X-ray motion analysis of the vertebral column during the startle response in striped bass, Morone saxatilis.

Whole-body stiffness has a substantial impact on propulsive wave speed during axial undulatory locomotion in fishes. The connective tissues of the vertebral column may contribute to body stiffness, but without mechanical and kinematic analysis it is unclear whether the in vivo range of motion of intervertebral joints (IVJs) is great enough to stress IVJ tissues, thus generating stiffness. The present study used 2D videoradiography and 3D X-ray reconstruction of moving morphology (XROMM) to quantify vertebral kinematics during the startle response in striped bass (Morone saxatilis). X-ray video revealed two distinct patterns of bending: pattern I begins in the abdominal region and then proceeds to maximum IVJ angles in the caudal region, whereas pattern II begins in the cervical region and proceeds to maximum IVJ angles in the abdominal and then the caudal joints. In pattern II bends, the cervical joints exhibit a greater in vivo range of motion than previously reported in other species. XROMM analysis of caudal IVJs suggests primarily lateral bending: mean axial and dorsoventral rotations were less than 2 deg and inconsistent across 51 sequences analyzed from five individuals, whereas mean maximum lateral bending angles were 10.4±3.57 deg. These angles, combined with previous investigations of mechanical properties, reveal that the maximum angles all occur within the neutral zone of bending, indicating that little stress is experienced about the joint. This suggests that the IVJs of striped bass are quite compliant and likely do not contribute significantly to whole-body stiffness or elastic recoil during swimming in vivo.