Functional regionalization of the pectoral fin of the benthic longhorn sculpin during station holding and swimming

Studies of the kinematics of the pectoral fins in fishes have focused on fins as devices for propulsion or maneuvering. Studying pectoral fin function in benthic fishes is an opportunity to understand how the fins are used in a broader range of fin-based behaviors, especially those involving substrate contact. Morphological specializations of the pectoral fins, hypothesized adaptations for substrate contact, have been described for several benthic fish groups. These specializations include, but are not limited to, reduced webbing between ventral rays as well as thickening and shortening of these rays compared with the dorsal rays. Our focal species, the benthic longhorn sculpin, Myoxocephalus octodecimspinosus, possesses these morphological specializations, which divide the fin loosely into dorsal and ventral regions. Our goal was to investigate the functional consequences of these specializations, if any. First, we used high-speed video to examine the motion of the pectoral fins during swimming and station holding on the bottom, the first such study for a benthic fish. We found that longhorn sculpin do not oscillate their pectoral fins during swimming. Rather, the pectoral fins are held in a steady laterally extended posture. Oscillations of the body, median fins and caudal fin are used for propulsion. The shape of the fin also changes dramatically as the fish moves from station holding to swimming. Second, we measured the curvature of the individual fin rays that support and control the shape and movement of the pectoral fins. We did this to examine whether morphological specialization of the fin rays influences fin ray curvature. Individual fin rays in different fin regions show consistently different patterns of bending regardless of behavior. We propose that the pectoral fin is divided into functional as well as morphological regions. The fin rays in each functional region have distinct roles during swimming and substrate contact.