Ontogenetic change in novel functions: waterfall climbing in adult Hawaiian gobiid fishes

Juveniles from three species of Hawaiian gobiid fishes climb waterfalls as part of an amphidromous life cycle, allowing them to re-penetrate adult upstream habitats after being swept out to the ocean upon hatching. The importance of climbing for juvenile stream gobies is well established, but adult fishes in upstream island habitats also face potential downstream displacement by catastrophic disturbances. Thus, retention of climbing ability could be advantageous for adult stream gobies. Climbing performance might be expected to decline among adults, however, due to the tendency for mass specific muscular power production to decrease with body size, and a lack of positively allometric growth among structures like the pelvic sucker that support body weight against gravity. To evaluate changes in waterfall climbing ability with body size in Hawaiian stream gobies, we compared climbing performance and kinematics between adults and juveniles from three species (Awaous guamensis, Sicyopterus stimpsoni, and Lentipes concolor). For species in which juveniles climbed using “powerbursts” of axial undulation, adult performance and kinematics showed marked changes: adult A. guamensis failed to climb, and adult L. concolor used multiple pectoral fin adductions to crutch up surfaces at slow speeds, rather than rapid powerbursts. Adult S. stimpsoni, like juveniles, still alternated attachment of oral and pelvic suckers to “inch” up surfaces and climbed at speeds comparable to those of juveniles. However, unlike juveniles, adult S. stimpsoni also add pectoral fin crutching to every climbing cycle. Thus, although powerburst species appear particularly susceptible to size-related declines in waterfall climbing performance, the addition of compensatory mechanisms prevents complete loss of this novel function in at least some species.