Ontogenetic scaling of disc width with total length in west African batoids

Morphological scaling describes changes in the size or shape of one morphological character (e.g. mass, length, width, area etc.) as another increases in size. Understanding how morphological characters scale with body size can shed light on how natural selection influences morphology, and the nature of ecomorphological relationships through ontogeny. Batoids (Elasmobranchii: Batoidea) are a highly specialised lineage of cartilaginous fishes displaying extreme dorsoventral flattening. Despite this, little is known about morphological scaling in batoids compared to sharks. In this study we test the relationship between disc width and total length in five batoid species (Torpedo torpedo, Mobula tarapacana, Fontitrygon margarita, Raja parva, Rhinobatos irvinei) representing four orders that differ in both ecology and morphology, measured from artisanal fisheries in Western and Central Ghana. Whilst a lack of existing ecological data presents some limitations, our results are broadly consistent with ecomorphological theory previously applied to sharks. Moreover, we find that for some lineages (including some myliobatiform taxa) total length may represent a valid proxy for estimating overall body size. This finding has applications for body size and shape estimation in partially processed batoids obtained from fishing camps, and extinct taxa known only from incomplete or fragmented remains.