Meeting Abstract
In sharks, the vertebral column is engaged as a spring: both intervertebral joints and centra strain significantly during bending. The vertebral column has nonlinear mechanical properties, allowing it to function as a spring or a brake depending on the kinematic inputs to the system. These findings were isolated to specific regions of the vertebral column in a single species. Here we examine the dynamic mechanical properties along the length of the body, and we examine the histological morphology in two superficially similar but phylogenetically distant, shark species, the spiny dogfish, Squalus acanthias, and the smooth dogfish, Mustelus canis. We tested fresh segments of centra in bending over a range of frequencies and curvatures, similar to those experienced by these species when swimming. E’ and E” (the elastic and viscous components of stiffness; respectively) were significantly stiffer in the caudal region than the precaudal, while S. acanthias was stiffer than M. canis. Second moment of area (I, a structural predictor of stiffness) was also greater in the precaudal region. However, work to bend (W, elastic energy) did not vary in either species. The morphology of the vertebral column varied among species dramatically. iS. acanthias had an open intracentral canal while the canal was closed in M. canis. S. acanthias central cartilage was a deep spindle shape and had long intervertebral joints compared to the shallow spindle and shorter joints of iM. canis. These data suggest that the caudal region, compared to the precaudal region, stores and releases more elastic spring energy during swimming. Since this pattern occurs in two phylogenetically distant species, these regional differences in vertebral column mechanics and functional morphology may be a general solution for thrust production in sharks.
