Biomechanics of the jaws of spotted ratfish

Elasmobranch fishes (sharks, skates and rays) consume prey of a variety of sizes and properties, and the feeding mechanism typically reflects diet. Spotted ratfish, Hydrolagus colliei (Holocephali, sister group of elasmobranchs), consume both hard and soft prey; however, the morphology of the jaws does not reflect the characteristics typical of durophagous elasmobranchs. This study investigated the mechanical properties and morphological characteristics of the jaws of spotted ratfish over ontogeny, including strain, stiffness and second moment of area, to evaluate the biomechanical function of the feeding structures. Compressive stiffness of the jaws (E=13.51-21.48 MPa) is similar to that of silicone rubber, a very flexible material. In Holocephali, the upper jaw is fused to the cranium; we show that this fusion reduces deformation experienced by the upper jaw during feeding. The lower jaw resists bending primarily in the posterior half of the jaw, which occludes with the region of the upper jaw that is wider and flatter, thus potentially providing an ideal location for the lower jaw to crush or crack prey. The mechanical properties and morphology of the feeding apparatus of spotted ratfish suggest that while the low compressive stiffness is a material limit of the jaw cartilage, spotted ratfish, and perhaps all holocephalans, evolved structural solutions (i.e. fused upper jaw, shape variation along lower jaw) to meet the demands of a durophagous diet.