SELECTIVE ADVANTAGES CONFERRED BY THE HIGH-PERFORMANCE PHYSIOLOGY OF TUNAS, BILLFISHES, AND DOLPHIN FISH

Tunas are extensively distributed throughout world's oceans and grow a nd reproduce fast enough to support one of the world's largest commerc ial fisheries. Yet they are apex predators living in the energy depaup erate pelagic environment. It is often presumed that tunas evolved the ir specialized anatomy, physiology, and biochemistry to be capable of (a) high maximum swimming speeds, (b) high sustained swimming speeds, and/or (c) very efficient swimming; all of which help account for thei r wide distribution and reproductive success. However, a growing body of data on the energetics and physiological abilities of tunas do not support these assumptions. The three things demonstratively ''high per formance'' about tunas, and probably other pelagic species such as mar lin (Makaira spp. and Tetrapturus spp.) and dolphin fish (Coryphaena s pp.), are (a) rates of somatic and gonadal growth, (b) rates of digest ion, (c) rates of recovery from exhaustive exercise (i.e., clearance o f muscle lactate and the concomitant acid load). All of these are ener gy consuming processes requiring rates of oxygen and substrate deliver y above those needed by the swimming muscles for sustained propulsion and for other routine metabolic activities. I hypothesize that the abi lity of high performance pelagic species (tunas, billfishes, and dolph in fish) to deliver oxygen and metabolic substrates to the tissues at high rates evolved to permit rapid somatic and gonadal growth, rapid d igestion, and rapid recovery from exhaustive exercise (abilities centr al to success in the pelagic environment), not exceptionally high sust ained swimming speeds.