SHADWICK, R.E.: . Structure and Function of Axial Tendons in Tunas.

The great lateral tendons in tunas and some other scombrid fish link mytotomal muscle directly to the caudal fin rays, and thus serve to transfer muscle power to the hydrofoil-like tail during swimming. These robust collagenous tendons have structural and mechanical similarity to tendons found in other vertebrates. Biochemical studies indicate that tuna tendon collagen is composed of the alpha -1/alpha -2 heterotrimer that is typical of vertebrate Type I collagen, while tuna skin collagen has the unusual alpha-1/alpha-2/alpha-3 trimer previously described in other fish skin. While changes in covalent crosslinking could be induced with in vitro incubation over several months (as with mammalian tendons), no differences were detected between tendons from fish ranging from 2-75 kg. Application of buckle-type force transducers on caudal tendons in skipjack and yellowfin yielded measurements of in vivo forces during steady and burst swimming. Tendons excised post-mortem were subjected to load cycling to determine the modulus of elasticity and energy dissipation (0.65-1.2 GPa and 7-25%, respectively). These material properties compare closely to those of mammalian leg tendons that are known to function as effective biological springs in terrestrial locomotion. However, peak forces during steady swimming impose strains of much less than 1% of tendon length because the tendons are relatively thick. Even the maximal burst forces recorded produced strains of only 1.5-2%. Consequently, the strain energy stored in the stretched tendon is insignificant compared to the work done by the muscle in producing thrust. Thus, the caudal tendons in tunas do not function as energy saving locomotor springs, even at maximal swimming effort.