Ontogenetic Shifts in Body Morphology of Demersal Sharks’ Species (Order: Squaliformes) Inhabiting the Western-Central Mediterranean Sea, with Implications for Their Bio-Ecological Role

Abstract

Several elasmobranch species undergo shifts in body proportions during their ontogenetic growth. Such morphological changes could reflect variation in diet, locomotion, or, more broadly, in the species’ interactions with their environment. However, to date, only a few studies have been conducted on this topic, and most of them focused on particular body regions. In the present study, the ontogenetic growth of five different demersal shark species was investigated by using both traditional linear morphometry of the entire body and shape analysis of the caudal fin. A total of 449 sharks were analysed: 95 little gulper sharks, 80 longnose spurdogs, 103 kitefin sharks, 124 velvet belly lanternsharks, and 47 angular roughsharks. From each specimen, 36 linear morphometric measurements were taken. While a first canonical analysis of principal coordinates ruled out the possibility of different growth patterns between males and females, the same analysis statistically discriminated between small and large individuals in every species based on their morphology. A Similarity Percentage analysis revealed that the most important measurements in distinguishing these two groups were those related to body lengths, indicating that large individuals are more elongated than small individuals. The shape analysis of caudal fins revealed allometric growth during ontogenetic development, with adult individuals having a wider fin (discriminant analysis, p < 0.05). These findings could be related to changes in predatory skills, supporting the hypothesis of a shift in the ecological role that these sharks play in their environment, thus providing new essential information for their conservation.

Ontogenetic shifts in body form in the bull shark Carcharhinus leucas

“…In the total data set, and data sets sorted by sex and maturity, there are multiple aspects of morphology that scale allometrically with body size, but others that scale isometrically (Tables 2-6). This is consistent with scaling coefficients reported for other shark species (Bellodi et al, 2023;Irschick et al, 2017). We also note the presence of unambiguous differences in morphometric scaling between the sexes and between size classes (Tables 3-6).…”

“…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 .…”