Meeting Abstract
As swimming speed varies, body curvature changes along the length of the body, presumably as a function of body stiffness, skeletal stiffness, or some combination of the two. Since body curvature varies regionally, we hypothesize that the mechanical properties of the vertebral column also vary along the length of the body. The mineral fraction found in vertebral cartilage of elasmobranchs (sharks, skates, and rays) has been shown to influence the tissue stiffness, which varies by species. Based on previous research, we hypothesize that mineral content increases in these tissues throughout ontogeny and, in concert; the stiffness of the vertebral column will increase. Here we examine the effects of regional variation and ontogeny on the mechanical properties of vertebral columns from blacktip sharks (Carcharhinus limbatus). Specimens ranged in size from 52 to 113 cm, representing more than a doubling in animal size. We used a custom built bending rig to translate axial movement into bending on a dynamic mechanical tester over a range of frequencies (0.01 to 2.0 Hz) and strains (0.1 to 2%). Testing was done in an environmental chamber filled with elasmobranch Ringers solution to maintain cartilage hydration. We measured the apparent composite modulus, E (in MPa), an estimate of the contribution of material to the stiffness of the structure. Regional variation and the interaction between regional variation and ontogeny significantly influenced the composite modulus, but the effect of ontogeny alone was not significant. We found the same results for flexural stiffness, EI (Nm2), which takes into account the composite modulus and the second moment of area, I, a metric of the structure’s cross-sectional distribution of material. We also found the same results for work out of the system (J), which is the energy released during elastic recoil.
