Darren Andrew Whitehead

Elasmobranchs (sharks and rays) exhibit a wide range of body forms adapted to various ecological niches. Body form differs not only between species, but between life stages of individual species as a result of ontogenetic allometry. In sharks, it has been proposed that these ontogenetic shifts in body form result from shifts in trophic and/or spatial ecology (the allometric niche shift hypothesis). Alternatively, it has been suggested that ontogenetic allometry may result from intrinsic morphological constraints associated with increasing body size – e.g. to counteract shifts in form-function relationships that occur as a function of size and could compromise locomotory performance. One major limitation affecting our understanding of ontogenetic scaling in sharks is that existing studies focus on postpartum ontogeny – ignoring the period of growth that occurs prior to birth/hatching. In this study, we report ontogenetic growth trajectories from 39 near-term brown smooth hound ( Mustelus henlei ) embryos taken from manually collected measurements. We found that unlike most other species and later ontogenetic stages of M. henlei , these embryos predominantly grow isometrically, and appear to display relatively high levels of morphological disparity. These results provide rudimentary support for the allometric niche shift hypothesis (as in the absence of ontogenetic niche shifts isometry dominates body-form scaling) and provide important insight into early shark ontogeny and morphological/developmental evolution.

Recent studies have uncovered mosaic patterns of allometric and isometric growth underlying ontogenetic shifts in the body form of elasmobranch species (shark and rays). It is thought that shifts in trophic and spatial ecology through ontogeny drive these morphological changes; however, additional hypotheses relating to developmental constraints have also been posed. The bull shark (Carcharhinus leucas) is a large‐bodied coastal shark that exhibits strong ontogenetic shifts in trophic and spatial ecology. In this study, we utilise a large data set covering a large number of morphological structures to reveal ontogenetic shifts in the body form of C. leucas, stratifying analyses by sex and size classes to provide fine‐scale, more ecomorphologically relevant results. Our results indicate shifts in functional demands across the body through ontogeny, driven by selective pressures relating to trophic and spatial ecology driving the evolution of allometry. We also find significant differences in scaling trends between life stages, and between the sexes, highlighting the importance of utilising large, diverse datasets that can be stratified in this way to improve our understanding of elasmobranch morphological evolution. Ultimately, we discuss the implications of these results for existing ecomorphological hypotheses regarding the evolution of specific morphological structures, and pose novel hypotheses where relevant.

The central tenet of ecomorphology links ecological and morphological variation through the process of selection. Traditionally used to rationalise morphological differences between taxa, an ecomorphological approach is increasingly being utilised to study morphological differences expressed through ontogeny. Elasmobranchii (sharks, rays and skates) is one clade in which such ontogenetic shifts in body form have been reported. Such studies are limited to a relatively small proportion of total elasmobranch ecological and morphological diversity, and questions remain regarding the extent to which ecological selection are driving observed morphometric trends. In this study, we report ontogenetic growth trajectories obtained via traditional linear morphometrics from a large data set of the brown smoothhound shark (Mustelus henlei). We consider various morphological structures including the caudal, dorsal and pectoral fins, as well as several girth measurements. We use an ecomorphological approach to infer the broad ecological characteristics of this population and refine understanding of the selective forces underlying the evolution of specific morphological structures. We suggest that observed scaling trends inM. henleiare inconsistent with migratory behaviour, but do not contradict a putative trophic niche shift. We also highlight the role of predation pressure and sex‐based ecological differences in driving observed trends in morphometry, a factor which has previously been neglected when considering the evolution of body form in sharks.

Despite being the world's largest fish, many questions remain regarding the ecology and behaviour of the whale shark (Rhincodon typus). We present the first direct evidence of whale sharks engaging in bottom‐feeding behaviour and provide potential explanations for this foraging strategy. We suggest that whale sharks actively feed on benthic prey, either predominantly in deepwater environments or where the abundance of such prey exceeds that of planktonic food sources. We also highlight the potential for ecotourism and citizen science projects to contribute to our understanding of marine megafauna behavioural ecology.

This study constitutes the first descriptions of morphological abnormalities in the silky shark Carcharhinus falciformis from a near term embryo obtained in the Gulf of California, Mexico. External exploration and X-ray images were taken to describe the specimen, which revealed that the embryos deformities were located in the cephalic region suggesting a holoprosencephaly disorder. The analyzed images evidenced a deformed oval chondrocranium with both nasal capsules pointing upwards and the rostrum pointing downwards, in which the palatoquadrate cartilage was deformed with signs of prognathism.

The aim of this study is to provide preliminary observations on the microanatomy ofRhincodon typusskin using histology and electron microscopy analyses. Skin biopsies were obtained from a deceased juvenile male shark (548 cm total length) stranded in La Paz, Mexico, during February 2018. The results of this study evidenced the basic structure of the dermal denticles in the epidermis of the trunk of the shark, as well as the composition of the connective tissue in the hypodermis. Histological images of the hypodermis showed a high concentration of collagen fibres, formed by a large number of fine and wavy fibres of compact shape and little intercellular substance.