SUMMARY Elasmobranchs, particularly sharks, function at speed and size extremes,exerting large forces on their cartilaginous skeletons while swimming. This casts doubt on the generalization that cartilaginous skeletons are mechanically inferior to bony skeletons, a proposition that has never been experimentally verified. We tested mineralized vertebral centra from seven species of elasmobranch fishes: six sharks and one axially undulating electric ray. Species were chosen to represent a variety of morphologies, inferred swimming speeds and ecological niches. We found vertebral cartilage to be as stiff and strong as mammalian trabecular bone. Inferred swimming speed was a good, but not infallible, predictor of stiffness and strength. Collagen content was also a good predictor of material stiffness and strength, although proteoglycan was not. The mineral fraction in vertebral cartilage was similar to that in mammalian trabecular bone and was a significant predictor of material properties.

26Chondrichthyians (sharks, ratfish, and rays) can function at extremes (growing big, swimming 27 fast, and eating hard-prey) suggesting their skeletons are experiencing loading regimes equal to 28 or greater than those of other fishes. In most vertebrates, cartilage is a soft connective tissue 29 serving two purposes; a low-friction bearing surface and contour filler; however, cartilaginous 30 fishes maintain a skeleton made of cartilage throughout life. We examined material properties 31 and biochemical components of cartilage from the jaws and/or chondrocranium of seven species 32 of shark. For each species cylindrical plugs were drilled from the specimen, mineralized tesserae 33were removed, and plugs tested in compression to ten percent of initial thickness (ε=0.10) at 34 2mm/sec. Stiffness and strength varied significantly among species and in both cases the 35 chondrocranial properties were greater than those of the jaws. After materials testing, cartilage 36 plugs were lyophilized to obtain water content; then collagen and proteoglycan was measured 37 with hydroxyproline and DMMB assays, respectively. Water content was greatest in the 38 chondrocranial cartilage while collagen content was consistent between the jaws and 39 chondrocrania. However, proteoglycan content was greater in the jaw cartilage. The average 40 values for water and proteoglycan content were consistent with mammalian cartilage, while 41 collagen content was much lower than mammalian cartilage. Material properties and 42 biochemical components were also similar to the mineralized cartilage found in elasmobranch 43 vertebral cartilage.

26Chondrichthyians (sharks, ratfish, and rays) can function at extremes (growing big, swimming 27 fast, and eating hard-prey) suggesting their skeletons are experiencing loading regimes equal to 28 or greater than those of other fishes. In most vertebrates, cartilage is a soft connective tissue 29 serving two purposes; a low-friction bearing surface and contour filler; however, cartilaginous 30 fishes maintain a skeleton made of cartilage throughout life. We examined material properties 31 and biochemical components of cartilage from the jaws and/or chondrocranium of seven species 32 of shark. For each species cylindrical plugs were drilled from the specimen, mineralized tesserae 33were removed, and plugs tested in compression to ten percent of initial thickness (ε=0.10) at 34 2mm/sec. Stiffness and strength varied significantly among species and in both cases the 35 chondrocranial properties were greater than those of the jaws. After materials testing, cartilage 36 plugs were lyophilized to obtain water content; then collagen and proteoglycan was measured 37 with hydroxyproline and DMMB assays, respectively. Water content was greatest in the 38 chondrocranial cartilage while collagen content was consistent between the jaws and 39 chondrocrania. However, proteoglycan content was greater in the jaw cartilage. The average 40 values for water and proteoglycan content were consistent with mammalian cartilage, while 41 collagen content was much lower than mammalian cartilage. Material properties and 42 biochemical components were also similar to the mineralized cartilage found in elasmobranch 43 vertebral cartilage.