Fish caudal fin control mechanisms and fluid dynamics

Fishes comprise approximately 35,000 of all known vertebrates and exhibit incredible diversity in fin morphology, which in many species is presumed as evidence for the evolution of novel locomotory modes, Bluegill sunfish (a teleost fish) possess a homocercal tail, meaning that the dorsal and ventral lobes of the tail are equal in size. The internal caudal musculature of teleosts does not retain this symmetry, due in particular to the large hypochordal longitudinalis muscle that acts upon the dorsal lobe. Under steady swimming conditions, bluegill sunfish exhibited speed-dependent modification of duty cycle and burst activity of tail muscles, displaying variable muscle recruitment patterns that had not previously been observed in any studies of fish body musculature. During maneuvering, intrinsic caudal muscles act in a more specialized manner compared to axial muscle counterparts and are capable of fine motor control of fin shape and stiffness resulting in an extensive locomotory repertoire. Sharks (Chondrichthyans are some of the most primitive fishes of which there are living representatives, and unlike teleosts they possess a heterocercal tail where the dorsal lobe is much larger than the lower lobe. While the kinematics and two-dimensional hydrodynamics of heterocercal tail locomotion had been studied previously, the internal morphology of the shark tail had been largely ignored. The radialis muscle, a small red muscle in the tail of sharks, acts independently of the axial musculature of swimming sharks. Instead of contributing to undulatory locomotion, the radialis muscle acts in a postural role modifying stiffness of the tail fin throughout the tail beat to provide resistance against hydrodynamic loading. The radialis muscle may be the earliest instance in vertebrates of using a muscle to modify hydrostatic pressure in order to locally control stiffness through its association with the stratum compactum of the dermis. Using a novel three-dimensional particle image velocimetry (PIV) system, instantaneous volumetric hydrodynamic wake structures formed by live swimming shark and robotic analogs were captured. This new method has revealed that some volumetric wake structures were incorrectly inferred in previous work using two-dimensional methods.