Meeting Abstract
Feeding performance in durophagous stingrays is typically associated with large jaw adductor muscles, a robust jaw skeleton, and rigid, interlocking teeth. All four genera within Myliobatidae have diets composed entirely of decapod crustaceans, bivalves, and gastropods. High bite forces allow juveniles to compete with adults and access these robust prey early in their ontogeny. Bullnose rays (Myliobatis freminvillii) and cownose rays (Rhinoptera bonasus) represent two evolutionary lineages of durophagous stingrays. Here we consider the anatomical basis of bite force generation in Myliobatis, and compare it to previously collected data for Rhinoptera. An ontogenetic series of 12 bullnose rays were dissected and the arrangement and size of their jaw muscles were used to estimate maximum bite force using a biomechanical model. Both species of ray have jaw adductor muscles that increase rapidly in size during ontogeny, driving positive allometry of bite force generation. We find that the primary jaw adductor muscle tendon functions as a biological pulley in both rays. This tendon redirects lateral muscle forces in a direction perpendicular to the occlusal plane. This tendon is less robust in bullnose rays compared to cownose rays. Cownose rays have a drastically wider gape, coercing the tendon to form a more acute angle over which muscle forces are redirected. We also detected differences in interactions between the tendon and jaw cartilage that likely effect the mechanical strength of the jaw.
