Meeting Abstract
The mechanical properties of an appendage influence its propulsive capabilities. In labrid fishes, which employ pectoral fin-based propulsion ranging from rowing to flapping, the fins of flappers are nearly an order of magnitude stiffer than rowers. Also, fin ray stiffness decreases along a fin’s proximodistal span and across the fin’s chord from the leading to trailing edge. The flexural stiffness of a simple structure is a product of its material’s Young’s modulus (E) and its second moment of area (I). However, fin rays are complex structures that are typically branched and segmented distally. To examine the impact of fin ray morphology on stiffness, we quantified intrinsic pectoral fin ray stiffness in similar-sized fins of two closely related species, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. We measured each fin ray’s E, I, unsegmented length, segments per unit length, average segment length, branching pattern, and the percent fin coverage by ray versus membrane (ray density). Within each species, bivariate analyses revealed that I, unsegmented length, average segment length, and number of segments per unit length were all significantly correlated with ray stiffness; E did not correlate with stiffness, but was significantly greater at 83.2% fin ray length in comparison to more proximal measurements. Multivariate analysis showed that the majority of the variation in stiffness was explained by variation in I. Branching pattern, fin ray density and I are the largest factors contributing to interspecific differences in fin stiffness. Variation in I, E, segmentation, and branching patterns combine to produce the overall stiffness field of a pectoral fin surface, contributing to its dynamic shape during locomotion.
