The mechanics of active fin-shape control in ray-finned fishes.

We study the mechanical properties of fin rays, which are a fundamental component of fish fin structure. We derive a linear elasticity model that predicts the shape of fin rays, given the input muscle actuation and external loading. We then test the model using experiments that measure (i) the ray deflection for a given actuation at the muscular interface, and (ii) the force-displacement response under conditions of actuation and non-actuation. The model agrees well with the experiment; both show a concentration of curvature at the ray base or at the point of an externally applied force, and a variation in ray stiffness over more than an order of magnitude depending on actuation at the bases of the fin rays.