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.

Biomechanics of the jaw of the durophagous bonnethead shark.

Durophagy in chondrichthyan fishes is thought to entail a set of morphological characteristics, such as hypertrophied adductor muscles, molariform teeth, and high bite forces. However, these characteristics are not common to all durophagous chondrichthyans. In some durophagous chondrichthyans, the jaws are better suited biomechanically to resist bending in the area where prey is processed. Resistance to bending is in part, quantified by second moment of area (I), which uses the neutral axis of an object to analyze the arrangement of material. This study investigated whether the lower jaw of the bonnethead shark, Sphyrna tiburo, is more resistant to bending under the crushing/molariform teeth compared to the grasping teeth. Using computerized tomography (CT) scanning, the jaws of ten bonnethead sharks were visualized, then digitally resliced at identical positions along the jaw for all specimens. I increased along the lower jaw from anterior to posterior as the teeth transform from grasping to crushing, with the largest absolute increase occurring about the transition from grasping to crushing teeth. When the lower jaw is compared to that of a rod of similar cross-sectional area, the shape exceeds that of a rod by 1.6 to 5.7 times, meaning the shape of the jaw is better suited to resist bending than if the same size jaw was shaped as a solid rod. These results suggest the lower jaw of S. tiburo is adapted to resist bending more under the molariform teeth where crushing occurs than at the anterior grasping teeth.