the 46 kDa band with faint 52.5 kDa staining. Both the 52.5 and 46 kDa bands were absent when the antibodies were pre-blocked with the peptide antigens used to make them. Across the three antibodies, there were many similarities in localization but differences in subcellular localization. Predominantly, antibody staining was greatest in the intermediate segment 1 (IS-I) and proximal (PIb) segments of the first sinus zone loop of the nephron, with reasonably strong expression also found at the start and middle of the late distal tubule (LDT; second sinus zone loop). While some expression in the collecting tubule (CT) could not be ruled out, the level of staining seemed to be low or non-existent in convoluted bundle zone nephron segments such as the CT. Hence, this suggests that spiny dogfish have a fundamentally different mode of urea absorption in comparison to that found in other shark species, potentially focused more on the nephron sinus zone loops than the CT.

Environmental stress reduces shark residency to coral reefs

Coral reef ecosystems are highly threatened and can be extremely sensitive to the effects of climate change. Multiple shark species rely on coral reefs as important habitat and, as such, play a number of significant ecological roles in these ecosystems. How environmental stress impacts routine, site-attached reef shark behavior, remains relatively unexplored. Here, we combine 8 years of acoustic tracking data (2013-2020) from grey reef sharks resident to the remote coral reefs of the Chagos Archipelago in the Central Indian Ocean, with a satellite-based index of coral reef environmental stress exposure. We show that on average across the region, increased stress on the reefs significantly reduces grey reef shark residency, promoting more diffuse space use and increasing time away from shallow forereefs. Importantly, this impact has a lagged effect for up to 16 months. This may have important physiological and conservation consequences for reef sharks, as well as broader implications for reef ecosystem functioning. As climate change is predicted to increase environmental stress on coral reef ecosystems, understanding how site-attached predators respond to stress will be crucial for forecasting the functional significance of altering predator behavior and the potential impacts on conservation for both reef sharks and coral reefs themselves.

Beneath the surface: DNA barcoding of shark fins in Singapore.

The global decline of shark populations, largely driven by overfishing to supply the shark fin trade, poses a significant threat to marine ecosystems. Southeast Asia, and particularly Singapore, is a key hub for the transit and trade of shark fins that contribute to the exploitation of these apex predators. Through the use of DNA barcoding techniques, this study aimed to determine what species of shark are involved in the Singapore shark fin trade. Fins were collected from markets, dried goods shops and traditional Chinese medicine halls throughout Singapore. In total, DNA was extracted from 684 fins collected in January 2024 and PCR amplification targeted a fragment of the mitochondrial COI gene for species identification. Results revealed fins from 24 species across 16 genera, with 19 species listed on CITES Appendices II, and 16 listed as threatened on the IUCN Red List (critically endangered = 2, endangered = 4, vulnerable = 10). The top five most frequently identified species were Carcharhinus falciformis, Galeorhinus galeus, Rhizoprionodon oligolinx, Sphyrna lewini and Rhizoprionodon acutus. Of these, four are listed on CITES Appendix II and four are listed as threatened on the IUCN Red List.

Patterns of dermal denticle loss in sharks.

As they grow, sharks both replace lost denticles and proliferate the number of denticles by developing new (de novo) denticles without prior denticle shedding. The loss and replacement of denticles has potential impacts on the energetic cost of maintaining the skin surface, the biomechanical functions of shark skin, as well as our ability to predict shark abundance from fossil denticle occurrence in sediment cores. Here, we seek to better understand patterns of denticle loss and to show how denticles are being replaced in mature sharks. We illustrate shark skin surfaces with missing denticles and quantify both within-species and between-species patterns of missing denticles using images from across regions of the body for two species and images at similar body regions for 16 species of sharks. Generally, sharks are missing similar numbers of denticles (0%-6%) between species and regions. However, there are exceptions: in the smooth dogfish, the nose region is missing significantly more denticles than most posterior-body and fin regions, and the common thresher shark is missing significantly more denticles than the smooth dogfish, leopard shark, angel shark, bonnethead, and gulper shark. Denticle regrowth starts with crown development and mineralization beneath the epidermis, followed by eruption of the crown, and finally the mineralization of the root. The pulp cavity of replacement denticles is initially large and surrounded by a thin shell of enameloid upon eruption of the denticle. After eruption of the denticle, the deposition of dentine continues internally after the denticle reaches its final position. Replacement of missing denticles, representing less than 6% of the skin surface at any one time, may not compromise hydrodynamic function, but by constantly updating the skin surface throughout life, sharks may reduce surface fouling and maintain a functional complex skin surface by repairing local damage to individual denticles.

The complete mitochondrial genome of the leopard shark Triakis semifasciata (Triakidae)

ABSTRACT The leopard shark Triakis semifasciata (family Triakidae) is threatened by habitat loss and targeted by recreational and commercial fishermen in the Eastern Pacific Ocean, from Oregon, USA, to the Gulf of California, Mexico. Despite environmental issues, there are few genetic and genomic resources available for this species. This study assembled the complete mitochondrial genome of T. semifasciata from a skin metagenomic sample and described it in detail. The phylogenetic position of T. semifasciata amongst closely related species was also examined using mitochondrial protein-coding genes (PCGs). The mitochondrial genome of T. semifasciata is 16,613 bp in length and consists of 13 PCGs, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. A 30 bp long region was identified as the origin of replication for the light strand (OL) between the trnN and trnC genes, and a 974 bp long putative control region (CR) contains the origin of replication for the heavy strand (OH). The gene order in T. semifasciata is identical to that of cofamilial species. An analysis of Ka/Ks ratios for all PCGs yielded values < 1, indicating that all PCGs experience strong purifying selection. All tRNAs exhibit a canonical ‘cloverleaf’ secondary structure except for tRNA-Ser1 which lacks the stem of the dihydrouridine (DHU) arm and in place possesses a simple loop. A phylomitogenomic maximum likelihood (ML) analysis did not support the monophyly of the family Trakidae and placed T. semifasciata in a sister position to Hemitriakis japanica. However, the aforementioned sister position was poorly supported by ML bootstrap values. This study represents a new genomic resource for this commercially and recreationally important species and confirms that mitochondrial genomes can be assembled from skin metagenomic samples as shown before in an unrelated shark species.