PhD student Tara Law; MSc. University of British Columbia
Fish swim by passing waves of bending posteriorly along their bodies, and these waves impart forward thrust by accelerating water through the shedding of vortices at the trailing edge of the fish's tail fin. This process is powered by muscles that are distributed along the full length of the fish body. Muscles near the head of the fish contract first and contraction proceeds posteriorly down the length of the fish to the caudal fin. Although researchers now have an idea of how muscle activation might be proceeding down the body, it is still unclear how the muscle force is transitted to the tail. If contraction occurs at a distance from bending, muscle force must be trasmitted down the body by other structures rather than by direct connection of the muscles to the vertebrae. Several structures have been suggested to transmit muscle force including tendons, myosepta, and skin.
The skin of fishes may play a substantial role in swimming mechanics. Fish dermis is composed largely of a crossed-helical array of collagen fibres which connect directly to the myosepta of the muscles and to the tail fin. The myosepta (collagen sheets that separate layers of muscle fibres along the fish body) attach directly to the skin as well as to the backbone. Thus, the skin forms a key component of the muscle-tendon-tail fin system in fish. The arrangement and mechanical properties of these collagen fibres may enable the skin to function in a number of ways that could decrease the cost of swimming. It is likely that the skin provides a tensile container for a hydrostatic structure that contributes to the overall skeletal support of these animals, and through this type of support system it may constitute an elastic structure that would provide energy conserving mechanisms that reduce the cost of swimming. The goal of my research is to develop a full understanding of the mechanical role of the skin.
Previous researchers have provided indications of the potential of the skin to play a major mechanical role in fish swimming, but they are far from providing clear evidence of the true role. Three major possibilities for the function of the helically-wound fibre lattice in the skin, which may not be mutually exclusive are:
(1) fibre-wound hose - the skin may provide a flexible container that allows easy bending of the animal's body without buckling, much like the action of helical reinforcing in many hoses.
(2) rope - the skin may act as a rigid linkage system that serves to transfer muscular forces generated in the anterior portion of the fish, backwards to the tail where the majority of the propulsive thrust is generated.
(3) spring - the skin may act as an elastic system that captures energy from the stretch applied to it at one stage of the locomotor cycle and then pays back this energy through elastic recoil at a later stage, in a process that might improve efficiency or enhance performance.