Upstream range expansion by invasive round gobies: Is functional morphology important?

Abstract

Range expansion by invasive taxa can be facilitated by a wide variety of life history traits, by behavioral repertoires, or via human-aided translocation. The round goby (Neogobius melanostomus) continues to expand its range in Europe and the Great Lakes watershed by moving upstream in tributary rivers and streams. It is not clear whether in-stream barriers (e.g., waterfalls, culverts, dams) will prevent further upstream expansion. Since other species within the Gobiidae are adept waterfall climbers, we investigated whether body scaling morphometrics, fused pelvic fin adhesion, or swimming patterns in round gobies were suggestive of climbing capability. Behaviorally, round gobies swam less frequently, limited their swimming mode to caudal thrusts, and sought shelter more as velocity increased. Attachment to the substrate via the fused pelvic fins was documented in a laboratory flume. Fish smaller than 5 cm or having a body mass less than 1.5 g had passive pelvic fin adhesion power sufficient to remain in place on a wetted, inclined plane, whereas larger fish did not. Small fish had significantly smaller pelvic fin area/length ratios relative to larger fish (P < 0.001), but significantly greater pelvic fin area/mass ratios (P < 0.001). Caudal fin area, but not fused pelvic fin area, exhibited an allometric growth pattern versus weight in large gobies, whereas neither caudal nor pelvic fin area showed allometric increases versus weight in small round gobies. Pelvic fin area in both large and small fish grew allometrically compared to length, whereas caudal fin area increased allometrically relative to length only in small fish. Fin morphometric results suggest, theoretically, that neither adults nor juveniles have pelvic fin areas large enough to support their weight against gravity forces, but the areas are sufficient to withstand hydraulic drag forces. Collectively, our results suggest that in-stream barriers which force fish to overcome gravity should be impassable, but that barriers allowing fish to remain submerged could be navigable.