Tail shape and the swimming speed of sharks

Trait-based ecology is a rapidly growing approach for developing insights and predictions for data-poor species. Caudal tail fin shape has the potential to reveal much about the energetics, activity and ecology of fishes and can be rapidly measured from field guides, which is particularly helpful for data-sparse species. One outstanding question is whether swimming speed in sharks is related to two morphological traits: caudal fin aspect ratio (CFAR, height²/tail area) and caudal lobe asymmetry ratio (CLAR). We derived both metrics from the species drawings in Sharks of the world (Ebert et al. 2013 Sharks of the world: a fully illustrated guide) and related fin shape to two published datasets of (1) instantaneous swimming speeds (Jacoby et al. 2015 Biol. Lett. 11, 20150781 (doi:10.1098/rsbl.2015.0781)) and (2) cruising speeds (Harding et al. 2021 Funct. Ecol. 35, 1951-1959 (doi:10.1111/1365-2435.13869)) for 28 total unique shark species. Both estimates of swimming speed were positively related to CFAR (and weakly negatively to CLAR). Hence, shark species with larger CFAR and more symmetric tails (low CLAR) tended to be faster-moving and have higher average speeds. This relationship demonstrates the opportunity to use tail shape as an easily measured trait to index shark swimming speed to broader trait-based analyses of ecological function and extinction risk.