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.

The effect of jaw suspension on cartilage strength in elasmobranchs.

The jaws and their supporting cartilages are tessellated in elasmobranchs and exhibit an abrupt increase in stiffness under compression. The major jaw-supporting cartilage, the hyomandibula, varies widely by shape and size and the extent of the load-bearing role is hypothesized to be inversely related to the number of craniopalatine articulations. Here, we test this hypothesis by evaluating the strength of the hyomandibular cartilage under compression in 13 species that represent all four jaw suspension systems in elasmobranchs (amphistyly, orbitostyly, hyostyly, and euhyostyly). The strength of the hyomandibular cartilages was measured directly using a material testing machine under compressive load, and indirectly by measuring morphological variables putatively associated with strength. The first measure of strength is force to yield (Fy), which was the peak force (N) exerted on the hyomandibula before plastic deformation. The second measure was compressive yield strength (σy, also called yield stress), which is calculated as peak force (N) before plastic deformation/cross-sectional area (mm2) of the specimen. Our results show that the load-bearing role of the hyomandibular cartilage, as measured by yield strength, is inversely related to the number of craniopalatine articulations, as predicted. Force to yield was lower for euhyostylic jaw suspensions and similar for the others. We also found that mineralization is associated with greater yield strength, while the second moment of area is associated with greater force to yield.

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.

Smooth muscle "microsphincters" in the gastric mucosa of stingrays (Elasmobranchii, Dasyatidae).

The occurrence of smooth muscle sphincters around gastric gland tubules in the cardiac stomach of some elasmobranch species is reported for the first time. These "microsphincters" took the form of a twisted torus, approximately 12-16 muscle fibres thick, that could fully constrict the gland tubules. However, their inconsistent positioning does not suggest a role in partitioning the tubules from the stomach lumen or in modulating secretory activity. Further research is required to ascertain the full taxonomic occurrence of these structures across the Elasmobranchii and to elucidate their function.

Manta-like planktivorous sharks in Late Cretaceous oceans.

The ecomorphological diversity of extinct elasmobranchs is incompletely known. Here, we describe Aquilolamna milarcae, a bizarre probable planktivorous shark from early Late Cretaceous open marine deposits in Mexico. Aquilolamna, tentatively assigned to Lamniformes, is characterized by hypertrophied, slender pectoral fins. This previously unknown body plan represents an unexpected evolutionary experimentation with underwater flight among sharks, more than 30 million years before the rise of manta and devil rays (Mobulidae), and shows that winglike pectoral fins have evolved independently in two distantly related clades of filter-feeding elasmobranchs. This newly described group of highly specialized long-winged sharks (Aquilolamnidae) displays an aquilopelagic-like ecomorphotype and may have occupied, in late Mesozoic seas, the ecological niche filled by mobulids and other batoids after the Cretaceous-Paleogene boundary.

Dental microwear texture analysis as a tool for dietary discrimination in elasmobranchs.

As abundant and widespread predators, elasmobranchs play influential roles in food-web dynamics of marine communities. Clearly, these trophic interactions have significant implications for fisheries management and marine conservation, yet elasmobranch diet is relatively understudied; for the majority of species little or no quantitative dietary data exist. This reflects the difficulties of direct observation of feeding and stomach contents analysis in wild elasmobranchs. Here, by quantifying the 3D surface textures that develop on tooth surfaces as a consequence of feeding, we show that tooth microwear varies with diet in elasmobranchs, providing a new tool for dietary analysis. The technique can be applied to small samples and individuals with no gut contents, and thus offers a way to reduce the impact on wild elasmobranch populations of analysing their dietary ecology, especially relevant in conservation of endangered species. Furthermore, because microwear accumulates over longer periods of time, analysis of texture overcomes the 'snapshot bias' of stomach contents analysis. Microwear texture analysis has the potential to be a powerful tool, complementing existing techniques such as stable isotope analysis, for dietary analysis in living and extinct elasmobranchs.

Visual discrimination and resolution in freshwater stingrays (Potamotrygon motoro).

Potamotrygon motoro has been shown to use vision to orient in a laboratory setting and has been successfully trained in cognitive behavioral studies using visual stimuli. This study explores P. motoro's visual discrimination abilities in the context of two-alternative forced-choice experiments, with a focus on shape and contrast, stimulus orientation, and visual resolution. Results support that stingrays are able to discriminate stimulus-presence and -absence, overall stimulus contrasts, two forms, horizontal from vertical stimulus orientations, and different colors that also vary in brightness. Stingrays tested in visual resolution experiments demonstrated a range of visual acuities from < 0.13 to 0.23 cpd under the given experimental conditions. Additionally, this report includes the first evidence for memory retention in this species.

Repeatability of baited remote underwater video station (BRUVS) results within and between seasons.

Baited remote underwater video stations (BRUVS) are increasingly being used to evaluate and monitor reef communities. Many BRUVS studies compare multiple sites sampled at single time points that may differ from the sampling time of another site. As BRUVS use grows in its application to provide data relevant to sustainable management, marine protected area success, and overall reef health, understanding repeatability of sampling results is vital. We examined the repeatability of BRUVS results for the elasmobranch community both within and between seasons and years, and explored environmental factors affecting abundances at two sites in Indonesia. On 956 BRUVS, 1139 elasmobranchs (69% rays, 31% sharks) were observed. We found consistent results in species composition and abundances within a season and across years. However, elasmobranch abundances were significantly higher in the wet season. The elasmobranch community was significantly different between the two sites sampled, one site being more coastal and easily accessed by fishermen. Our results demonstrate that while BRUVS are a reliable and repeatable method for surveying elasmobranchs, care must be taken in the timing of sampling between different regions to ensure that any differences observed are due to inherent differences amongst sampling areas as opposed to seasonal dissimilarities.

Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture.

Cartilaginous fish are located at a pivotal point in phylogeny where the adaptive immune system begins to resemble that of other, more-derived jawed vertebrates, including mammals. For this reason, sharks and other cartilaginous fish are ideal models for studying the natural history of immunity. Insights from such studies may include distinguishing the (evolutionarily conserved) fundamental aspects of adaptive immunity from the (more recent) accessory. Some lymphoid tissues of sharks, including the thymus and spleen, resemble those of mammals in both appearance and function. The cartilaginous skeleton of sharks has no bone marrow, which is also absent in bony fish despite calcified bone, but cartilaginous fish have other Leydig's and epigonal organs that function to provide hematopoiesis analogous to mammalian bone marrow. Conserved across all vertebrate phylogeny in some form is gut-associated lymphoid tissues, or GALT, which is seen from agnathans to mammals. Though it takes many forms, from typhlosole in lamprey to Peyer's patches in mammals, the GALT serves as a site of antigen concentration and exposure to lymphocytes in the digestive tract. Though more complex lymphoid organs are not present in agnathans, they have several primitive tissues, such as the thymoid and supraneural body, that appear to serve their variable lymphocyte receptor-based adaptive immune system. There are several similarities between the adaptive immune structures in cartilaginous and bony fish, such as the thymus and spleen, but there are mechanisms employed in bony fish that in some instances bridge their adaptive immune systems to that of tetrapods. This review summarizes what we know of lymphoid tissues in cartilaginous fishes and uses these data to compare primary and secondary tissues in jawless, cartilaginous, and bony fishes to contextualize the early natural history of vertebrate mucosal immune tissues.

Social media posts reveal the geographic range of the Critically Endangered clown wedgefish, Rhynchobatus cooki.

The shark-like rays of the family Rhinidae (wedgefishes) are globally threatened with extinction. The poorly known clown wedgefish, Rhynchobatus cooki has historically been recorded only from fish markets in Singapore and Jakarta, Indonesia. Its natural geographic range has until now gone undocumented. Social media posts revealed the first wild records of this Critically Endangered species; six records were located between 2015 and 2020 from small-scale fisheries in Lingga and Singkep Islands, Indonesia. These results demonstrate the utility of social media searches to identify biogeographic records of cryptic and data-poor species.

Paratrygon aiereba irradiated anti-mucus serum reduce edematogenic activity induced in experimental model.

Accidents by freshwater stingrays are common in northern Brazil, there is no specific therapy for high morbidity and local tissue destruction. The irradiation of venoms and toxins by ionizing radiation has been used to produce appropriate immunogens for the production of antisera. We planned to study the efficacy of stinging mucus irradiation in the production of antisera, with serum neutralization assays of edematogenic activity and quantification of cytokines performed in animal models of immunization with native and irradiated mucus of Paratrygon aiereba, a large freshwater stingray. Antiserum potency and its cross-reactivity with mucus from other freshwater stingrays were detected by ELISA. Immunization models demonstrated the ability to stimulate a strong humoral response with elevated levels of serum IgG detectable by ELISA, and both native and irradiated mucus were immunogenic and capable of recognizing mucus proteins from other freshwater neotropical stingrays. Mucus P. aiereba causes cellular and humoral adaptive immune responses in cells of immunized mice producing antibodies and cytokines such as TNF-α, IL-6 and IL-17. Rabbit antisera immunized with mucus from P. aiereba irradiated at 2 kGy showed a significant reduction of mucus-induced edematogenic activity in mice. Our data suggest that the use of antisera against freshwater stingray mucus show the possibility of specific therapy for these accidents.

Contrasting life-history traits of two sympatric smooth-hound species: implication for vulnerability.

In this study, life-history traits (maximum and average size, size at maturity and fecundity) of two congeneric smooth-hounds, Mustelus mustelus and Mustelus punctulatus, which share a geographical distribution and experience a similar fishing exploitation, were estimated and compared between species. The results indicated a lower maximum and average size, a lower size at maturity and a higher fecundity in M. punctulatus compared with those in M. mustelus. Considering that these two species co-occur in the same areas and are caught by the same fishing gears, the results indicate a higher vulnerability to exploitation of M. mustelus compared with that of M. punctulatus.

The geographic distribution of reef and oceanic manta rays (Mobula alfredi and Mobula birostris) in Australian coastal waters.

The known distribution of manta rays in Australian waters is patchy, with records primarily centred around tourism hotspots. We collated 11,614 records of Mobula alfredi from photo-ID databases (n = 10,715), aerial surveys (n = 378) and online reports (n = 521). The study confirms an uninterrupted coastal distribution from north of 26°S and 31°S on the west and east coasts, respectively. More southerly M. alfredi records relate to warm-water events with a southernmost extent at 34°S. Coastal sightings of Mobula birostris were rare (n = 32), likely reflecting a preference for offshore waters, but encompass a wider latitudinal extent than M. alfredi of 10-40°S.

Diet composition of starry smooth-hound Mustelus asterias and methodological considerations for assessing the trophic level of predatory fish.

The stomach contents of 640 starry smooth-hound Mustelus asterias from the north-east Atlantic were examined. The diet was dominated by crustaceans (98.8% percentage of index of relative importance, %IRI), with the two main prey species being hermit crab Pagurus bernhardus (34% IRI) and flying crab Liocarcinus holsatus (15% IRI). Ontogenetic dietary preferences showed that smaller individuals [20-69 cm total length (LT ) n = 283] had a significantly lower diversity of prey than larger individuals (70-124 cm LT , n = 348); however, 18 prey species were found exclusively in smaller individuals and eight prey taxa were found exclusively in larger individuals. Larger commercially important brachyurans such as edible crab Cancer pagurus and velvet swimming crab Necora puber were more prevalent in the diet of larger individuals. Specimens from the North Sea ecoregion had a lower diversity of prey types for a given sample size than fish from the Celtic Seas ecoregion. Whilst cumulative prey curves did not reach an asymptote, this was primarily due to the high taxonomic resolution utilized and 95% of the diet was described by just seven crustacean taxa. The trophic level (TL) was calculated as 4.34 when species-level prey categories were used. This fine-scale taxonomic resolution resulted in a TL estimate close to a whole level above that estimated using wider taxonomic groupings. This large bias has important methodological implications for TL studies based on categorized prey data, particularly those of predatory fish.

Feeding habits of the cockfish, Callorhinchus callorynchus (Holocephali: Callorhinchidae) from off northern Argentina

The feeding habits of Callorhinchus callorynchus were investigated in coastal waters off northern Argentina. The effect of body size, seasons and regions was evaluated on female diet composition using a multiple-hypothesis modelling approach. Callorhinchus callorynchus fed mainly on bivalves (55.61% PSIRI), followed by brachyuran crabs (10.62% PSIRI) and isopods (10.13% PSIRI). Callorhinchus callorynchus females showed changes in the diet composition with increasing body size and also between seasons and regions. Further, this species is able to consume larger bivalves as it grows. Trophic level was 3.15, characterizing it as a secondary consumer. We conclude that C. callorynchus showed a behavior of crushing hard prey, mainly on bivalves, brachyuran, gastropods and anomuran crabs. Females of this species shift their diet with increasing body size and in response to seasonal and regional changes in prey abundance or distribution.(AU)

RESUMEN Los hábitos alimentarios de Callorhinchus callorynchus fueron investigados en las aguas costeras del norte de Argentina. Se evaluó el efecto del tamaño del cuerpo, la temporada y la región sobre la composición de la dieta de las hembras mediante un enfoque de modelado de múltiples hipótesis. Callorhinchus callorynchus se alimentó principalmente de bivalvos (55,61% PSIRI), seguido de cangrejos brachyuras (10,62% PSIRI) y de isópodos (10,13% PSIRI). Las hembras de C. callorynchus presentaron cambios en la dieta con incremento del tamaño del cuerpo, la temporada y la región. Además, esta especie es capaz de consumir bivalvos de mayor tamaño a medida que incrementa el tamaño del cuerpo. El nivel trófico fue calculado en 3,15, caracterizando a esta especie como un consumidor secundario. Concluimos que C. callorynchus presentó un comportamiento de triturador de presas duras, principalmente bivalvos, cangrejos y gasterópodos. Las hembras de esta especie cambian su dieta con el incremento del tamaño del cuerpo y en respuesta a cambios temporales y regionales en la abundancia y distribución de sus presas.(AU)

Twins! Microsatellite analysis of two embryos within one egg case in oviparous elasmobranchs.

Elasmobranchs display various reproductive modes, which have been key to their evolutionary success. In recent decades there has been a rise in the number of reported cases of foetal abnormalities including fertilised, double-embryos held within one egg capsule, hereafter referred to as twins. Previously, the occurrences of twin egg cases have been reported in two batoid and one shark species. We report the first cases of twins in three species of oviparous elasmobranchs: the undulate ray (Raja undulata), the nursehound (Scyliorhinus stellaris), and the small-spotted catshark (Scyliorhinus canicula). We investigated the genetic relationships between the twins in S. stellaris, and S. canicula using microsatellite markers. Whilst the S. stellaris twins displayed the same genotypes, we found that the S. canicula twin individuals arose through heteropaternal superfecundation. This is the first reported incidence of such a paternity in elasmobranchs. The relationship between environmental change and reproductive strategy in elasmobranchs is unclear and further research is needed to determine its effect on the prevalence and mechanisms of formation of elasmobranch twins.

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.

Shearing overbite and asymmetrical jaw motions facilitate food breakdown in a freshwater stingray, Potamotrygon motoro.

Many species of fish process their prey with cyclic jaw motions that grossly resemble those seen in mammalian mastication, despite starkly different tooth and jaw morphologies. The degree of similarity between the processing behaviors of these disparate taxa has implications for our understanding of convergence in vertebrate feeding systems. Here, we used XROMM (X-ray reconstruction of moving morphology) to investigate prey processing behavior of Potamotrygon motoro, the ocellate river stingray, which has recently been found to employ asymmetrical, shearing jaw motions to break down its prey. We found that P. motoro modulates its feeding kinematics to produce two distinct types of chew cycles: compressive cycles and overbite cycles. The latter are characterized by over-rotation of the upper jaw relative to the lower jaw, past the expected occlusal limit, and higher levels of bilateral asymmetry as compared with compressive chews. We did not find evidence of the mediolateral shearing motions typical of mammalian mastication, but overbite cycles appear to shear the prey item between the upper and lower toothplates in a propalinal fashion. Additionally, comparison of hyomandibular and jaw motions demonstrates that the angular cartilages decouple jaw displacement from hyomandibular displacement in rostrocaudal and mediolateral directions. The multiple similarities between mammalian mastication and the dynamic processing behavior of P. motoro support the use of sub-family Potamotrygoninae as a model for studying evolutionary convergence of mastication-like processing.

Electroreception in marine fishes: chondrichthyans.

Electroreception in marine fishes occurs across a variety of taxa and is best understood in the chondrichthyans (sharks, skates, rays, and chimaeras). Here, we present an up-to-date review of what is known about the biology of passive electroreception and we consider how electroreceptive fishes might respond to electric and magnetic stimuli in a changing marine environment. We briefly describe the history and discovery of electroreception in marine Chondrichthyes, the current understanding of the passive mode, the morphological adaptations of receptors across phylogeny and habitat, the physiological function of the peripheral and central nervous system components, and the behaviours mediated by electroreception. Additionally, whole genome sequencing, genetic screening and molecular studies promise to yield new insights into the evolution, distribution, and function of electroreceptors across different environments. This review complements that of electroreception in freshwater fishes in this special issue, which provides a comprehensive state of knowledge regarding the evolution of electroreception. We conclude that despite our improved understanding of passive electroreception, several outstanding gaps remain which limits our full comprehension of this sensory modality. Of particular concern is how electroreceptive fishes will respond and adapt to a marine environment that is being increasingly altered by anthropogenic electric and magnetic fields.

Elephantfish.

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