Ontogenetic Change in Performance: Do Innovations Constrain Performance in Gobiid Fishes?

Recent innovations in a species may expand opportunities to obtain new resources or previously inaccessible habitats. Over time, the novel behavior may diversify in response to an adaptive landscape or become canalized by convergent selective pressures into a narrowed performance window. Selective pressures also change across life stages, which may further enhance diversification or constrain performance. To test these alternative hypotheses, we compared ontogenetic changes in the performance of a novel locomotor behavior-waterfall-climbing-across five amphidromous species of gobiid fishes from the Pacific, Caribbean, and Indian Oceans. Two species climb waterfalls using an inching motion, a derived functional innovation that alternates movements of pelvic and oral suckers. The remaining species climb waterfalls using short "powerbursts" of swimming, followed by long rest periods in which the pelvic sucker is attached to the waterfall substrate. Among adults, net climbing speed (including rest) was similar across the three powerburst species and one inching species, but a derived, faster speed was observed in one inching taxon, suggesting greater functional diversity in the younger innovation. However, kinematic data indicate that similar performance levels can be achieved through multiple functional pathways. Moreover, patterns of functional diversity across species observed in juveniles do not track neatly into adulthood, highlighting the range of potential influences that can shape the trajectory of ontogenetic change in function.