Locomotor muscle of high performance fishes: What do comparisons of tunas with ectothermic sister taxa reveal?

Many studies of high-performance fishes focus on tunas because they are morphologically adapted for efficient swimming, they migrate long distances and accelerate rapidly, and they successfully exploit the epipelagic marine environment. Tunas have also received attention because, by conserving metabolically generated heat, they elevate the temperature of certain tissues above ambient water temperature; that is, they are endothermic. However, few studies have used phylogenetically based comparisons to determine what is unique about tunas and to elucidate evolutionary trends. This paper compares tunas with their closest relatives, ectothermic fishes of the family Scombridae, and examines characteristics of the myotomal musculature that are related to swimming performance. White muscle anaerobic capacity is significantly greater in tunas chan in ectothermic scombrid fishes, indicating that energy generation for bursts may be greater in tunas. Because tunas have high capacities to buffer metabolic acids within the white muscle and blood, and to recover from exhaustive exercise rapidly, they may be able to burst repeatedly. The white muscle of tunas also has a higher aerobic capacity than that of ectothermic scombrids, which may indicate chat at lease some tuna white muscle fibers are recruited at sustainable speeds or it may be related to rapid lactate turnover or growth races. In contrast, many intracellular characteristics of the red myotomal muscle do not differ significantly between tunas and ectothermic scombrid fishes. Many of these, including mitochondrial ATP production rate and muscle contraction speed, are temperature dependent and are higher at. the higher in vivo red muscle temperatures of tunas. Thus, endothermy may result in elevated aerobic performance levels in tunas, and their high blood hemoglobin concentrations, muscle myoglobin contents, and red muscle capillary densities would ensure adequate oxygen delivery to the active, warm red muscle fibers. However, direct tests of this hypothesis are needed. Much more remains to be learned about tunas and ectothermic scombrids so that more complete phylogenetic analyses can be used to understand the adaptive strategies that have resulted in tunas' success as active epipelagic predators.