Role of Red and White Muscles in the Swimming of the Skipjack Tuna

Abstract

IN several respects the skipjack (Katsuwonus pelamis) can be considered the most highly specialized of the scombroid fishes: in this species, the swimbladder has been lost, the branchial musculature is extremely reduced, and the deep red muscle, found only in the tunas, assumes the largest proportion of the trunk musculature of any of the species investigated by Kafuku¹ in his comparative study of the distribution of red muscle. Constant forward motion is thus required, not only to give hydrodynamic lift but also to provide a flow of water over the gill surfaces; it seems reasonable to suppose that the large amount of deep red muscle is in some way associated with these requirements. Braekkan², on the basis of a primarily biochemical investigation, has concluded that this deep red muscle functions as a store of energy for the adjacent region of white rather than as a contractile tissue in its own right. Recent work on the red muscles of other teleosts^3,4 has shown that they are fully contractile and have neuromuscular properties similar to the slow fibres of terrestrial vertebrates. These muscles are found in situations in which prolonged activity is required. By analogy with other fish, it might therefore be thought that the constant “basal swimming” of the scombroids would be a result of red muscle contraction. There remained the possibility that in the highly specialized scombroids the deep red muscle might indeed be found to perform a maintenance function; Braekkan² has stated that … “The anatomical situation of the red muscle prevents its taking part in the main muscular work”.