Distribution and relative proportions of red muscle in scombrid fishes: consequences of body size and relationships to locomotion and endothermy
The scaling of red muscle with body weight and the distribution of red muscle within the body were compared in seven scombrid fish species to determine relationships between red muscle function and the maintenance of endothermy by tunas. In ectothermic Sarda chiliensis and Scomber japonicus, red muscle occurs along the body edge, is concentrated posteriorly, and the total amount of this tissue is proportional to body weight raised to a power significantly greater than 1.0. In five endothermic tunas, Auxis thazard, Euthynnus lineatus, Katsuwonus pelamis, Thunnus albacares, and T. alalunga, red muscle scaling coefficients are 1.0 or less, and red muscle is positioned deep and anterior in the body. The power needed to overcome drag increases with fish body size (weight and length) and velocity and is reflected in the red muscle scaling relationships of both Sarda and Scomber. By contrast, decreasing relative amounts of red muscle in larger tunas suggest these fishes increase propulsion efficiency as they grow. This may be a result of either or both greater muscle efficiency and reduced division of labor between red and white muscle to which both endothermy and thermoregulation could contribute.