LAUDER, G.V.*; MADDEN, P.; MCHENRY, M.J.; Harvard University; Harvard University; Univ. California, Irvine: Fin rays of ray-finned fishes: biology and mechanics of remarkable structures

The median and paired fins of ray-finned fishes (Actinopterygii) are supported by fin rays with a characteristic structure. Each ray consists of two elements (hemitrichs) that articulate with pectoral girdle radial bones and are held together by collagen fibers which allow the hemitrichs to slide past each other. This design suggests that the shape and mechanical properties of a fin can be actively controlled by muscles attached to the hemitrichs. We examined the stiffness of fin rays in different regions of a fin and explored how stiffness and curvature change when fin rays are actuated at the base of the hemitrichs. Using sunfish (Lepomis macrochirus), we measured stiffness in both 3-point bending and cantilever tests and tracked fin ray shape change with high-resolution image acquisition. The 3-D structure of fins was measured with micro-CT in order to separate structural from material components of stiffness. Additional experiments compared known force inputs at the base of hemitrichs to force output at the distal third of fin rays. Stiffness ranged by nearly two orders of magnitude, with rays in the middle of the fin exhibiting greater stiffness than dorsal or ventral rays. The proximal portions of rays were as much as 30 times greater in stiffness than distal portions. Actuating hemitrichs with a displacement of 100 microns produced average forces of 10-15 mN at the distal third of middle fin rays. Preventing relative hemitrich displacement increased fin ray stiffness nearly 12 times. These results demonstrate that fish fins have tremendous regional variation in their mechanics and that their mechanical properties may be modulated by muscular actuation at the base of the hemitrichs. This allows active control over propulsor stiffness and curvature during locomotion.