Meeting Abstract
8.2 Thursday, Jan. 3 Ontogeny, morphology and mechanics of the tessellated skeleton of cartilaginous fishes DEAN, MN*; SUMMERS, AP; Univ. of California, Irvine; Univ. of California, Irvine mdean@uci.edu
The cyclic loading of the feeding and swimming modes of elasmobranch fishes (sharks, rays and relatives) is not compatible with the fact that cartilage cannot repair. Materials counteract the gradual build-up of fatigue damage through either being overbuilt (with an excessive safety factor) or resistant to fatigue. As the former is unlikely in active animals, we posit that elasmobranch skeletons are inherently fatigue-resistant and that this is a function of the calcification of the tissue. The uncalcified hyaline-like cartilage core of each element is overlain by a tessellated bark of abutting mineralized tiles (tesserae), adjoined by a fibrous phase. We employ a diversity of imaging techniques and ontogenetic tissue series to investigate the development, ultra-scale morphology and mechanics of the tessellated skeleton in a species of stingray. Tesserae form in histotroph embryos and gradually widen and thicken with ontogeny. Chondrocytes flatten and are engulfed by tesserae to form cell-rich laminae with communicating passageways between entombed lacunae. Elasmobranch chondrocytes decrease in size and density with age as in endochondral ossification, yet do not hypertrophy and die as in tetrapods. We combine materials testing and synchrotron radiation microtomography to define empirical parameters for functional hypotheses and determine the response of tesserae and tessellated elements to load. Nanoindentation tests show that the mineralized tissue behaves nearly elastically and is an order of magnitude stiffer than the uncalcified layer, which is highly viscoelastic. Mathematical models suggest that, during skeletal bending, this layered biological composite acts to distribute damaging tensile stresses to the compressive portion of the mineralized phase where the elastic modulus is more than three times higher and therefore better able to resist applied forces.
