UNIQUE ADAPTATIONS OF THE METABOLIC BIOCHEMISTRY OF TUNAS AND BILLFISHES FOR LIFE IN THE PELAGIC ENVIRONMENT

Virtually all characteristics of tunas and billfishes reflect their highly charged lifestyles as apex predators in the oceanic pelagic environment. The adaptations they possess for efficient and rapid swimming, efficient and rapid food processing, turnover of nutrients and storage and mobilization of internal fuel supplies, and for rapid recovery rates, are discussed. Overall, tunas, and billfishes are designed for high performance, at both sustainable and burst swimming speeds, but there are several differences between tunas and billfishes. Tunas' aerobic metabolic capacities exceed those of ectothermic fishes, including billfishes and other scombrids, by virtue of their elevated red muscle temperatures, and because heart and white muscle aerobic capacities are significantly greater in tunas. The adaptations for high performance involve some costs, including the need for a constant high energy input to sustain high metabolic rates, high activity levels, and endothermy, Yet, tunas and billfishes have adopted successful lifestyles, as evidence by their large numbers and biomass within the marine environment. Although our knowledge of these fishes has increased dramatically during the past 15 years, there are major gaps in our understanding of the metabolic biochemistry and physiology of these fishes, and these are highlighted so that additional research can be directed towards filling these gaps.