Chondrichthyes occupy a key position in the phylogeny of vertebrates. The complete sequence of the mitochondrial genome (mitogenome) of four species of sharks and five species of rays was obtained by ...whole genome sequencing (DNA-seq) in the Illumina HiSeq2500 platform. The arrangement and features of the genes in the assembled mitogenomes were identical to those found in vertebrates. Both Maximum Likelihood (ML) and Bayesian Inference (BI) analyses were used to reconstruct the phylogenetic relationships among 172 species (including 163 mitogenomes retrieved from GenBank) based on the concatenated dataset of 13 individual protein coding genes. Both ML and BI analyses did not support the "Hypnosqualea" hypothesis and confirmed the monophyly of sharks and rays. The broad notion in shark phylogeny, namely the division of sharks into Galeomorphii and Squalomorphii and the monophyly of the eight shark orders, was also supported. The phylogenetic placement of all nine species sequenced in this study produced high statistical support values. The present study expands our knowledge on the systematics, genetic differentiation, and conservation genetics of the species studied, and contributes to our understanding of the evolutionary history of Chondrichthyes.
Anatomical knowledge of early chondrichthyans and estimates of their phylogeny are improving, but many taxa are still known only from microremains. The nearly cosmopolitan and regionally abundant ...Devonian genus
has long been known solely from isolated teeth and fin spines. Here, we report the first skeletal remains of
from the Famennian (Late Devonian) of the Maïder region of Morocco, revealing an anguilliform body, specialized braincase, hyoid arch, elongate jaws and rostrum, complementing its characteristic dentition and ctenacanth fin spines preceding both dorsal fins. Several of these features corroborate a likely close relationship with the Carboniferous species
, and phylogenetic analysis places both taxa securely as members of the elasmobranch stem lineage. Identified as such, phoebodont teeth provide a plausible marker for range extension of the elasmobranchs into the Middle Devonian, thus providing a new minimum date for the origin of the chondrichthyan crown-group. Among pre-Carboniferous jawed vertebrates, the anguilliform body shape of
is unprecedented, and its specialized anatomy is, in several respects, most easily compared with the modern frilled shark
. These results add greatly to the morphological, and by implication ecological, disparity of the earliest elasmobranchs.
Abstract
Elasmobranchs are highly diverse in movement, sensory capacity, and behavioural responses to the environment, leading to differences in the function and nature of associations and ...interactions between individuals, populations, and species. The term “aggregation” has been widely and variably used to describe co-occurrence across elasmobranch species, but its defining criteria remain vague and indeterminate. The lack of standardized terminology hinders comparisons across studies, species, and systems. It can cause misinterpretation about the drivers of aggregation formation or grouping behaviour and limits the impact of elasmobranch behavioural research. Here, we propose generalizable definitions of aggregation and supporting terms, adapted from more mature socioecological theory in other systems, which can be applied across elasmobranchs. We discuss the drivers and types of elasmobranch aggregations, detail the criteria required to identify an aggregation based on our definition, and offer direction on future methods and reporting to advance the field. These proposed definitions and reporting protocols standardize the study of elasmobranch socioecology, will generate greater understanding of the mechanisms underlying behaviour and distribution across species, and support more cross-system comparisons. Finally, this information can offer greater predictive power into when and how aggregations may form, leading to more proactive management strategies to mitigate anthropogenic threats.
The sustainable mitigation of human–wildlife conflicts has become a major societal and environmental challenge globally. Among these conflicts, large marine predators feeding on fisheries catches, a ...behaviour termed “depredation,” has emerged concomitantly with the expansion of the world’s fisheries. Depredation poses threats to both the socio‐economic viability of fisheries and species conservation, stressing the need for mitigation. This review synthesizes the extent and socio‐ecological impacts of depredation by sharks and marine mammals across the world, and the various approaches used to minimize it. Depredation was reported in 214 fisheries between 1979 and 2019 (70% post‐2000) and affected fleets from 44 countries, in all sectors (commercial, artisanal and recreational), and in all major fishing techniques (nets, traps and hook‐and‐lines). A total of 68 predator species were involved in depredation (20 odontocetes, 21 pinnipeds and 27 sharks), and most (73%) were subject to either by‐catch and/or retaliatory killing from fishers when interacting with gear. Impacts on fishers were primarily associated with catch losses and gear damage but often lacked assessments. Deterrence was a major mitigation approach but also the least effective. Gear modifications or behavioural adaptation by fishers were more promising. This review highlights the need for improved monitoring, and interdisciplinary and integrated research to quantify the determinants and impacts of depredation in the socio‐ecological dimension. More importantly, as the conflict is likely to escalate, efforts directed towards changing perceptions and integrating knowledge through adaptive co‐management are raised as key directions towards coexistence between fisheries and large marine predators.
Rhaetian seas in the latest Triassic transgressed from west to east over the southwest of the UK, reaching parts of South Wales and the North Somerset coast first. Evidence comes from marine ...conditions in the pre-Rhaetian Williton Member, a unit not seen further east. Here, we confirm this hypothesis with reports of diverse, Rhaetian-style fish faunas in the Williton Member, as well as evidence that the Westbury Formation bonebeds are from deeper waters than most others in the region. Our study focuses on the classic coastal section at Lilstock, which shows the entire Penarth Group and the Triassic-Jurassic boundary. The Williton Member fossil beds yield Rhaetian-type chondrichthyans (Lissodus, denticles), osteichthyan teeth (Gyrolepis, Sargodon, Saurichthys), and bivalves. The basal and higher bone beds of the Westbury Formation are dominated by osteichthyans (86.8 %, 84.7 %), with chondrichthyans relatively rare (13.2 %, 15.3 %), the opposite of what is seen at other locations in the southwest of the UK (16–59 % osteichthyans; 41–84 % chondrichthyans). The similarity of the faunal composition in the basal and higher Rhaetian bone beds is also unusual, and the dominance by bony fishes can be interpreted as evidence for deeper water than further to the east.
Numerous studies have now demonstrated that the most common method of ageing sharks and rays, counting growth zones on calcified structures, can underestimate true age. I reviewed bomb carbon dating ...(n = 15) and fluorochrome chemical marking (n = 44) age validation studies to investigate the frequency and magnitude of this phenomenon. Age was likely to have been underestimated in nine of 29 genera and 30% of the 53 populations studied, including 50% of those validated using bomb carbon dating. Length and age were strongly significant predictors of occurrence, with age typically underestimated in larger and older individuals. These characteristics suggest age underestimation is likely a systemic issue associated with the current methods and structures used for ageing. Where detected using bomb carbon dating, growth zones were reliable up to 88% of asymptotic length (L∞) and 41% of maximum age (AMax). The maximum magnitude of age underestimation, ΔMax, ranged from five to 34 years, averaging 18 years across species. Current perceptions of shark and ray life histories are informed to a large extent by growth studies that assume calcified ageing structures are valid throughout life. The widespread age underestimation documented here shows this assumption is frequently violated, with potentially important consequences for conservation and management. In addition to leading to an underestimation of longevity, the apparent loss of population age‐structure associated with it may unexpectedly bias growth and mortality parameters. Awareness of these biases is essential given shark and ray population assessments often rely exclusively on life history parameters derived from ageing studies.
Any attempt to describe the spatial ecology of sharks and rays should consider the drivers responsible for movement. Research has shown fluctuations in the environment (abiotic factors) can trigger ...movement and changes in behaviour and habitat use for many elasmobranch species. Most studies to date have selectively focused on a small number of abiotic factors (i.e. temperature, salinity); however, other factors such as dissolved oxygen, tide, photoperiod, barometric pressure and pH have also been documented to act as drivers of movement in shark and ray species. Although usually examined individually, abiotic factors rarely act in isolation and often differ in their level of influence between species, sex, ontogenetic stage, season and geographic location. This paper reviews the role of abiotic factors as a driver of movement and changes in behaviour and habitat use in elasmobranchs. In the context of a changing climate, insight into how sharks and rays may respond to fluctuating environmental conditions projected under future scenarios is required.
•Novel eDNA survey for detecting critically endangered scalloped hammerhead sharks.•Enables the first detection of scalloped hammerheads in Guam since 1971.•A single-day eDNA sampling effort ...outperforms five decades of visual surveys.•The eDNA survey method is non-invasive, cost-effective, and user friendly.•Survey application allows for rapid data generation for science-based conservation.
Among the hammerhead sharks, scalloped hammerheads (Sphyrna lewini) have undergone the steepest population declines worldwide. Due to their high susceptibility to exploitation, the species is now classified as ‘critically endangered’, the most threatened category listed by the International Union for Conservation of Nature. There is an urgent need for data on the distribution of S. lewini to inform the design and implementation of effective conservation management strategies, and mitigate the risk of global extinction. Environmental DNA (eDNA) is emerging as a powerful method to monitor the geographic distribution, population trends, and habitat usage of rare and endangered species. In comparison to traditional survey methods, eDNA methods offer lower cost, higher detection rates, and are non-invasive. At present, there is no targeted eDNA assay for the detection of S. lewini and existing methods to assess their distribution are either fisheries-dependent, leading to bias, or costly and laborious, leading to impracticality in regions of low or unknown abundance. Here we present an optimised workflow for the detection of S. lewini presence using eDNA methods, and apply these to successfully detect scalloped hammerhead sharks in Guam, of the western Pacific Ocean, where their presence has not been scientifically reported since the 1970s. The detection of S. lewini by eDNA survey methods was achieved from a single-day sampling effort, demonstrating the efficacy of the technique and workflow. If implemented, the eDNA survey methods developed here will enable the rapid generation of information on the distribution of scalloped hammerhead sharks in the western Pacific, and likely globally, and assist in the accurate placement of no-take reserves to best enable the species’ recovery.
Teeth and denticles belong to a specialized class of mineralizing epithelial appendages called odontodes. Although homology of oral teeth in jawed vertebrates is well supported, the evolutionary ...origin of teeth and their relationship with other odontode types is less clear. We compared the cellular and molecular mechanisms directing development of teeth and skin denticles in sharks, where both odontode types are retained. We show that teeth and denticles are deeply homologous developmental modules with equivalent underlying odontode gene regulatory networks (GRNs). Notably, the expression of the epithelial progenitor and stem cell marker sex-determining region Y-related box 2 (sox2) was tooth-specific and this correlates with notable differences in odontode regenerative ability. Whereas shark teeth retain the ancestral gnathostome character of continuous successional regeneration, new denticles arise only asynchronously with growth or after wounding. Sox2+ putative stem cells associated with the shark dental lamina (DL) emerge from a field of epithelial progenitors shared with anteriormost taste buds, before establishing within slow-cycling cell niches at the (i) superficial taste/tooth junction (T/TJ), and (ii) deep successional lamina (SL) where tooth regeneration initiates. Furthermore, during regeneration, cells from the superficial T/TJ migrate into the SL and contribute to newteeth, demonstrating persistent contribution of taste-associated progenitors to tooth regeneration in vivo. This data suggests a trajectory for tooth evolution involving cooption of the odontode GRN from nonregenerating denticles by sox2+ progenitors native to the oral taste epithelium, facilitating the evolution of a novel regenerative module of odontodes in the mouth of early jawed vertebrates: the teeth.
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