7. Mechanical design for swimming: muscle, tendon, and bone

Abstract

Abstract The muscle, tendon, and bone of tunas and mackerels form a mechanical system specialized for both high-performance swimming and long-distance cruising. The axial musculature, connective tissues, and skeleton of scombrid fishes exhibit modifications of the basic actinopterygian design for swimming that highlight mechanisms for the generation and transmission of force from muscle to caudal fin. This chapter illustrates the axial morphology of scombrids and analyzes the mechanics of thunniform locomotion. The external body shape and tail morphology are described for a variety of scombrid fishes. We review the complex shapes of myomeres and myosepta in the mackerels and tunas and reveal the orientation of two major systems of collagen fibers in myosepta and horizontal septa with respect to points of attachment to skeleton and skin. Locomotor muscle is arranged in serially nested cones that are associated with complex arrays of connective tissue. The collagen fibers of myosepta, horizontal septa, and skin are the tensile elements that transfer locomotor forces from the contraction of myomeres to the backbone and caudal fin during locomotion. Locomotor muscle pulls against a three-dimensional structure of tendons, septa, and skin that is kept in tension by the radial expansion of the contracting muscle. The main horizontal septum is formed by the convergence of myosepta and is likely to be the major transmitter of muscle force to the axial skeleton. The vertical septum is formed from the serial neural spines and a fabric of collagen fibers connecting them. We propose a biomechanical model for the function of the neural spines and vertical septum in energy storage and return in scombrids. A posterior system of muscles and tendons operates the Innate caudal fin in tunas and mackerels. A new mechanical model is proposed for the function of the caudal muscles in fine tuning the shape and motion of the caudal fin. The construction of biomechanical models allows the identification of several musculoskeletal mechanisms of locomotion in scombrid fishes that 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.