Flow gradient drives morphological divergence in an Amazon pelagic stream fish

Abstract

Body shape and size variations are common in stream fishes, and morphological differences can have either a genetic or non-genetic basis. Flow has been indicated as one of the causes of intraspecific variation, and shifts in stream-fish body morphology are related to swimming performance and to individual fitness. Although populations in lotic versus lentic habitats have been compared, the effects of a flow gradient on fish shape are little studied. We tested differences in size, body shape and caudal-peduncle morphology of a pelagic fish that inhabits streams with different velocities in two basins, using geometric morphometrics to evaluate shifts in body morphology. Fish from lower-flow velocities had larger bodies that were deeper posteriorly; fish from higher-flow velocities were smaller and more streamlined. Shape variation among specimens was significantly influenced by the local velocity, with similar responses in fish body shape in the different basins. We showed that selective pressures generated by flow velocities affect fish shape in the same way in both basins. Fish body size, shape and caudal-peduncle morphology affect swimming performance, which could influence the energy costs for survival. Our results with a small pelagic characid help to elucidate questions on morphological predictions for fishes across flow regimes.