Functional diversity in extreme environments: effects of locomotor style and substrate texture on the waterfall-climbing performance of Hawaiian gobiid fishes

Juveniles of three species of Hawaiian gobiid fishes climb waterfalls during migration to adult habitats using two kinematically distinct patterns: Awaous guamensis and Lentipes concolor use rapid, intermittent bouts of axial undulation ('powerbursts'), whereas Sicyopterus stimpsoni inches up waterfalls by alternately attaching oral and pelvic suckers to the substrate. Despite the differing kinematics and speed of these behaviors, the extreme demands of locomotion up waterfalls might require similar levels of performance from all climbing goby species. However, the roughness of climbing surfaces might affect performance differently between climbing styles, with rough surfaces decreasing inching performance by impairing the grip of sucking discs on the substrate, but improving powerburst performance by allowing the pectoral fins better purchase as they push against the substrate to start climbing bouts. To test whether species of Hawaiian climbing gobies differ in climbing performance, we filmed multiple individuals of each species as they ascended distances of several body lengths over substrates with three different degrees of roughness. Substantial differences in climbing performance among species were independent of climbing style. For example, the powerburst climber L. concolor always had a faster net climbing speed (speed over 20 cm, including rests between climbing bouts) than the inching climber S. stimpsoni; however, the powerburst climber A. guamensis was similar in net speed to S. stimpsoni, or, on smooth substrates, climbed more slowly. Yet, some differences in performance did emerge between the climbing styles. Substrate roughness did not affect climbing speed in S. stimpsoni, but, as predicted, both powerburst climbers climbed faster on rougher surfaces. In addition, the intermittent motion and rest periods for both powerburst climbers seem of appropriate durations to facilitate an increase in the distance that these species travel before fatigue, but the inching climber S. stimpsoni spent more time in motion than either powerburst climber, and the relative durations of its motion and rest periods seem less likely to convey any performance advantages. These results indicate considerable functional diversity among Hawaiian climbing gobies beyond the two varieties of climbing styles, and show that a wide range of performance capabilities can persist among members of a fauna even under extreme environmental conditions.