From wings to tails: the aspect ratio of propulsive surfaces as a general proxy for vertebrate dispersal potential

Dispersal influences many fundamental processes, however, direct measures of dispersal are scarce for most taxa, limiting our ability to evaluate ecological theory at broad scales. In birds, the aspect ratio of the wing has become a widely accepted proxy for dispersal, but whether morphology can be used to index dispersal in aquatic systems is unknown. Here, we show that the aspect ratio of the caudal fin (CFAR) or tail predicts swimming speed and geographic range size in sharks, and we use CFAR to map dispersal potential across 452 (82%) shark species. Contrary to the latitudinal gradient in dispersal seen in birds, shark dispersal shows no latitudinal structure and instead follows a coastal-pelagic gradient. By combining phylogenetic path analysis with Bayesian phylogenetic multilevel models, we find the coastal-pelagic pattern is driven by metabolically-relevant morphology (body size and gill slit height) and water column position, with no direct effect of environmental temperature. Since CFAR can be measured from anatomically accurate illustrations and museum specimens, dispersal potential and connectivity-related vulnerability can be estimated for data-poor and newly described species without sampling. With the avian precedent, these results establish the aspect ratio of propulsive surfaces as a general proxy for dispersal across fluid-dwelling vertebrates.