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 hi ghly charged lifestyles as apex predators in the oceanic pelagic envir onment. The adaptations they possess for efficient and rapid swimming, efficient and rapid food processing, turnover of nutrients and storag e and mobilization of internal fuel supplies, and for rapid recovery r ates, are discussed. Overall, tunas, and billfishes are designed for h igh performance, at both sustainable and burst swimming speeds, but th ere are several differences between tunas and billfishes. Tunas' aerob ic 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 a re significantly greater in tunas. The adaptations for high performanc e involve some costs, including the need for a constant high energy in put to sustain high metabolic rates, high activity levels, and endothe rmy, Yet, tunas and billfishes have adopted successful lifestyles, as evidence by their large numbers and biomass within the marine environm ent. Although our knowledge of these fishes has increased dramatically during the past 15 years, there are major gaps in our understanding o f the metabolic biochemistry and physiology of these fishes, and these are highlighted so that additional research can be directed towards f illing these gaps.