Vertebral column morphology, C-start curvature, and the evolution of mechanical defenses in tetraodontiform fishes

Maximum body curvature during the initial phase of escape swimming (stage I of C-start) was measured in four species of tropical marine fishes. A line ar correlation between maximum curvature and number of functional intervert ebral joints was found (range for number of joints, 17-25), A biomechanical model of vertebral column bending predicts that, if intervertebral joint a ngles are held constant, increasing the number of joints should produce a l inear decrease in the measured curvature coefficient (curvature coefficient is inversely related to curvature), The measured curvature coefficients fi t this model closely, indicating that, within the range of 17-25 joints, ve rtebral number is an important determinant of vertebral column flexibility. The study species with the lowest vertebral number, a filefish, Monacanthu s hispidus, is a member of the Tetraodontiformes, a group characterized by the lowest vertebral numbers found among fishes. Elaborate antipredator def enses, such as a carapace and the ability to inflate the body, have evolved six times within the Tetraodontiformes, and some form of mechanical defens e is present in all families of this group, We propose an evolutionary scen ario in which low vertebral number reduced the escape swimming performance of ancestral tetraodontiforms, thus increasing their vulnerability to preda tors and driving the repeated evolution of mechanical defenses in this grou p. Our finding that lower vertebral numbers are correlated with lower C-sta rt curvature suggests that low vertebral number may impair escape performan ce; thus, one necessary condition for the proposed scenario is met.