Stiffness without mineral: material properties and biochemical components of jaws and chondrocrania in the Elasmobranchii (sharks, skates, and rays)

Chondrichthyians (sharks, ratfish, and rays) can function at extremes (growing big, swimming fast, and eating hard-prey) suggesting their skeletons are experiencing loading regimes equal to or greater than those of other fishes. In most vertebrates, cartilage is a soft connective tissue serving two purposes; a low-friction bearing surface and contour filler; however, cartilaginous fishes maintain a skeleton made of cartilage throughout life. We examined material properties and biochemical components of cartilage from the jaws and/or chondrocranium of seven species of shark. For each species cylindrical plugs were drilled from the specimen, mineralized tesserae were removed, and plugs tested in compression to ten percent of initial thickness (ε=0.10) at 2mm/sec. Stiffness and strength varied significantly among species and in both cases the chondrocranial properties were greater than those of the jaws. After materials testing, cartilage plugs were lyophilized to obtain water content; then collagen and proteoglycan was measured with hydroxyproline and DMMB assays, respectively. Water content was greatest in the chondrocranial cartilage while collagen content was consistent between the jaws and chondrocrania. However, proteoglycan content was greater in the jaw cartilage. The average values for water and proteoglycan content were consistent with mammalian cartilage, while collagen content was much lower than mammalian cartilage. Material properties and biochemical components were also similar to the mineralized cartilage found in elasmobranch vertebral cartilage.