BACKGROUND: Studying habitat use and vertical movement patterns of individual fish over continuous time and space is innately challenging and has therefore largely remained elusive for a wide range of species. Amongst sharks, this applies particularly to smaller-bodied and less wide-ranging species such as the spurdog (Squalus acanthias Linnaeus, 1758), which, despite its importance for fisheries, has received limited attention in biologging and biotelemetry studies, particularly in the North-East Atlantic.
METHODS: To investigate seasonal variations in fine-scale niche use and vertical movement patterns in female spurdog, we used archival data from 19 pregnant individuals that were satellite-tagged for up to 365 days in Norwegian fjords. We estimated the realised niche space with kernel densities and performed continuous wavelet analyses to identify dominant periods in vertical movement. Triaxial acceleration data were used to identify burst events and infer activity patterns.
RESULTS: Pregnant females frequently utilised shallow depths down to 300 m at temperatures between 8 and 14 °C. Oscillatory vertical moments revealed persistent diel vertical migration (DVM) patterns, with descents at dawn and ascents at dusk. This strict normal DVM behaviour dominated in winter and spring and was associated with higher levels of activity bursts, while in summer and autumn sharks predominantly selected warm waters above the thermocline with only sporadic dive and bursts events.
CONCLUSIONS: The prevalence of normal DVM behaviour in winter months linked with elevated likely foraging-related activity bursts suggests this movement behaviour to be foraging-driven. With lower number of fast starts exhibited in warm waters during the summer and autumn months, habitat use in this season might be rather driven by behavioural thermoregulation, yet other factors may also play a role. Individual and cohort-related variations indicate a complex interplay of movement behaviour and habitat use with the abiotic and biotic environment. Together with ongoing work investigating fine-scale horizontal movement as well as sex- and age-specific differences, this study provides vital information to direct the spatio-temporal distribution of a newly reopened fishery and contributes to an elevated understanding of the movement ecology of spurdog in the North-East Atlantic and beyond.

Mating system variation refers to the spectrum between genetic monogamy and polyandry, and has important consequences for sexual conflict, sexual selection and individual fitness in animals. Theoretically this variation could also have substantial population-level effects, influencing population viability and extinction risk. Evidence for these effects is mixed, in part due to the fact that substantial environmental change is thought to be required for them to have visible demographic consequences. In this study we test for the presence of relationships between polyandry and population status in Elasmobranchii (sharks and rays). Elasmobranchii is a large vertebrate clade that exhibits substantial interspecific variation in both genetic mating system and population status, as well as being subject to intense anthropogenically-mediated environmental change. We also predict past macroevolutionary shifts in genetic mating system through elasmobranch phylogeny. Our results show that both genetic monogamy and polyandry have evolved multiple times independently within Elasmobranchii, and we suggest that both of these extremes represent alternative adaptive strategies that are favoured under discrete ecological and biological conditions. Nevertheless, there is no evidence of population-level consequences of mating system variation in elasmobranchs. These results are significant as they suggest that mating system variation in this clade is unlikely to be a major determinant of extinction vulnerability. Ultimately additional work will be required, however this study improves our understanding of the evolutionary dynamics underlying mating system variation in elasmobranchs, and the potential for resultant population-level consequences.

Elucidating cellular architecture and cell-type evolution across species is central to understanding immune system function and susceptibility to disease. Adaptive immunity is a shared trait of the common ancestor of cartilaginous and bony fishes. However, evolutionary features of lymphocytes in these two jawed vertebrates remain unclear. Here, we present a single-cell RNA sequencing atlas of immune cells from cartilaginous (white-spotted bamboo shark) and bony (zebrafish and Chinese tongue sole) fishes. Cross-species comparisons show that the same cell types across different species exhibit similar transcriptional profiles. In the bamboo shark, we identify a phagocytic B cell population expressing several pattern recognition receptors, as well as a T cell sub-cluster co-expressing both T and B cell markers. In contrast to a division by function in the bony fishes, we show close linkage and poor functional specialization among lymphocytes in the cartilaginous fish. Our cross-species single-cell comparison presents a resource for uncovering the origin and evolution of the gnathostome immune system.

Maternal investment theory is the study of how breeding females allocate resources between offspring size and brood size to achieve reproductive success. In classical trade-off models, r/K-selection and bet-hedging selection, the primary predictors of maternal investments in offspring are population density and resource stability. In crowded, stable environments, K-selected females invest in large offspring at an equivalent cost in brood size. In uncrowded, unstable environments, r-selected females invest in large broods at an equivalent cost in offspring size. In unpredictable resource environments, bet-hedging females invest moderately in brood size and offspring size. The maternal risk-management model represents a profound departure from classical trade-off models. Maternal investments in offspring size, brood size, and brood number are shaped independently by autonomous risk factors: the duration of gaps in resources during seasonal cycles, rates of predation, and unpredictable catastrophic events. To date, no single model has risen to a position of preeminence. Here in sharks, we show that maternal investments within and across species do not agree with the predictions of trade-off models and instead agree with the predictions of the maternal risk-management model. Within and across shark species, offspring size and brood size were independent maternal investment strategies. The risk of starvation favored investments in larger offspring. The risk of predation favored investments in larger broods. If empirical studies continue to confirm its predictions, maternal-risk management may yet emerge as a unifying model of diverse reproductive adaptations by means of natural selection.

Requiem sharks (genus Carcharhinus) have previously been reported to form large aggregations around marine infrastructures in the eastern Mediterranean Sea. While this behaviour may offer fitness advantages at the individual level, the implications of extended residency at human-altered habitats for population persistence have yet to be assessed. In this work, we investigated the phylogeographic and demographic composition of sharks near a coal-fired power and desalination station in Israel. Our aim was to infer habitat use and the mechanisms underlying the aggregation behaviour, and to highlight potential conservation impacts. We sampled, measured, and released 70 individuals between 2016 and 2022 to assess genetic variability within the cytochrome C oxidase I (COI) region and to analyse the aggregation\'s structure based on the sharks\' size and sex distribution. In addition, we performed meristic counts on a reference specimen collected dead at another power station in Israel to supplement species identification using the abovementioned techniques. Our findings indicate size-based sex segregation of adult female dusky and male sandbar sharks (Carcharhinus obscurus and Carcharhinus plumbeus, respectively), with each species comprising two COI haplotypes. In the dusky shark, one haplotype corresponded to an Indo-Pacific lineage, and the other matched an Atlantic lineage. In the sandbar shark, we observed a haplotype previously sampled in the Mediterranean Sea, the Red Sea, the Northwest Indian Ocean, and South Africa, and another haplotype that was unique to our study site and genetically closer to the former than to sequences sampled in other ocean basins. This study provides the first indication of sympatric aggregation amongst phylogeographically distinct dusky and sandbar shark lineages, suggesting that human-altered habitats in the eastern Mediterranean Sea may influence the distribution of these species. Based on the observed segregation pattern, we conclude that the site does not function as a nursery, parturition, or mating area, and discuss other plausible explanations that warrant further research. Finally, we highlight important directions for future research and the implications of our findings for management and conservation.

The ridges of the dermal denticles of migratory sharks have inspired riblets to reduce the frictional drag of a fluid. In particular, the dermal denticles of white sharks (Carcharodon carcharias) are characterized by a high middle ridge and low side ridges. The detailed morphology of their denticles and their variation along the body, however, have never been investigated. Moreover, the hydrodynamic function of high-low combinations of ridges is unknown. In this article, the ridge spacings and heights of the white shark denticles were three-dimensionally quantified using microfocus X-ray computed tomography. Then, the swimming speed at which the ridges would reduce drag was hydrodynamically calculated with a flat plate body model and previous riblet data. High ridges with a large spacing were found to effectively reduce drag at a migration speed of 2.3 m s-1, while adjacent high and low ridges with a small spacing reduced drag at a burst hunting speed of 5.1 m s-1. Moreover, the above hydrodynamic calculation method was also applied to the shortfin mako shark and an extinct giant shark (called megalodon) with known ridge spacings, resulting in the estimated hunting speeds of 10.5 m s-1and 5.9 m s-1, respectively.

Parthenogenesis, or virgin birth, describes a mode of reproduction where an egg develops into an offspring without fertilization, and is observed across various vertebrate taxa, excluding mammals. Obligate parthenogenesis, found in around 100 vertebrate species and 1000 invertebrate species, is relatively rare. Conversely, facultative parthenogenesis, where females can reproduce both sexually and parthenogenetically, is observed in some vertebrates, including elasmobranchs. Notably, this phenomenon in elasmobranchs is mainly documented in captivity, allowing for detailed long-term observation. Specifically, this study reports the first case of facultative parthenogenesis in the common smooth-hound shark Mustelus mustelus, a species classified by IUCN as endangered. Here we show that the juvenile M. mustelus were born through parthenogenesis, exhibiting homozygosity at each genetic marker, consistent with terminal fusion automixis. Remarkably, this finding reveals that parthenogenesis can occur annually in these sharks, alternating between two females, and conclusively excludes long-term sperm storage as a cause. Consequently, this enhances our understanding of parthenogenesis in elasmobranchs and highlights the reproductive flexibility of M. mustelus. Overall, these results contribute to our broader understanding of reproductive strategies in elasmobranchs, which could inform conservation efforts for endangered species.

BACKGROUND: The oceanic whitetip shark Carcharhinus longimanus (family Carcharhinidae) is one of the largest sharks inhabiting all tropical and subtropical oceanic regions. Due to their life history traits and mortality attributed to pelagic longline fishing practices, this species is experiencing substantial population decline. Currently, C. longimanus is considered by the IUCN Red List of Threatened Species as \"vulnerable\" throughout its range and \"critically endangered\" in the western north Atlantic. This study sequences and describes the complete mitochondrial genome of C. longimanus in detail.
RESULTS: The mitochondrial genome of C. longimanus was assembled through next-generation sequencing and then analyzed using specialized bioinformatics tools. The circular, double-stranded AT-rich mitogenome of C. longimanus is 16,704 bp long and contains 22 tRNA genes, 2 rRNA genes, 13 protein coding genes and a 1,065 bp long control region (CR). Out of the 22 tRNA genes, only one (tRNA-Ser1) lacked a typical \'cloverleaf\' secondary structure. The prevalence of TTA (Leu), ATT (Ile) and CTA (Leu) codons in the PCGs likely contributes to the AT-rich nature of this mitogenome. In the CR, ten microsatellites were detected but no tandem repeats were found. Stem-and-loop secondary structures were common along the entire length of the CR. Ka/Ks values estimated for all PCGs were < 1, indicating that all the PCGs experience purifying selection. A phylomitogenomic analysis based on translated PCGs confirms the sister relationship between C. longimanus and C. obscurus. The analysis did not support the monophyly of the genus Carcharhinus.
CONCLUSIONS: The assembled mitochondrial genome of this pelagic shark can provide insight into the phylogenetic relationships in the genus Carcharhinus and aid conservation and management efforts in the Central Pacific Ocean.

The frequency of unprovoked shark bites is increasing worldwide, leading to a growing pressure for mitigation measures to reduce shark-bite risk while maintaining conservation objectives. Personal shark deterrents are a promising and non-lethal strategy that can protect ocean users, but few have been independently and scientifically tested. In Australia, bull (Carcharhinus leucas), tiger (Galeocerdo cuvier), and white sharks (Carcharodon carcharias) are responsible for the highest number of bites and fatalities. We tested the effects of two electric deterrents (Ocean Guardian\'s Freedom+ Surf and Freedom7) on the behaviour of these three species. The surf product reduced the probability of bites by 54% across all three species. The diving product had a similar effect on tiger shark bites (69% reduction) but did not reduce the frequency of bites from white sharks (1% increase), likely because the electrodes were placed further away from the bait. Electric deterrents also increased the time for bites to occur, and frequency of reactions and passes for all species tested. Our findings reveal that both Freedom+ Surf and Freedom7 electric deterrents affect shark behaviour and can reduce shark-bite risk for water users, but neither product eliminated the risk of shark bites entirely. The increasing number of studies showing the ability of personal electric deterrents to reduce shark-bite risk highlights personal protection as an effective and important part of the toolbox of shark-bite mitigation measures.

Per- and polyfluoroalkyl substances (PFAS) enter the marine food web, accumulate in organisms, and potentially have adverse effects on predators and consumers of seafood. However, evaluations of PFAS in meso-to-apex predators, like sharks, are scarce. This study investigated PFAS occurrence in five shark species from two marine ecosystems with contrasting relative human population densities, the New York Bight (NYB) and the coastal waters of The Bahamas archipelago. The total detected PFAS (∑PFAS) concentrations in muscle tissue ranged from 1.10 to 58.5 ng g-1 wet weight, and perfluorocarboxylic acids (PFCAs) were dominant. Fewer PFAS were detected in Caribbean reef sharks (Carcharhinus perezi) from The Bahamas, and concentrations of those detected were, on average, ∼79% lower than in the NYB sharks. In the NYB, ∑PFAS concentrations followed: common thresher (Alopias vulpinus) > shortfin mako (Isurus oxyrinchus) > sandbar (Carcharhinus plumbeus) > smooth dogfish (Mustelus canis). PFAS precursors/intermediates, such as 2H,2H,3H,3H-perfluorodecanoic acid and perfluorooctanesulfonamide, were only detected in the NYB sharks, suggesting higher ambient concentrations and diversity of PFAS sources in this region. Ultralong-chain PFAS (C ≥ 10) were positively correlated with nitrogen isotope values (δ15N) and total mercury in some species. Our results provide some of the first baseline information on PFAS concentrations in shark species from the northwest Atlantic Ocean, and correlations between PFAS, stable isotopes, and mercury further contextualize the drivers of PFAS occurrence.