Morphologically driven swimming dynamics of the longnose skate

Abstract

Skates propel themselves using winglike pectoral fins that generate undulatory waves along the body. We hypothesize that their skeletal morphology, parallel rows of fin rays composed of mineralized radials, enhances locomotion by passively directing waves through stiffness. Stiffness decreases distally down each fin ray due to bifurcation, and the space between parallel fin rays reduces stiffness along the anterior to posterior axis of the fin. We compared swimming in a live longnose skate (Raja rhina) to the passive motion of a deceased specimen driven by a vertical linear actuator. We used motion tracking to quantify frequency, wavenumber, and amplitude. We investigated whether an undulatory wave can be passively generated without muscle activation, and at what frequencies it most closely resembled live swimming. We observed that for live swimming, small increases in frequency were able to significantly increase swimming velocity, with the skate having a preferred frequency and wavenumber range. This range also matched the observed values calculated from the dead skate’s response, with similar frequencies of maximum amplitude for the live and deceased skate. At these peak frequencies, the output wavenumbers also matched. These results indicated that morphology alone supports wave propagation without active muscle input. Musculoskeletal structure constrains and optimizes the swimming frequency and wavenumber, highlighting the role of passive mechanics in undulatory swimming in the longnose skate.