Comparative morphology and mechanical properties of the lepidotrichia of climbing and non-climbing Hawaiian gobioid fishes

Many gobioid fishes from tropical islands exhibit amphidromous life histories, in which juveniles migrate from the ocean to upstream freshwater habitats. However, upstream reaches are blocked by waterfalls on many islands. In some species, juveniles climb these waterfalls with the aid of an adhesive sucker formed from fusion of the pelvic fins. Adults of some species may also climb waterfalls to return upstream if they are dislodged by pulses of high flow. Several structural features of gobies have been examined to assess how they contribute to successful climbing. In this study, we compared the morphology and flexural stiffness of lepidotrichia from the pelvic and caudal fins for four of the five native species of Hawaiian stream fishes, including both climbing and non-climbing taxa, to evaluate how the structure and mechanical properties of the fin skeleton might facilitate climbing success. We found that pelvic lepidotrichia were uniformly more flexible than caudal lepidotrichia, and that the lepidotrichia of climbing taxa were shorter and more flexible than those Of non-climbing taxa. These lepidotrichial properties may allow climbing taxa to maximize the pressure differentials and adhesive strength that can be achieved with gobioid suckers, without requiring excessively large suckers that could incur increased hydrodynamic drag. These results highlight how variation in skeletal mechanical properties can contribute to the remarkable functional capacities and ecological distribution of tropical island stream fishes.