WESTNEAT, M.W.*: Mechanical design for swimming in big fish: Locomotor function in tunas and relatives.

The axial morphology of big marine fishes is a mechanical system specialized for both high performance swimming and long-distance cruising. The axial musculature, connective tissues, and skeleton of tunas, mackerels, billfishes and their relatives exhibit modifications of the basic actinopterygian design for swimming that reveal mechanisms for the generation and transmission of force from muscle to caudal fin. The main horizontal septum is formed by the convergence of myosepta and is the major transmitter of muscle force to the axial skeleton. The vertical septum is formed from the serial neural and hemal spines and a fabric of collagen fibers connecting them. Using polarized light, the collagen fiber matrix of the vertical septum is shown to be a crossed-fiber array in which fibers are oriented in two primary directions, one coursing anterodistally and the other posterodistally from the backbone at angles ranging from 45-55� to the vertebral axis. A biomechanical model is proposed for the function of the neural spines and vertical septum inenergy storage and return in scombrids. In the tail, a posterior system of muscles and tendons operates the caudal fin of tunas, billfishes, and other taxa. Comparative morphology of the hypochordal longitudinalis, flexor dorsalis, flexor ventralis, infracarinalis, supracarinalis, and interradialis muscles is presented. A new mechanical model is proposed for the function of the caudal muscles in fine tuning the shape and motion of the caudal fin. Biomechanical models make predictions about muscle function and force transfer in locomotion of big marine fishes that may direct future research on muscle function. Key features of the locomotor design of tunas, mackerels and outgroups are highlighted on a phylogeny to identify the major evolutionary stages in the functional morphology of scombrid locomotion. ONR N000149910184 and NSF DEB- 9815614.