Divergent dogfish: morphological correlates of vertebral column bending mechanics in two species of distantly related sharks.

backgroundDogfish sharks from two different Orders - Squalus acanthias and Mustelus canis - offer the opportunity to compare whether known differences in vertebral morphology correlate with differences in the dynamic mechanical properties of the vertebral column. Histology of a caudal intervertebral joint in both species, combined with morphometrics of the entire column, provide the morphological information. Dynamic bending tests of ten-vertebrae sections of the vertebral column from the head to the base of the caudal fin are conducted by varying the amplitude of lateral curvature and bending frequency in a factorial design. Using multivariate modeling, this question is addressed: How do species, anatomical region, and vertebral column morphology correlate with the dynamic mechanical properties of the vertebral column in bending?

resultsVertebral column morphology differs in the ratio of precaudal to caudal vertebrae, the form of the intracentral canal, and the lateral position of the intervertebral ligaments. Six quantitative features of the vertebral column - dimensions and ratios of the centra and the intervertebral joints - vary between species. The dynamic mechanical properties, in turn, differ between species at three structural levels, the intact vertebral column, the cross-sectional level, and as apparent material properties. The two species have similar mechanical behavior: a strong response to changes in curvature and almost no response to changes in bending frequency; in both species, the loss modulus increases dramatically at the highest bending frequency. In spite of those similarities, most of the morphological and mechanical features are different. In terms of morphological correlates, the diameter and length of the centrum significantly and robustly predict the apparent material properties of storage and loss moduli, with coefficients of determination above 0.50 in all multivariate models for those properties.

conclusionsThe increase in the loss modulus at higher bending frequencies is consistent with the hypothesis that the vertebral column of dogfish sharks functions as a spring-brake mechanism during swimming. The mechanical properties are predicted, to a large degree, by variation in the morphology of the centrum within and between species, suggesting a possible link between vertebral morphology and bending mechanics, with morphological differences between species indicative of possible evolutionary trajectories for the alteration of the mechanical behavior of the vertebral column.