Ontogenetic morphometry of the brown smoothhound sharkMustelus henleiwith implications for ecology and evolution

Abstract

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 in M. henlei are 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.

Ontogenetic shifts in body form in the bull shark Carcharhinus leucas

“…This result mirrors that found in Mustelus henlei, where it was speculated that ecological differences between the sexes could result in different functional demands with regard to thrust generation and/ or stability, thus imparting different selective regimes upon each sex (Gayford, Godfrey, et al, 2023). Regrettably, this hypothesis cannot be proven nor discounted on the basis of our results as to the best of our knowledge these are the only two studies to date to have considered sex-based differences in ontogenetic scaling coefficients among elasmobranchs.…”

“…Moreover, there are known sexual differences in spatial ecology and habitat, with females exhibiting philopatry and reduced dispersal compared to males (Devloo-Delva et al, 2023). For this reason, we expect to detect sexual dimorphism in scaling coefficients, as recovered in recent studies of other shark species (Gayford, Godfrey, et al, 2023). Although one previous study has addressed ontogenetic scaling in bull sharks (Irschick & Hammerschlag, 2015), the sample size used in this study was small (N = 29), and may not reflect true scaling trends observed across size classes and sexes.…”

“…Five separate analyses were conducted: one using the full data set, two using male-only and female-only datasets, respectively, and two more using adult-only and juvenile-only datasets. Separate analyses were conducted in this way to provide greater resolution regarding the ontogenetic expression of scaling trends and reveal and sex-based morphological differences differences (Gayford, Godfrey, et al, 2023).…”

“…There appears to be taxonomic variation in the functional role of the first dorsal fin amongst sharks, such that some species rely on the structure predominantly for thrust generation, whilst in others, it performs a stabilising function (Lingham-Soliar, 2005b;Maia & Wilga, 2013;Maia et al, 2017). Despite this dichotomy, it appears that there is a clear ontogenetic trend amongst some shark species towards relatively tall and narrow dorsal fins in later ontogeny, regardless of hypothesised functionality (Gayford, Godfrey, et al, 2023;Irschick & Hammerschlag, 2015;Sternes & Higham, 2022). Our results are consistent with this trend, both in the total data set, and in each of the sexes (Figures 2, 5 and 6; Tables 2, 5 and 6).…”

“…vertebrate clade occupying a diverse range of ecomorphological niches (Navia et al, 2017;Sternes & Shimada, 2020). Allometric growth has been recorded in varying proportions in several elasmobranch taxa, with functional hypotheses for observed trends proposed (Ahnelt et al, 2020;Bellodi et al, 2023;Gayford, Godfrey, et al, 2023;Irschick & Hammerschlag, 2015;Irschick et al, 2017;Reiss & Bonnan, 2010;Sternes & Higham, 2022). As of yet, the only formalised hypothesis concerning body form in elasmobranchs rather than specific structures, the allometric niche shift hypothesis suggests that species that exhibit ontogenetic shifts in habitat usage and/or trophic ecology are more likely to display allometric growth than species exhibiting relatively static ecologies through ontogeny .…”