THE HORIZONTAL SEPTUM - MECHANISMS OF FORCE TRANSFER IN LOCOMOTION OFSCOMBRID FISHES (SCOMBRIDAE, PERCIFORMES)

We describe the complex shapes of myomeres and myosepta in the mackere ls and tunas (Scombridae: Teleostei), and we 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. Our goal i s to identify the likely pathways of the transmission of muscle forces during locomotion. Our primary conclusions are 1) that the collagen f ibers of myosepta, horizontal septa, and skin are the organs that tran sfer locomotor forces from the contraction of myomeres to the backbone and caudal fin during locomotion, and 2) that 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 a nd is likely to be the major transmitter of muscle force to the axial skeleton. The geometry of the myomeres, the position of red muscle, an d particularly the geometric conformation of crossed-fiber arrays of c ollagen in the main horizontal septum suggest specific mechanisms for the transfer of muscle force to the backbone among scombrid fishes. Mo rphometrics and the construction of physical models help us to identif y musculoskeletal mechanisms of locomotion, and we present two quantit ative models of locomotor mechanics in fishes. (C) 1993 Wiley-Liss, In c.