Functional implications of morphological specializations among the pectoral fin rays of the benthic longhorn sculpin

Fin ray structure in ray-finned fishes (Actinopterygii) largely defines fin function. Fin rays convert the muscle activity at the base of the fin to shape changes throughout the external fin web. Despite their critical functional significance, very little is known about the relationship between form and function in this key vertebrate structure. In this study we demonstrate that morphological specializations of the pectoral fin rays of the benthic longhorn sculpin (Myoxocephalus octodecimspinosus) have specific functional consequences both within and among individual rays. The fin rays of longhorn sculpin have an elongate unjointed region with a cylindrical shape in cross-section proximally, and are jointed with a crescent-shaped cross-section distally. Variation in the relative length of the proximal versus distal regions affects the location of maximum curvature as well as the mean curvature along the length of individual rays. We experimentally manipulated fin rays to mimic the differential muscle activity that generates curvature of fin rays in living animals. We found that the shape of the fin rays in cross-section affects their curvature. Among fin rays, the most ventral fin rays with relatively longer proximal unjointed regions have a more distal location of maximum curvature. These ventral rays also have higher mean curvature, likely because of a combination of features including the cross-sectional shape, area and diameter of the distal segments as well as their relative size and number, which were not examined in detail here. Because these rays are used routinely for substrate contact, this higher curvature could contribute to increased flexibility for substrate contact behaviors such as clinging or gripping the substrate. These morphological and functional differences among fin rays are correlated with the functional regionalization of the fin. Specifically, the ventral fin rays that are used during substrate contact are more stiff proximally and more highly curved distally than the pectoral rays in the dorsal region, which are longer and used during slow swimming. This study highlights the importance of examining morphological and functional variation both within and among complex structures such as fin rays.