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.

In this study, I compare the morphology from the pectoral fin rays from the benthic longhorn sculpin (Myoxocephalus octodecimspinosus) to those from a species that does not use its fins for substrate contact, the yellow perch (Perca flavescens). I use CT scanning technology to compare the shape and structure of the paired hemitrichia that make up the pectoral fin rays between these species. I found that the structure of hemitrichia of the fin rays in yellow perch is consistent with previous descriptions for pelagic fishes. They are almost completely segmented, have a crescent shape in cross section, and are branched distally. In contrast, longhorn sculpin hemitrichia exhibit morphological regionalization along the proximo-distal length of the ray. The most proximal 20-50% of the length of the hemitrichia is unsegmented and cylindrical in cross section. Distally, the fin rays of longhorn sculpin are segmented and crescent-shaped but do not branch. I measured the second moment of area of the hemitrichia at distances of 10%, 30%, 50%, and 70% distance along the length of the fin rays. The cylindrical regions of the sculpin hemitrichia had a higher second moment of area than the crescent-shaped regions in either species. I hypothesize that that this regionalization of individual fin rays provides resistance to bending proximally and flexibility distally, features that may be useful during substrate contact. This combination of an elongate, unsegmented proximal region and segmented distal region in fin rays has not yet been described among extant ray-finned fishes. However, this structure is reminiscent of that of the elongate cylindrical region found in the fossil sarcopterygian fish Eusthenopteron.

SUMMARYFin 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 crosssectional 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.

The fin-to-limb transition was marked by the origin of digits and the loss of dermal fin rays. Paleontological research into this transformation has focused on the evolution of the endoskeleton, with little attention paid to fin ray structure and function. To address this knowledge gap, we study the dermal rays of the pectoral fins of 3 key tetrapodomorph taxa—Sauripterus taylori (Rhizodontida), Eusthenopteron foordi (Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)—using computed tomography. These data show several trends in the lineage leading to digited forms, including the consolidation of fin rays (e.g., reduced segmentation and branching), reduction of the fin web, and unexpectedly, the evolution of asymmetry between dorsal and ventral hemitrichia. In Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more than ventral rays. In Tiktaalik, dorsal rays fully cover the third and fourth mesomeres, while ventral rays are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip analogous to a fleshy “palm.” Asymmetry is also observed in cross-sectional areas of dorsal and ventral rays. Eusthenopteron dorsal rays are slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and degree of asymmetry appears to be greater at larger sizes. Analysis of extant osteichthyans suggests that cross-sectional asymmetry in the dermal rays of paired fins is plesiomorphic to crown group osteichthyans. The evolution of dermal rays in crownward stem tetrapods reflects adaptation for a fin-supported elevated posture and resistance to substrate-based loading prior to the origin of digits.

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