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 1 of C-start) was measured in four species of tropical marine fishes. A linear correlation between maximum curvature and number of functional intervertebral joints was found (range for number of joints, 17-25). A biomechanical model of vertebral column bending predicts that, if intervertebral joint angles are held constant, increasing the number of joints should produce a linear decrease in the measured curvature coefficient (curvature coefficient is inversely related to curvature). The measured curvature coefficients fit this model closely, indicating that, within the range of 17-25 joints, vertebral number is an important determinant of vertebral column flexibility. The study species with the lowest vertebral number, a filefish, Monacanthus hispidus, is a member of the Tetraodontiformes, a group characterized by the lowest vertebral numbers found among fishes. Elaborate antipredator defenses, such as a carapace and the ability to inflate the body, have evolved six times within the Tetraodontiformes, and some form of mechanical defense is present in all families of this group. We propose an evolutionary scenario in which low vertebral number reduced the escape swimming performance of ancestral tetraodontiforms, thus increasing their vulnerability to predators and driving the repeated evolution of mechanical defenses in this group. Our finding that lower vertebral numbers are correlated with lower C-start curvature suggests that low vertebral number may impair escape performance; thus, one necessary condition for the proposed scenario is met.