Functional and comparative morphology of the pectoral fins among scorpaeniform fishes

Abstract

Many benthic fishes use their pectoral fins for both swimming and substrate contact. Each behavior imposes different, potentially conflicting functional demands on the pectoral fins. It has been hypothesized that benthic fish decouple these demands through morphological and functional regionalization of the fins, but this hypothesis had not been experimentally tested. In this study, I demonstrated functional regionalization of the pectoral fin in the longhorn sculpin, Myoxocephalus octodecimspinosus, a benthic scorpaeniform fish. The ventral region of the pectoral fin is used during substrate contact; the dorsal region is used during swimming. I characterize a previously undescribed swimming behavior during which the pectoral fins are held in a steady, laterally extended posture while the dorsal, anal and caudal fins are used for propulsion. I also investigate the functional morphology of individual pectoral fin rays. I find that the curvature of the fin rays is higher in the ventral versus dorsal region of the fin, regardless of behavior. The fin rays of longhorn sculpin possess a previously undescribed morphology in cross-section. Specifically, the hemitrichia, or bony halves that make up the fin rays, are cylindrical in cross-section proximally and crescent-shaped distally. The cylindrical segments in the proximal regions of the fin rays of longhorn sculpin provide resistance to bending, which may confer strength and support for weight-bearing substrate-contact behaviors. Further, the ventral fin rays, which are used for substrate contact, have a higher proportion of their total length made up of cylindrical segments. Finally, I used a comparative approach to examine the distribution of morphological specializations and habitat (benthic or pelagic) of the pectoral fins among scorpaeniform fishes. I identified three discrete characters of the pectoral fins that are associated with a benthic habitat for analysis; (1) an asymmetrical fin membrane among ventral fin rays, (2) the presence of fin rays that are unbranched distally, and (3) the presence of ventral pectoral fin rays that are free of fin membrane. Each of these characters has evolved more than once among scorpaeniform fishes and is more common among benthic than pelagic fishes