Integument-based inferences on the swimming ability and prey hunting strategy of the bigeye thresher shark, Alopias superciliosus (Lamniformes: Alopiidae)

Abstract

Alopias is a group of lamniform sharks characterized by a highly elongate caudal fin with three known extant species: Alopias pelagicus (pelagic thresher shark), Alopias superciliosus (bigeye thresher shark), and Alopias vulpinus (common thresher shark). Alopias pelagicus and A. vulpinus are considered fast swimmers and use their caudal fin to hunt for small schooling fish by stunning them, but this feeding behavior has never been directly observed for A. superciliosus. In this study, we examined four integumentary variables of selected fast swimming (e.g. A. pelagicus, A. vulpinus, and Lamna) and slow swimming (e.g. Mitsukurina and Megachasma) lamniform sharks to determine whether A. superciliosus is a fast swimmer or a slow swimmer. Our data indicate that A. superciliosus is a slow swimming lamniform, but it probably employs a simple laterally directed tail slap to capture its prey. Overall, our study points to an interpretation that A. superciliosus is an ambush predator, rather than an active prey-pursuing hunter. Mapping of scale data onto previously published phylogenetic trees indicate that slow swimming is a plesiomorphic condition in the order Lamniformes. Our work represents the most extensive comparative study of the morphology and variation of integumentary structures, especially placoid scales, conducted so far for Lamniformes.