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First published online July 20, 2006
Journal of Experimental Biology 209, 2920-2928 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02325

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Material properties and biochemical composition of mineralized vertebral cartilage in seven elasmobranch species (Chondrichthyes)

Marianne E. Porter^1,*, Jennie L. Beltrán¹, Thomas J. Koob² and Adam P. Summers¹

¹ Department of Ecology and Evolutionary Biology, 321 Steinhaus Hall, University of California, Irvine, CA 92697-2525, USA
² Shriners Hospital for Children, 12502 Pine Drive, Tampa, FL 33612-9499, USA

View larger version (106K): [in a new window]   Fig. 1. Radiograph of an anterior view of a mako shark (I. oxyrinchus) and silky shark (C. falciformis) vertebral centra with excised neural and hemal arches. Shark vertebral cartilages vary in extent and pattern of mineralization (Ridewood, 1921). I. oxyrinchus has many plates of mineralization radiating from the centrum whereas Ca. falciformis has essentially solid mineral around the perimeter of the vertebra. The lateral view of gulper shark vertebrae (Ce. granulosus) illustrates the mineralized double cone configuration.

View larger version (21K): [in a new window]   Fig. 2. Phylogeny of a diverse group of seven species varying in their ecology and inferred swimming speeds [phylogeny is adapted from Maisey et al. (Maisey et al., 2004)]. The ordinal color scheme is maintained in this paper. We sampled from the batoids and both lineages of sharks. The number to the right of the icon is the number of species we used from each order.

View larger version (23K): [in a new window]   Fig. 3. Material properties of mineralized cartilage in shark vertebral centra. (A) Ultimate strength (MPa) of vertebral cartilages from seven elasmobranch species showing significant differences (F6,151=182.8; P<0.001). The broken horizontal line represents the lower limits of trabecular bone (Rho et al., 1994). Letters above the box and whisker plot denote significant differences between species. (B) Material stiffness was significantly different among the species (F6,151=54.4; P<0.001). T. californica was less stiff than all shark species (P<0.001). The horizontal line shows the lower limits of stiffness for trabecular bone (Hodgskinson and Currey, 1992). (C) Yield strain was significantly different among the species (F6,147=27.6; P<0.001). T. californica had the greatest yield strain of all species (P<0.007).

View larger version (26K): [in a new window]   Fig. 4. Biochemical composition of vertebrae from seven species of elasmobranch. Letters above the box and whisker plot denote significant differences between species. (A) Water content (% WM) is significantly different among species (F6,153=70.483; P<0.001). (B) Mineral content (% DM) varied significantly among species (F6,63=27.836; P<0.001). (C) Proteoglycan (PG) content, expressed as percentage of dry mass (DM), varied among species (F6,82=10.531; P<0.001). The highest PG content was 28% found in I. oxyrinchus and the lowest was only 12% found in Ce. granulosus. (D) There were significant differences in collagen content, expressed as percentage of dry mass, among species (F6,85=4.054; P=0.001). Overall, the collagen content of the species examined ranged from 17% (T. californica) to 27% (S. zygaena).

View larger version (20K): [in a new window]   Fig. 5. Linear regressions of biochemical content on ultimate strength from seven species of elasmobranch. Ultimate strength of the shark vertebral cartilage was significantly correlated to water, mineral and collagen content (P<0.001) (A,C,D), but not proteoglycan content (B). Each point represents the material test and subsequent biochemical assay on a single vertebra.

View larger version (21K): [in a new window]   Fig. 6. Linear regressions of biochemical content on material stiffness from seven species of elasmobranch. Proteoglycan content (B) is the only biochemical component that appears to not influence material stiffness. Each point represents a single material test and subsequent biochemical assays on a single vertebra.

View larger version (50K): [in a new window]   Fig. 7. Radiograph of a portion of a vertebral column from Ce. granulosus (Gulper). Two vertebrae appear to be fused together (white arrow) as a possible result of material failure. This may also be the result of some pathology, but we suspect not based on the overall health of the rest of this vertebral column. Spinal deformities in elasmobranchs generally affect much larger portions of the column (Heupel et al., 1999). This may be an example of healing in a cartilaginous skeleton, which is contrary to information in the literature (Ashhurst, 2004).