Bridging disciplines to advance elasmobranch conservation: applications of physiological ecology

Lectin binding to pectoral fin of neonate little skates reared under ambient and projected-end-of-century temperature regimes

The glycosylation of macromolecules can vary both among tissue structural components and by adverse conditions, potentially providing an alternative marker of stress in organisms. Lectins are proteins that bind carbohydrate moieties and lectin histochemistry is a common method to visualize microstructures in biological specimens and diagnose pathophysiological states in human tissues known to alter glycan profiles. However, this technique is not commonly used to assess broad-spectrum changes in cellular glycosylation in response to environmental stressors. In addition, the binding of various lectins has not been studied in elasmobranchs (sharks, skates, and rays). We surveyed the binding tissue structure specificity of 14 plant-derived lectins, using both immunoblotting and immunofluorescence, in the pectoral fins of neonate little skates (Leucoraja erinacea). Skates were reared under present-day or elevated (+5°C above ambient) temperature regimes and evaluated for lectin binding as an indicator of changing cellular glycosylation and tissue structure. Lectin labeling was highly tissue and microstructure specific. Dot blots revealed no significant changes in lectin binding between temperature regimes. In addition, lectins only detected in the elevated temperature treatment were Canavalia ensiformis lectin (Concanavalin A) in spindle cells of muscle and Ricinus communis agglutinin in muscle capillaries. These results provide a reference for lectin labeling in elasmobranch tissue that may aid future investigations.

Population ecology and juvenile density hotspots of thornback ray (Raja clavata) around the Shetland Islands, Scotland

Elasmobranchs are facing global decline, and so there is a pressing need for research into their populations to inform effective conservation and management strategies. Little information exists on the population ecology of skate species around the British Isles, presenting an important knowledge gap that this study aimed to reduce. The population ecology of thornback ray (Raja clavata) around the Shetland Islands, Scotland, was investigated in two habitats: inshore (50-150 m deep) and shallow coastal (20-50 m deep), from 2011 to 2022, and 2017 to 2022, respectively. Using trawl survey data from the annual Shetland Inshore Fish Survey, the size composition of R. clavata catches was compared between shallow and inshore habitats across 157 trawl sets, and 885 individuals, over the years 2017-2022. Catch per unit effort (CPUE) of R. clavata was significantly higher in shallow than that in inshore areas (ANOVA, F = 72.52, df = 1, 5, p < 0.001). Size composition also significantly differed between the two habitats (analysis of similarities, R = 0.96, p = 0.002), with R. clavata being smaller in shallow areas and juveniles (<60 cm) occurring more frequently. Spatial distribution maps confirmed density hotspots of juveniles in shallow habitats, with repeated use of certain locations consistent over time. The results of this study provide the first evidence for R. clavata using shallow areas for potential nurseries in Shetland, which can inform the IUCN's Important Shark and Ray Area process. Furthermore, this study provides important new population ecology information for R. clavata around Shetland, which may have important conservation implications and be valuable for informing species and fisheries stock assessments in this region.

Tail shape and the swimming speed of sharks

Trait-based ecology is a rapidly growing approach for developing insights and predictions for data-poor species. Caudal tail fin shape has the potential to reveal much about the energetics, activity and ecology of fishes and can be rapidly measured from field guides, which is particularly helpful for data-sparse species. One outstanding question is whether swimming speed in sharks is related to two morphological traits: caudal fin aspect ratio (CFAR, height2/tail area) and caudal lobe asymmetry ratio (CLAR). We derived both metrics from the species drawings in Sharks of the world (Ebert et al. 2013 Sharks of the world: a fully illustrated guide) and related fin shape to two published datasets of (1) instantaneous swimming speeds (Jacoby et al. 2015 Biol. Lett. 11, 20150781 (doi:10.1098/rsbl.2015.0781)) and (2) cruising speeds (Harding et al. 2021 Funct. Ecol. 35, 1951-1959 (doi:10.1111/1365-2435.13869)) for 28 total unique shark species. Both estimates of swimming speed were positively related to CFAR (and weakly negatively to CLAR). Hence, shark species with larger CFAR and more symmetric tails (low CLAR) tended to be faster-moving and have higher average speeds. This relationship demonstrates the opportunity to use tail shape as an easily measured trait to index shark swimming speed to broader trait-based analyses of ecological function and extinction risk.

Almost nothing is known about the tiger shark in South Atlantic waters

The tiger shark (Galeocerdo cuvier) has been relatively well assessed concerning biology and ecology aspects in both Atlantic and Pacific North America and in Caribbean waters. The amount of data in these regions has led to the species protection under capture quotas and with the creation of sanctuaries. The reality in developing countries, however, is the exact opposite, with scarce information on the species in the southern hemisphere, namely South American and African waters. In these regions, protection measures are insufficient, and studies on tiger shark biology and ecology are scarce, significantly hindering conservation and management efforts. Thus, the aim of this study was to compile scientific literature on the tiger shark in the South Atlantic and discuss the impact of these data (or lack thereof) distributed within a total of ten research categories for guiding management plans. In total, 41 scientific publications on different G. cuvier biology and ecology aspects were obtained. The most studied topics were Feeding Ecology (n = 12), followed by Human Interactions (n = 8), and Movements and Migration (n = 7). Northeastern Brazil (Southwest Atlantic) was the most researched area, probably due to the higher coastal abundance of tiger sharks in this area, alongside a high number of recorded attacks, justifying funding for studies in the region. No studies carried out in other South American or African countries were found. It is important to mention that even though some research topics are relatively well covered, a severe knowledge gap is noted for risk assessments and fisheries management, with a proposition for the implementation of sanctuaries noted. This is, however, particularly worrisome, as the South Atlantic is mostly unexplored in this regard for tiger sharks. It is also important to note how different the attention given to this species is in the North Atlantic when compared to the South region. Lastly, we highlight that the existence of sub-populations, the lack of migratory corridors geographically connecting distinct areas used by the species, and the lack of fisheries statistics on tiger shark landings, all increase the vulnerability of this species in the South Atlantic.

Variation of body condition and plasma energy substrates with life stage, sex, and season in wild-sampled nurse sharks Ginglymostoma cirratum

Reported here are the relationships among morphological (i.e., body condition) and biochemical (i.e., plasma concentrations of triglycerides, cholesterol, free fatty acids, and ketone bodies and ketone body ratios) parameters related to energy storage and use, as well as the variation of such parameters, for 107 free-ranging nurse sharks Ginglymostoma cirratum sampled off South Florida. Immature G. cirratum exhibited a higher variance in body condition, plasma free fatty acid concentrations and ketone body ratios compared to adults. Mature female G. cirratum had significantly higher body condition than mature males, driven by a seasonal increase in mature female body condition during the wet season. Mature male G. cirratum showed a decrease in the ketone body β-hydroxybutyric acid during the dry season. Taken together, this study provides a baseline assessment of body condition and internal physiological state for a data-poor marine species and demonstrates significant ontogenetic, sexual and seasonal variation in G. cirratum energetic state. As concluded by other studies of energy metabolism in free-ranging sharks, this research highlights the importance of considering intraspecific patterns and sampling context for inferring the drivers of variation.

Gill slits provide a window into the respiratory physiology of sharks

Metabolically important traits, such as gill surface area and metabolic rate, underpin life histories, population dynamics and extinction risk, as they govern the availability of energy for growth, survival and reproduction. Estimating both gill surface area and metabolic rate can be challenging, especially when working with large-bodied, threatened species. Ideally, these traits, and respiratory physiology in general, could be inferred from external morphology using a faster, non-lethal method. Gill slit height is quick to measure on live organisms and is anatomically connected to the gill arch. Here, we relate gill slit height and gill surface area for five Carcharhiniform sharks. We compared both total and parabranchial gill surface area to mean and individual gill slit height in physical specimens. We also compared empirical measurements of relative gill slit height (i.e. in proportion to total length) to those estimated from field guide illustrations to examine the potential of using anatomical drawings to measure gill slit height. We find strong positive relationships between gill slit height and gill surface area at two scales: (i) for total gill surface area and mean gill slit height across species and (ii) for parabranchial gill surface area and individual gill slit height within and across species. We also find that gill slit height is a consistent proportion of the fork length of physical specimens. Consequently, relative gill slit height measured from field guide illustrations proved to be surprisingly comparable to those measured from physical specimens. While the generality of our findings needs to be evaluated across a wider range of taxonomy and ecological lifestyles, they offer the opportunity that we might only need to go to the library and measure field guide illustrations to yield a non-lethal, first-order approximation of the respiratory physiology of sharks.

Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)

Understanding how rising temperatures, ocean acidification, and hypoxia affect the performance of coastal fishes is essential to predicting species-specific responses to climate change. Although a population's habitat influences physiological performance, little work has explicitly examined the multi-stressor responses of species from habitats differing in natural variability. Here, clearnose skate (Rostaraja eglanteria) and summer flounder (Paralichthys dentatus) from mid-Atlantic estuaries, and thorny skate (Amblyraja radiata) from the Gulf of Maine, were acutely exposed to current and projected temperatures (20, 24, or 28 °C; 22 or 30 °C; and 9, 13, or 15 °C, respectively) and acidification conditions (pH 7.8 or 7.4). We tested metabolic rates and hypoxia tolerance using intermittent-flow respirometry. All three species exhibited increases in standard metabolic rate under an 8 °C temperature increase (Q₁₀ of 1.71, 1.07, and 2.56, respectively), although this was most pronounced in the thorny skate. At the lowest test temperature and under the low pH treatment, all three species exhibited significant increases in standard metabolic rate (44-105%; p < 0.05) and decreases in hypoxia tolerance (60-84% increases in critical oxygen pressure; p < 0.05). This study demonstrates the interactive effects of increasing temperature and changing ocean carbonate chemistry are species-specific, the implications of which should be considered within the context of habitat.