Life in the flow lane: differences in pectoral fin morphology suggest transitions in station-holding demand across species of marine sculpin.

Aquatic organisms exposed to high flow regimes typically exhibit adaptations to decrease overall drag and increase friction with the substrate. However, these adaptations have not yet been examined on a structural level. Sculpins (Scorpaeniformes: Cottoidea) have regionalized pectoral fins that are modified for increasing friction with the substrate, and morphological specialization varies across species. We examined body and pectoral fin morphology of 9 species to determine patterns of body and pectoral fin specialization. Intact specimens and pectoral fins were measured, and multivariate techniques determined the differences among species. Cluster analysis identified 4 groups that likely represent differences in station-holding demand, and this was supported by a discriminant function analysis. Primarily, the high-demand group had increased peduncle depth (specialization for acceleration) and larger pectoral fins with less webbed ventral rays (specialization for mechanical gripping) compared to other groups; secondarily, the high-demand group had a greater aspect ratio and a reduced number of pectoral fin rays (specialization for lift generation) than other groups. The function of sculpin pectoral fins likely shifts from primarily gripping where demand is likely low, to an equal dependence on gripping and negative lift generation where demand is likely high. Specialization of the ventral pectoral fin region for gripping likely contributes to the recent diversification of some species into high-demand habitats.