Thermoregulation or Habitat Selection? Environmental Predictors of the Body Shape Variation in Sharks (Chondricthyes: Selachimorpha)

Abstract

Ecomorphology is the study of the relation between organisms' shape and environmental factors. It assumes that variations in the organisms' shape lead to functional differences, resulting in changes in resource and habitat use. Here, we aimed to evaluate the effects of environmental variables, representing hypotheses of thermoregulation and habitat selection, over the sharks' body shape on a global scale. According to the thermoregulation hypothesis, the more fusiform species should be more commonly found in colder waters, and regarding the habitat selection hypothesis, we expected that the flatter species would occur nearest to the shore, and in the shallowest waters. Based on the body shape and occurrences of the shark species, we estimated the mean and median body shape index (body height: length ratio) of shark species on each cell in a 400 x 400 km grid, and applied a model selection by AIC approach to identify the relative importance of four environmental factors faced by the shark species: minimum temperature of the coldest month (Tc), maximum temperature of the warmest month (Tw), bathymetry (Bathy), and distance to shore (DistShr). Our results indicate a consistent trend of dorsoventral flattening towards coastal zones and shallow areas. A second detected trend was the increase in the body shape index values towards the tropical regions all over the globe, while flattened species were more common above 30º latitude in both hemispheres. Minimum temperature of the coldest month was included in all the best fitted models, but it did not follow our initial predictions of negative relation to body shape index, thus we did not find support for the thermoregulation hypothesis. On the other hand, Bathy and DistShr presented a positive relation to body shape index, thus consistent with our initial predictions. Our results indicate that the sharks body shape is not a response to a selective pressure for heat conservation in cold environments, but rather to where they live and how they obtain their food, contrary to what has been reported to ecologically equivalent marine mammals (suborder Odontoceti).