The biomechanics, kinematics, and hydrodynamics of swimming in the porcupine pufferfish (Diodon holocanthus)

Historical descriptions of tetraodontiform (pufferfish) locomotion are oversimplified and persist in current literature. This study was inspired by an effort to examine various types of tetraodontiform locomotion and modify their descriptions. The porcupine puffer, Diodon holocanthus, is a median and paired fin swimmer (MPF). Balances of multiple varying forces must be the basis for the unusually great dynamic stability of swimming pufferfishes. I used high-speed digital video recordings to study biomechanics and kinematics of rectilinear swimming at different speeds of five porcupine puffers in a water tunnel. I measured critical swimming speeds (Ucrit); fin biomechanics, kinematics, and coordination; recoil movements; and gait changes. Major propulsors were pectoral fins at lower speeds; dorsal, anal, and caudal fins at higher speeds. Precise coordination of fin movements produced small recoil movements at speeds below Ucrit. I developed a thrust model from kinematic data to describe thrust production on a per-fin basis, centers of effort, and lengths of corresponding moment arms. The thrust model indicated a force imbalance between the dorsal-anal complex and the caudal fin. Major thrust producers were the pectoral and caudal fins at lower speeds; dorsal, anal, and caudal fins at higher speeds. I used two-dimensional (2D) computational fluid dynamics (CFD) to examine hydrodynamic effects of the body of Diodon holocanthus on the flow environment. Simulations showed significant low pressure areas generated by the operculum, causing large force coefficients. The dynamic stability of the porcupine pufferfish is likely due to coordinated fin movements coupled with thrust and stability supplementation from fin and body-generated vortex rings.