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

Abstract

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.

Divergent functional traits in three sympatric Arctic charr Salvelinus alpinus morphs are not coupled with the age of the lineage divergence

“…Thus, in a general sense, fins rays are important for fin function. Despite this, there is little known about the effect of variation in fin ray form or number on the subtleties of ultimate fin function (Taft & Taft, 2012).…”

Functional morphology of the fin rays of teleost fishes

“…Bluegill sunfish, like many bony fish, swim by moving and changing the shape of their very flexible fins via muscular control of the lepidotrichia (Walker, ; Lauder et al, ; Alben et al, ; Lauder and Madden, ; Chadwell et al, ). Each lepidotrich is made up of two hemitrichs (Geerlink and Videler, ; Videler, ; Taft and Taft, ); the hemitrichs are controlled individually by muscles at their base which control curvature and stiffness of each fin ray (Alben et al, ). Fin rays are flexible, yet are formed from acellular bone (Moss, ) which is more mineralized than the cellular bone in the skeleton, but has a similar Young's modulus (Cohen et al, ).…”

“…Previous research has investigated the anatomy, mechanics, and active control of fin rays and their flexibility during normal locomotion, and studied the mechanical properties of isolated fin rays (Lauder et al, ; Alben et al, ; Taft et al, ; Taft and Taft, ). However, there has been no detailed investigation into the flexibility of ray‐finned fish fin rays under unsteady locomotor conditions such as turning, and no study to our knowledge has investigated the effect of fluid dynamic perturbations on fin ray function.…”

Developmental Change in the Function of Movement Systems: Transition of the Pectoral Fins between Respiratory and Locomotor Roles in Zebrafish

“…For example, the morphology and mechanics of the pectoral fins of longhorn sculpins are adapted for benthic station-holding (Taft and Taft 2012). The pectoral fins of sea robins include three free fin rays that have been modified as sensory substrate probes used in foraging (Bardach and Case 1965).…”