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Abstract The notion that closely related species resemble each other in ecological niche space (i.e., phylogenetic dependence) has been a long‐standing, contentious paradigm in evolutionary biology, the incidence of which is important for predicting the ecosystem‐level effects of species loss. Despite being examined across a multitude of terrestrial taxa, many aspects of niche conservatism have yet to be explored in marine species, especially for characteristics related to resource use and trophic behavior (Eltonian niche characteristics, ENCs). We combined ENCs derived from stable isotope ratios at assemblage‐ and species‐levels with phylogenetic comparative methods, to test the hypotheses that benthic marine fishes (1) exhibit similar assemblage‐wide ENCs regardless of geographic location and (2) display phylogenetically dependent ENCs at the species level. We used a 12‐species sub‐set of the monophyletic group Rajidae sampled from three independent assemblages (Central California, Gulf of Alaska, and Northwest Atlantic), which span two ocean basins. Assemblage‐level ENCs implied low trophic diversity and high evenness, suggesting that Rajidae assemblages may exhibit a well‐defined trophic role, a trend consistent regardless of geographic location. At the species level, we found evidence for phylogenetic dependence of ENCs relating to trophic diversity (i.e., isotopic niche width; SEAc). Whether individuals can be considered functional equivalents across assemblages is hard to ascertain because we did not detect a significant phylogenetic signal for ENCs relating to trophic function (e.g., trophic position). Thus, additional, complimentary approaches are required to further examine the phylogenetic dependence of species functionality. Our approach illustrates the potential of stable isotope‐derived niche characteristics to provide insight on macroecological processes occurring across evolutionary time, which could help predict how assemblages may respond to the effects of species loss.

The notion that closely related species resemble each other in ecological niche space (i.e., phylogenetic dependence) has been a long-standing, contentious paradigm in evolutionary biology, the incidence of which is important for predicting the ecosystem-level effects of species loss. Despite being examined across a multitude of terrestrial taxa, many aspects of niche conservatism have yet to be explored in marine species, especially for characteristics related to resource use and trophic behavior (Eltonian niche characteristics, ENCs). We combined ENCs derived from stable isotope ratios at assemblage- and species-levels with phylogenetic comparative methods, to test the hypotheses that benthic marine fishes (1) exhibit similar assemblage-wide ENCs regardless of geographic location and (2) display phylogenetically dependent ENCs at the species level. We used a 12-species sub-set of the monophyletic group Rajidae sampled from three independent assemblages (Central California, Gulf of Alaska, and Northwest Atlantic), which span two ocean basins. Assemblage-level ENCs implied low trophic diversity and high evenness, suggesting that Rajidae assemblages may exhibit a well-defined trophic role, a trend consistent regardless of geographic location. At the species level, we found evidence for phylogenetic dependence of ENCs relating to trophic diversity (i.e., isotopic niche width; SEAc). Whether individuals can be considered functional equivalents across assemblages is hard to ascertain because we did not detect a significant phylogenetic signal for ENCs relating to trophic function (e.g., trophic position). Thus, additional, complimentary approaches are required to further examine the phylogenetic dependence of species functionality. Our approach illustrates the potential of stable isotope-derived niche characteristics to provide insight on macroecological processes occurring across evolutionary time, which could help predict how assemblages may respond to the effects of species loss.

This article explores the reproductive behavior of bluntnose sixgill sharks in the deep sea. The researchers used remote deep-sea camera systems to observe the sharks and found evidence of mating scars on one female shark. However, they acknowledge that there could be other explanations for the scarring. The study emphasizes the need for further research on the reproductive habitats of deep-sea sharks and the importance of conservation efforts in these areas. Additionally, the article discusses the capture of a pregnant bluntnose sixgill shark in the Gulf of Tunis and highlights the importance of deep diving by large marine predators. It also emphasizes the need for diverse perspectives in deep-sea ocean field programs. [Extracted from the article]

The abundances of migratory shark species observed throughout the Mid‐Atlantic Bight (MAB) during productive summer months suggest that this region provides critical habitat and prey resources to these taxa. However, the principal prey assemblages sustaining migratory shark biomass in this region are poorly defined. We applied high‐throughput DNA metabarcoding to shark feces derived from cloacal swabs across nine species of Carcharhinid and Lamnid sharks to (1) quantify the contribution of broad taxa (e.g., invertebrates, fishes) supporting shark biomass during seasonal residency in the MAB and (2) determine whether the species displayed distinct dietary preference indicative of resource partitioning. DNA metabarcoding resulted in high taxonomic (species‐level) resolution of shark diets with actinopterygian and elasmobranch fishes as the dominant prey categories across the species. DNA metabarcoding identified several key prey groups consistent across shark taxa that are likely integral for sustaining their biomass in this region, including Atlantic menhaden (Brevoortia tyrannus), Atlantic mackerel (Scomber scombrus), and benthic elasmobranchs, including skates. Our results are consistent with previously published stomach content data for the shark species of similar size range in the Northwest Atlantic Ocean, supporting the efficacy of cloacal swab DNA metabarcoding as a minimally invasive diet reconstruction technique. The high reliance of several shark species on Atlantic menhaden could imply wasp‐waist food‐web conditions during the summer months, whereby high abundances of forage fishes sustain a diverse suite of migratory sharks within a complex, seasonal food web. [ABSTRACT FROM AUTHOR]

Understanding the factors shaping patterns of ecological resilience is critical for mitigating the loss of global biodiversity. Throughout aquatic environments, highly mobile predators are thought to serve as important vectors of energy between ecosystems thereby promoting stability and resilience. However, the role these predators play in connecting food webs and promoting energy flow remains poorly understood in most contexts. Using carbon and nitrogen isotopes, we quantified the use of several prey resource pools (small oceanic forage, large oceanics, coral reef, and seagrass) by 17 species of elasmobranch fishes (n = 351 individuals) in The Bahamas to determine their functional diversity and roles as ecosystem links. We observed remarkable functional diversity across species and identified four major groups responsible for connecting discrete regions of the seascape. Elasmobranchs were responsible for promoting energetic connectivity between neritic, oceanic and deep-sea ecosystems. Our findings illustrate how mobile predators promote ecosystem connectivity, underscoring their functional significance and role in supporting ecological resilience. More broadly, strong predator conservation efforts in developing island nations, such as The Bahamas, are likely to yield ecological benefits that enhance the resilience of marine ecosystems to combat imminent threats such as habitat degradation and climate change. [ABSTRACT FROM AUTHOR]

Elasmobranch fishes (sharks, skates and rays) are some of the most morphologically and behaviourally diverse vertebrates on the planet, demonstrating a wide range of feeding strategies. The nurse shark (Ginglymostoma cirratum) is a large, widely distributed shark species which is commonly associated with tropical and subtropical reefs worldwide; yet it remains vastly understudied relative to other large species. To advance our understanding of nurse shark behaviour and ecology, we used opportunistic video observations gathered throughout the islands of Turks and Caicos from September 2020 to April 2021. We made 233 observations from 78 camera deployments and identified five behaviours, four of which were attributed to foraging. Stationary feeding behaviour was most commonly observed and strongly influenced by habitat type with a greater number of observations occurring on sand banks relative to reef habitat. Unique to this study was the first empirical description of pectoral positioning, by which individuals can position their body relative to a food-source through mobilization of the pectoral fins. We discuss these findings in relation to mechanical processes and kinematics, and the implications of expanding our knowledge of the functional role nurse sharks play in the transfer of energy across tropical seascapes. [ABSTRACT FROM AUTHOR]

Caribbean ocean environments house several large deep-sea basins that remain poorly surveyed. Here we report observations of benthic faunal communities attracted to remote, deep-sea video landers deployed at depths between 262 and 1100 m in two deep basins in The Bahamas, the Tongue of the Ocean (n = 18 deployments) and Exuma Sound (n = 11 deployments). The video comprises >8000 min of survey data across five years of sampling (2018–2022). We estimated regional deep-sea particulate organic carbon (POC) flux using satellite-derived observations and a model of POC decay with depth to assess potential food availability to benthic communities living deeper than 800 m in these basins. The benthic POC flux helped to contextualize potential drivers of faunal biodiversity and abundances estimated from the lander measurements. Throughout twilight zone depths of The Bahamas (defined here as approximately 200–1000 m) we identified taxa from 22 families across invertebrates, teleost fishes, and elasmobranchs. Faunal communities were largely dominated by giant isopods (Bathynomus sp.), gulper sharks (Centrophorus sp.), and swimming sea cucumbers (Enypniastes eximia). Despite sampling biases toward larger individuals, our findings suggest that Bahamian twilight zone communities comprise a diversity of large predator species that are potentially sustained through high energetic connectivity with shallow neritic sources of organic carbon. Our findings suggest that the Central and Southern Tongue of the Ocean should be the focus of future sampling efforts given a lack of historical sampling combined with high export productivity to depth. This study provides new insight into community composition, assemblage structure, and POC flux in Caribbean deep-sea ecosystems, shedding light on previously unrecognized patterns of biodiversity. • We surveyed poorly studied Bahamian deep-sea basins using novel landers. • Communities were dominated by large sharks and giant isopods. • Satellite derived POC flux was highly variable across space. • Further work is needed to document cryptic biodiversity in these regions. [ABSTRACT FROM AUTHOR]

Hypomelanosis refers to a suite of skin pigment abnormalities, including albinism, leucism and piebaldism. While documented across many vertebrate species, examples of hypomelanosis are rarely seen in chondrichthyans, with little insight into the potential effects on survival. Here, we report the first observation of abnormal skin pigmentation indicative of piebaldism in the Atlantic nurse shark Ginglymostoma cirratum, representing only the second reported case of skin aberrations for this species. This extremely rare observation is discussed in the broader context of fitness variation and long‐term survival. [ABSTRACT FROM AUTHOR]

The purpose of this study was to test the effects of high CO2 exposure on wound healing rates in an elasmobranch fish (Urobatis jamaicensis). Small dermal injuries (8 mm biopsy) closed by 22 days post wounding with a decrease in haematocrit. High CO2 exposure (ΔpH = 1.4) did not influence healing rate or haematocrit. Combined, these data provide evidence that minimally invasive scientific procedures have short‐term impacts on elasmobranch fishes even during exposure to a chronic stressor. Therefore, wound healing rates may not be strongly impacted by ocean acidification (ΔpH = 0.4). [ABSTRACT FROM AUTHOR]

Skates (class Chondrichthyes; subclass Elasmobranchii; order Rajiformes; family Rajidae) comprise one quarter of extant chondrichthyans, yet have received little attention in the scientific literature likely due to their relatively low economic value and difficulties in species identification. The absence of species‐specific information on catch, life history and migration of skates has often precluded the development of single‐species stock assessments and led to the use of cursory multispecies assessments, which lack the ability to track species‐specific catch and abundance trends. This has resulted in undetected local extirpations, as has been previously reported for common (Dipturus batis, Rajidae), white (Rostroraja alba, Rajidae) and long‐nose (Dipturus oxyrhinchus, Rajidae) skates in the Irish Sea. Here, we (a) use case studies to illustrate how the perception of skate population structure and stock status has historically been masked through multispecies assessment and management practices, (b) review current information on the movement of skates and identify gaps in knowledge, and (c) identify biases associated with the use of various tagging technologies, which have confounded our understanding of movement and migration ecology of skates. Considering that over 40% of extant Rajidae species are listed as "Data Deficient" by the International Union for Conservation of Nature, we illustrate a critical need to broaden the current understanding of skate life history, movement and migration ecology by providing recommendations on the further application of electronic tags and biogeochemical natural tags in movement studies and highlight the benefits that studies using these approaches have for novel management frameworks. [ABSTRACT FROM AUTHOR]

Using a baited remote underwater video system (BRUV), we provide the first recorded in situ observation of the sharpnose sevengill shark, Heptranchias perlo (Bonnaterre, 1788), from the Tongue of the Ocean, Bahamas. The individual was recorded at a depth of 718 meters, allowing for visual analysis of behavior in its natural environment. Temperature recordings of about 9° C at this depth indicate that H. perlo is physiologically capable of thriving within the lower mesophotic zone. This observation underscores the need to conduct further explorations of elasmobranch diversity and distribution in the Bahamas, which can be readily facilitated by BRUV-based methods. [ABSTRACT FROM AUTHOR]

Acquiring movement data for small-bodied, deep-water chondrichthyans is challenged by extreme effects of capture and handling stress, and post-release predation, however, it is urgently required to examine important fisheries interactions and assess the ecological role of these species within deep-water food webs. Here we suggest a novel release-cage mechanism to deploy pop-up satellite archival tags, as well as present vertical habitat data for a data-deficient, small-bodied, deep-water bycatch species, the Cuban dogfish ( Squalus cubensis ). Data were gathered from seven of eight High Rate X-Tags deployed on mature Cuban dogfish in the Exuma Sound, The Bahamas. Recovery periods appeared variable between individuals and are likely driven by capture-and-handling stress and tag burden. Application of the cross-correlation function to time-series depth and temperature data indicated three of the seven individuals suffered mortality through predation, which occurred during daytime, and suggests Cuban dogfish may constitute a proportion of deep-water apex predator diet in the Exuma Sound. Two animals were successfully released via a novel release-cage mechanism and displayed either no, or rapid (<15 mins) vertically stationary recovery periods and were not consumed by predators; data for these individuals were recorded for the entire deployment duration (14 days). Vertical habitat data suggests Cuban dogfish are diel-vertical migrators, similar to other deep-water taxa, and exhibit a relatively broad temperature and depth range, which may be driven by preference for specific bathymetric structures. These techniques provide an important first step into acquiring and presenting vertical habitat data for small-bodied, deep-water chondrichthyans, which can be directly applied to fisheries and ecosystem-based management approaches. [ABSTRACT FROM AUTHOR]