The relationship between pectoral fin ray stiffness and swimming behavior in Labridae: insights into design, performance, and ecology

Abstract

The functional capabilities of flexible, propulsive appendages are directly influenced by their mechanical properties. The fins of fishes have undergone extraordinary evolutionary diversification in structure and function, which raises questions of how fin mechanics relate to swimming behavior. In the fish family Labridae, pectoral fin swimming behavior ranges from rowing to flapping. Rowers are more maneuverable than flappers, but flappers generate greater thrust at high speeds and achieve greater mechanical efficiency at all speeds. Interspecific differences in hydrodynamic capability are largely dependent on fin kinematics and deformation, and are expected to correlate with fin stiffness. Here we examine fin ray stiffness in two closely related species that employ divergent swimming behaviors, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. To determine the spatial distribution of flexural stiffness across the fin, we performed three-point bending tests at the center of the proximal, middle and distal regions of four equally spaced fin rays. Pectoral fin ray flexural stiffness ranged from 0.0001 to 1.5109 µN m 2 , and the proximal regions of G. varius fin rays were nearly an order of magnitude stiffer than those of H. bivittatus. In both species, fin ray flexural stiffness decreased exponentially along the proximodistal span of fin rays, and flexural stiffness decreased along the fin chord from the leading to the trailing edge. Furthermore, the proportion of fin area occupied by fin rays was significantly greater in G. varius than in H. bivittatus, suggesting that the proportion of fin ray to fin area contributes to differences in fin mechanics.

The evolution of underwater flight: The redistribution of pectoral fin rays, in manta rays and their relatives (Myliobatidae)

“…The anterior region of myliobatid pectoral fins is stiff relative to other batoids, with cross‐bracing, increased radial thickness in the leading edge of the wing (Schaefer & Summers, ), and the compagibus laminam where propterygial pectoral fin rays lack terminal branching. This parallels the case in Labridae reef fishes, species with high aspect ratio (wing‐like) pectoral fins used in the flapping (oscillatory) swimming mode are stiffer relative to species with broader (paddle‐like) fins (Aiello et al, ; Thorsen & Westneat, ).…”

Variation in flexural stiffness of the lepidotrichia within and among the soft fins of yellow perch under different preservation techniques

“…In live animals, the curvature and position of the fin rays is generated by the contraction of the muscles that insert onto the proximal heads of the hemitrichia at the base of the fins. Although fins are not subjected to forces that would approximate three‐point bending in nature, this type of materials testing approach gives us an idea of the relative flexibility both within and among fin rays (Aiello et al, ; Alben et al, ; Taft et al, ; Tangorra et al, ) . By comparing the relative stiffness of the individual fin rays, we can learn about how functional variation could affect whole‐fin function.…”

“…Muscle contraction at the base of the fin can generate forces that control the position of the rays relative to one another, as well as the curvature along the length of individual fin rays (Alben et al, ; Arita, ; Geerlink & Videler, ; Chadwell & Ashley‐Ross, ; Lauder, ). The fact that fin rays provide support and define fin function as a whole has led to a burgeoning interest in the relationship between fin ray morphology and fin performance (Aiello, Westneat, & Hale, ; Aiello et al, ; Alben et al, ; Chadwell & Ashley‐Ross, ; Chadwell et al, ; Flammang, Alben, Madden, & Lauder, ; Geerlink & Videler, ; Taft & Taft, ).…”

“…There are two key reasons for using a three‐point bending approach. First, although fin rays are complex, bilaminar, and composite structures, we can still use this method to directly measure the stiffness of the whole structure (Aiello et al, ; Lauder et al, ; Tangorra et al, ). Second, studies of the material properties of vertebrate bony structures vary widely in methodology and taxonomic sampling (Currey, ).…”

“…Many previous studies of fin ray morphology and function have focused on isolated rays, or only one or two fins from each species, (Alben et al, ; Arita, ; Geerlink & Videler, ). There are several studies that have expanded the sample of fin rays analyzed, demonstrating substantial intra‐fin variation in both fin ray performance (curvature and stiffness) and morphology (Aiello et al, ; Chadwell & Ashley‐Ross, ; Chadwell et al, ; Lundberg & Marsh, ; Taft, ; Taft, Lauder, & Madden, ; Taft & Taft, ; Taft et al, ). However, we know of no studies that have examined multiple fin rays from every fin within a species.…”