Seasonality and relative abundance within an elasmobranch assemblage near a major biogeographic divide.

Nearshore waters are utilized by elasmobranchs in various ways, including foraging, reproduction, and migration. Multiple elasmobranch species have been previously documented in the nearshore waters of North Carolina, USA, which has a biogeographic break at Cape Hatteras on the Atlantic coast. However, comprehensive understanding of the elasmobranch community in this region is still lacking. Monthly year-round trawling conducted along two ocean transects (near Cape Lookout and Masonboro Inlet in 5 to 18 m depth) in Onslow Bay, North Carolina provided the opportunity to examine the dynamics and seasonal patterns of this community using a multivariate approach, including permutational multivariate analysis of variance and nonparametric BIO-ENV analysis. From November 2004 to April 2008, 21,149 elasmobranchs comprised of 20 species were caught, dominated by spiny dogfish (Squalus acanthias) and clearnose skate (Rostroraja eglanteria). All species exhibited seasonal variation in abundance, but several key species contributed the most to seasonal differences in species composition within each transect. Spiny dogfish was most abundant in the winter at both locations, comprised mainly of mature females. Although clearnose skate was caught in all seasons, the species was most abundant during the spring and fall. Atlantic sharpnose (Rhizoprionodon terraenovae) was one of the most abundant species in the summer, and two distinct size cohorts were documented. Temperature appeared to be the main abiotic factor driving the community assemblage. The extensive year-round sampling provided the ability to better understand the dramatic seasonal variation in species composition and provides new information on the relative abundance of several understudied elasmobranch species that may be of significant ecological importance. Our results underscore the importance of inner continental shelf waters as important elasmobranch habitat and provide baseline data to examine for future shifts in timing and community structure at the northern portion of the biogeographic break at Cape Hatteras.

Fishing profile and commercial landings of shark and batoids in a global elasmobranchs conservation hotspot.

This paper describes the fishing profile and the temporal variation in the commercial landings of elasmobranchs in a global hotspot for their conservation and investigates the variables that influenced the landings. Census data on commercial catches were obtained between April 2008 and October 2010 from nine landing sites in Bragança (Pará, northern Brazil). Five vessel types, four fishing gears, and eight fishing techniques engaged with elasmobranch capture were identified. A total of 2,357 landings were recorded, with a total production of 354 t. The highest yields were recorded in 2009, with sharks being harvested mostly by small and medium-sized vessels, and batoids, by small vessels and canoes. Drifting nets and longlines played a prominent role in elasmobranch fisheries. The results show that the landings were influenced by days at sea, which is common in tropical fisheries. The elasmobranch data series is discontinuous as statistics are absent for most fishing sites albeit imperative for proper management, as well as relevant for decision-makers focusing on their conservation.

Evolutionary trends in the elasmobranch neurocranium.

The neurocranium (braincase) is one of the defining vertebrate characters. Housing the brain and other key sensory organs, articulating with the jaws and contributing to the shape of the anteriormost portion of the body, the braincase is undoubtedly of great functional importance. Through studying relationships between braincase shape and ecology we can gain an improved understanding of form-function relationships in extant and fossil taxa. Elasmobranchii (sharks and rays) represent an important case study of vertebrate braincase diversity as their neurocranium is simplified and somewhat decoupled from other components of the cranium relative to other vertebrates. Little is known about the associations between ecology and braincase shape in this clade. In this study we report patterns of mosaic cranial evolution in Elasmobranchii that differ significantly from those present in other clades. The degree of evolutionary modularity also differs between Selachii and Batoidea. In both cases innovation in the jaw suspension appears to have driven shifts in patterns of integration and modularity, subsequently facilitating ecological diversification. Our results confirm the importance of water depth and biogeography as drivers of elasmobranch cranial diversity and indicate that skeletal articulation between the neurocranium and jaws represents a major constraint upon the evolution of braincase shape in vertebrates.

Can denticle morphology help identify southeastern Australian elasmobranchs?

Elasmobranchs are covered in scale-like structures called dermal denticles, comprising dentine and enameloid. These structures vary across the body of an individual and between species, and are frequently shed and preserved in marine sediments. With a good understanding of denticle morphology, current and historical elasmobranch diversity and abundance might be assessed from sediment samples. Here, replicate samples of denticles from the bodies of several known (deceased) shark species were collected and characterized for morphology before being assigned morphotypes. These data were used to expand the established literature describing denticles and to investigate intra- and interspecific variability, with the aim of increasing the viability of using sediment samples to assess elasmobranch diversity and abundance. Denticle morphology was influenced more by life-history traits than by species, where demersal species were largely characterized by generalized function and defense denticles, whereas pelagic and benthopelagic species were characterized by drag-reduction denticles. Almost all species possessed abrasion strength or defense denticles on the snout, precluding their utility for separating species. In a separate manipulative experiment, samples of denticles were collected from sediments in two aquaria with known elasmobranchs to determine their utility for reliably separating species. Visual examination of denticles, morphometric measurements, scaled photographs, and reference collections allowed for some precise identification, but not always to the species level. Ongoing work to develop denticle reference collections could help to identify past and present families and, in some cases, species.

Global patterns in functional rarity of marine fish.

Rare species, which represent a large fraction of the taxa in ecological assemblages, account for much of the biological diversity on Earth. These species make substantial contributions to ecosystem functioning, and are targets of conservation policy. Here we adopt an integrated approach, combining information on the rarity of species trait combinations, and their spatial restrictedness, to quantify the biogeography of rare fish (a taxon with almost 13,000 species) in the world's oceans. We find concentrations of rarity, in excess of what is predicted by a null expectation, near the coasts and at higher latitudes. We also observe mismatches between these rarity hotspots and marine protected areas. This pattern is repeated for both major groupings of fish, the Actinopterygii (bony fish) and Elasmobranchii (sharks, skates and rays). These results uncover global patterns of rarity that were not apparent from earlier work, and highlight the importance of using metrics that incorporate information on functional traits in the conservation and management of global marine fishes.

Phylogeography, genetic diversity and population structure of the freshwater stingray, Paratrygon aiereba (Müller & Henle, 1841) (Myliobatiformes: Potamotrygonidae) in the Colombian Amazon and Orinoco basins.

The freshwater stingray Paratrygon aiereba have coloration, osteological and morphometric variations that could suggest the existence of more than one species in Colombia. In order to evaluate the phylogeography, population structure and genetic diversity for P. aiereba distributed in the Amazon and Orinoco basins, we amplified Cytochrome oxidase subunit 1 (COI) partial region of mitochondrial DNA (mtDNA) in 50 samples from eight different sub-basins. Our results suggest three phylogroups and a vicariance event occurred 43 million years ago proposing how Paratrygon diverged into the basins. A high population structure (ΦST = 0.692; p < 0.005) and a value of (K) of 3 were defined. A high genetic diversity within phylogroups was found: Phylogroup A (h = 0.64; π% = 2.48), Phylogroup B (h = 0.552; π% = 1.67), and Phylogroup C (h = 0.49; π% = 0.73). These results should be considered in local management plans, conservation programs and reclassification in at least Amazon and Orinoco.

The fossil record of extant elasmobranchs.

Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various anthropogenic pressures. The abundant fossil record of fossil taxa has allowed the tracing of the evolutionary history of modern elasmobranchs to at least 250 MYA; nonetheless, exactly how far back the fossil record of living taxa goes has never been collectively surveyed. In this study, the authors assess the representation and extent of the fossil record of elasmobranchs currently living in our oceans by collecting their oldest records and quantifying first appearance dates at different taxonomic levels (i.e., orders, families, genera and species), ecological traits (e.g., body size, habitat and feeding mechanism) and extinction risks (i.e., threatened, not threatened and data deficient). The results of this study confirm the robust representation of higher taxonomic ranks, with all orders, most of the families and over half of the extant genera having a fossil record. Further, they reveal that 10% of the current global species diversity is represented in the geological past. Sharks are better represented and extend deeper in time than rays and skates. While the fossil record of extant genera (e.g., the six gill sharks, Hexanchus) goes as far back as c. 190 MYA, the fossil record of extant species (e.g., the sand shark, Carcharias taurus Rafinesque 1810) extends c. 66 MYA. Although no significant differences were found in the extent of the fossil record between ecological traits, it was found that the currently threatened species have a significantly older fossil record than the not threatened species. This study demonstrate that the fossil record of extant elasmobranchs extends deep into the geologic time, especially in the case of threatened sharks. As such, the elasmobranch geological history has great potential to advance the understanding of how species currently facing extinction have responded to different stressors in the past, thereby providing a deep-time perspective to conservation.

Evolutionary developmental genetics of teeth and odontodes in jawed vertebrates: a perspective from the study of elasmobranchs.

Most extant vertebrates display a high variety of tooth and tooth-like organs (odontodes) that vary in shape, position over the body and nature of composing tissues. The development of these structures is known to involve similar genetic cascades and teeth and odontodes are believed to share a common evolutionary history. Gene expression patterns have previously been compared between mammalian and teleost tooth development but we highlight how the comparative framework was not always properly defined to deal with different tooth types or tooth developmental stages. Larger-scale comparative analyses also included cartilaginous fishes: sharks display oral teeth and dermal scales for which the gene expression during development started to be investigated in the small-spotted catshark Scyliorhinus canicula during the past decade. We report several descriptive approaches to analyse the embryonic tooth and caudal scale gene expressions in S. canicula. We compare these expressions wih the ones reported in mouse molars and teleost oral and pharyngeal teeth and highlight contributions and biases that arise from these interspecific comparisons. We finally discuss the evolutionary processes that can explain the observed intra and interspecific similarities and divergences in the genetic cascades involved in tooth and odontode development in jawed vertebrates.

Two new species of freshwater stingrays of the genus Paratrygon (Chondrichthyes: Potamotrygonidae) from the Orinoco basin, with comments on the taxonomy of Paratrygon aiereba

The genus Paratrygon, currently recognized as the sole monotypic genus of the family Potamotrygonidae, has a considerably greater diversity than previously indicated, including molecular studies, which supported P. aiereba (hitherto the only recognized species in the genus) as a possible species complex. Here we describe two new species of the genus that are both endemic to and sympatric in the Orinoco basin. Paratrygon aiereba, type species of the genus, is now restricted to the Amazon basin. Both new species are identified and defined through morphological characters such as coloration, dermal denticle morphology, arrangement of thorns, distribution and morphology of ventral lateral line canals, morphology of skeletal elements, and morphometrics. An extensive comparison of these characters between the new species herein described and P. aiereba is presented. Finally, a taxonomic reappraisal of P. aiereba is provided through a revision of preserved material and its original description, plus new evidence about its type-locatity, collectors, and a reconsideration of the destination of its type-specimen.(AU)

O gênero Paratrygon, reconhecido atualmente como o único gênero monotípico da família Potamotrygonidae, possui uma considerável alta diversidade do que previamente indicado, incluindo estudos moleculares, que corroboravam P. aiereba (a única espécie reconhecida para o gênero até então) como um possível complexo de espécies. Aqui descrevemos duas novas espécies do gênero para a bacia do Orinoco, ambas endêmicas e simpátricas para esta bacia. Paratrygon aiereba, espécie tipo do gênero, agora está restrita para a bacia Amazônica. Ambas novas espécies são identificadas e definidas através de caracteres morfológicos tais como coloração, morfologia dos dentículos dérmicos, arranjo dos espinhos pontiagudos, distribuição e morfologia dos canais ventrais da linha lateral, morfologia dos elementos do esqueleto e morfometria. Uma comparação extensiva destes caracteres entre as novas espécies aqui descritas e P. aiereba é apresentada. Finalmente uma reavaliação taxonômica de P. aiereba é fornecida através da revisão de espécimes preservados e de sua descrição original, além de novas evidências sobre sua possível localidade-tipo, coletores, e uma reconsideração do destino de seu espécime-tipo.(AU)

Development of the ventricular myocardial trabeculae in Scyliorhinus canicula (Chondrichthyes): evolutionary implications.

The development of the ventricular myocardial trabeculae occurs in three steps: emergence, trabeculation and remodeling. The whole process has been described in vertebrates with two different myocardial structural types, spongy (zebrafish) and compact (chicken and mouse). In this context, two alternative mechanisms of myocardial trabeculae emergence have been identified: (1) in chicken and mouse, the endocardial cells invade the two-layered myocardium; (2) in zebrafish, cardiomyocytes from the monolayered myocardium invaginate towards the endocardium. Currently, the process has not been studied in detail in vertebrates having a mixed type of ventricular myocardium, with an inner trabecular and an outer compact layer, which is presumptively the most primitive morphology in gnathostomes. We studied the formation of the mixed ventricular myocardium in the lesser spotted dogfish (Scyliorhinus canicula, Elasmobranchii), using light, scanning and transmission electron microscopy. Our results show that early formation of the mixed ventricular myocardium, specifically the emergence and the trabeculation steps, is driven by an endocardial invasion of the myocardium. The mechanism of trabeculation of the mixed ventricular myocardium in chondrichthyans is the one that best reproduces how this developmental process has been established from the beginning of the gnathostome radiation. The process has been apparently preserved throughout the entire group of sarcopterygians, including birds and mammals. In contrast, teleosts, at least those possessing a mostly spongy ventricular myocardium, seem to have introduced notable changes in their myocardial trabeculae development.

Optimal soak times for Baited Remote Underwater Video Station surveys of reef-associated elasmobranchs.

Effective sampling of marine communities is essential to provide robust estimates of species richness and abundance. Baited Remote Underwater Video Stations (BRUVS) are a useful tool in assessment of fish assemblages, but research on the optimal sampling period required to record common and rare elasmobranch species is limited. An appropriate 'soak time' (time elapsed between settlement of the BRUVS on the seabed and when it is hauled off the seabed) requires consideration, since longer soak times may be required to record species rare in occurrence, or sightings in areas of generally low elasmobranch abundance. We analysed 5352 BRUVS deployments with a range of soak times across 21 countries in the Coral Triangle and Pacific Ocean, to determine the optimal soak time required for sampling reef-associated elasmobranchs, considering species rarity, and community abundance at each site. Species were categorised into 4 'rarity' groups (very rare to common), by their relative occurrence in the dataset, defined simply by the proportion of BRUVS on which they occurred. Individual BRUVS were categorised into 3 'abundance' groups (low to high) by overall relative elasmobranch abundance, defined as total number of all elasmobranchs sighted per unit of sampling effort. The effects of BRUVS soak times, and levels of rarity and abundance groupings, on the time to first sighting (TFS) and time to maximum number of elasmobranchs observed (tMaxN) were examined. We found that TFS occurred earlier for species groups with high occurrence, and on BRUVS with high elasmobranch abundance, yet longer soak times were not essential to observe rarer species. Our models indicated an optimum of 95% of both sighting event types (TFS, tMaxN) was recorded within 63-77 minutes, and a soak time of 60 minutes recorded 78-94% of the elasmobranch sighting events recorded (78-94% of TFS events and 82-90% of tMaxN events), when species rarity and abundance on BRUVS was accounted for. Our study shows that deployments of ~ 77 minutes are optimal for recording all species we observed, although 60 minutes soak time effectively samples the majority of elasmobranch species in shallow coral reef habitats using BRUVS.

Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan.

Recent sampling efforts in Madagascar and Zanzibar, as well as examinations of six-gilled sawsharks in several museum collections provided evidence for a complex of species within Pliotrema warreni Regan. The present manuscript contains a redescription of P. warreni involving the syntypes and additional material, as well as formal descriptions of two new species of Pliotrema Regan. All specimens of both new species were found in the western Indian Ocean. Individuals of the first new species, hereafter referred to as P. kajae sp. nov., were identified originating from Madagascar and the Mascarene Ridge. Specimens of the second new species, hereafter referred to as P. annae sp. nov., were only found off Zanzibar. Pliotrema kajae sp. nov. appears to inhabit upper insular slopes and submarine ridges at depths of 214-320 m, P. annae sp. nov. so far is only known from shallow waters (20-35 m). Both new species differ from P. warreni in a number of characteristics including the known distribution range and fresh coloration. Taxonomical differences include barbels that are situated approximately half way from rostral tip to mouth, with prebarbel length equidistant from barbel origin to symphysis of the upper jaw in P. kajae sp. nov. and P. annae sp. nov. (vs. about two thirds way from rostral tip to mouth, with prebarbel length about twice the distance from barbel origin to symphysis of upper jaw in P. warreni) and rostra that are clearly and slightly constricted between barbel origin and nostrils, respectively (vs. rostrum not constricted). Pliotrema kajae sp. nov. differs from P. annae sp. nov. in a longer snout, more numerous large lateral rostral teeth and upper jaw tooth rows, jaw teeth with (vs. without) sharp basal folds, and coloration, particularly pale to light brown (vs. medium to dark brown) dorsal coloration with (vs. without) two indistinct yellowish stripes. A revised diagnosis of Pliotrema and a key to the species are provided.

The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution.

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 Phoebodus has long been known solely from isolated teeth and fin spines. Here, we report the first skeletal remains of Phoebodus 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 Thrinacodus gracia, 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 Phoebodus is unprecedented, and its specialized anatomy is, in several respects, most easily compared with the modern frilled shark Chlamydoselachus. These results add greatly to the morphological, and by implication ecological, disparity of the earliest elasmobranchs.

Genetic and historical evidence of common sawsharks Pristiophorus cirratus in the waters of southern Queensland.

In 2011, a male pristiophorid was caught by a prawn trawler north east of Cape Moreton, Queensland, Australia. Molecular analyses confirmed the specimen to be the common sawshark Pristiophorus cirratus. Historical catch data indicate the occurrence of the species in the region but this is the first verified record of P. cirratus occurring in the waters of southern Queensland. Together, these records extend the recognised northern limit of P. cirratus by c. 500 km, which suggests that further investigation of its distribution is warranted.

Evolutionary Plasticity in Detoxification Gene Modules: The Preservation and Loss of the Pregnane X Receptor in Chondrichthyes Lineages.

To appraise how evolutionary processes, such as gene duplication and loss, influence an organism's xenobiotic sensitivity is a critical question in toxicology. Of particular importance are gene families involved in the mediation of detoxification responses, such as members of the nuclear receptor subfamily 1 group I (NR1I), the pregnane X receptor (PXR), and the constitutive androstane receptor (CAR). While documented in multiple vertebrate genomes, PXR and CAR display an intriguing gene distribution. PXR is absent in birds and reptiles, while CAR shows a tetrapod-specific occurrence. More elusive is the presence of PXR and CAR gene orthologs in early branching and ecologically-important Chondrichthyes (chimaeras, sharks and rays). Therefore, we investigated various genome projects and use them to provide the first identification and functional characterization of a Chondrichthyan PXR from the chimaera elephant shark (Callorhinchus milii, Holocephali). Additionally, we substantiate the targeted PXR gene loss in Elasmobranchii (sharks and rays). Compared to other vertebrate groups, the chimaera PXR ortholog displays a diverse expression pattern (skin and gills) and a unique activation profile by classical xenobiotic ligands. Our findings provide insights into the molecular landscape of detoxification mechanisms and suggest lineage-specific adaptations in response to xenobiotics in gnathostome evolution.

Elephantfish.

Justin Rizzari and Brittany Finucci introduce elephant fish, a small group of shark relatives.

DNA Barcode Reveals the Bycatch of Endangered Batoids Species in the Southwest Atlantic: Implications for Sustainable Fisheries Management and Conservation Efforts.

Today, elasmobranchs are one the most threatened vertebrate groups worldwide. In fact, at least 90% of elasmobranch species are listed in the International Union for Conservation of Nature (IUCN) Red List, while more than 40% are data-deficient. Although these vertebrates are mainly affected by unsustainable fishery activities, bycatch is also one of the major threats to sharks and batoids worldwide, and represents a challenge for both sustainable fishery management and for biodiversity and conservational efforts. Thus, in this study, DNA barcode methodology was used to identify the bycatch composition of batoid species from small-scale industrial fisheries in the southwest Atlantic and artisanal fisheries from southeast Brazil. A total of 228 individuals belonging to four Chondrichthyes orders, seven families, and at least 17 distinct batoid species were sequenced; among these individuals, 131 belonged to species protected in Brazil, 101 to globally threatened species, and some to species with trade restrictions provided by Appendix II of the Convention on International Trade in Endangered Species (CITES). These results highlight the impacts on marine biodiversity of bycatch by small-scale industrial and unmanaged artisanal fisheries from the southwest Atlantic, and support the implementation of DNA-based methodologies for species-specific identification in data-poor fisheries as a powerful tool for improving the quality of fisheries' catch statistics and for keeping precise bycatch records.

Environmental DNA reveals tropical shark diversity in contrasting levels of anthropogenic impact.

Sharks are charismatic predators that play a key role in most marine food webs. Their demonstrated vulnerability to exploitation has recently turned them into flagship species in ocean conservation. Yet, the assessment and monitoring of the distribution and abundance of such mobile species in marine environments remain challenging, often invasive and resource-intensive. Here we pilot a novel, rapid and non-invasive environmental DNA (eDNA) metabarcoding approach specifically targeted to infer shark presence, diversity and eDNA read abundance in tropical habitats. We identified at least 21 shark species, from both Caribbean and Pacific Coral Sea water samples, whose geographical patterns of diversity and read abundance coincide with geographical differences in levels of anthropogenic pressure and conservation effort. We demonstrate that eDNA metabarcoding can be effectively employed to study shark diversity. Further developments in this field have the potential to drastically enhance our ability to assess and monitor elusive oceanic predators, and lead to improved conservation strategies.

Biodiversity, Life History, and Conservation of Northeastern Pacific Chondrichthyans.

The sharks, batoids, and chimaeras, collectively the class Chondrichthyes, are one of the most successful groups of fishes, with over 1250 species globally. Recent taxonomic revisions have increased their diversity by about 20% over the past 17 years (2000-2016). The Northeast Pacific Ocean is one of the top 20 most diverse regions/countries on the globe with 77 chondrichthyan species, a number less than a quarter that of the most species-rich area (Australia) but that has increased by 10% since 2000 to include three new species (two skates and a chimaera). In this chapter we discuss the species richness of chondrichthyans occurring in the Northeast Pacific Ocean, characterize their life histories, briefly review several fisheries, and summarize the conservation status of those chondrichthyans occurring in the region. Detailed descriptions and evaluations of fisheries can be found in Chapter 7 of AMB Volume 78.