Insights into ontogenetic scaling and morphological variation in sharks from near-term brown smooth-hound (Mustelus henlei) embryos

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.