WHY DO TUNA MAINTAIN ELEVATED SLOW MUSCLE TEMPERATURES - POWER OUTPUTOF MUSCLE ISOLATED FROM ENDOTHERMIC AND ECTOTHERMIC FISH

It has been hypothesised that regional endothermy has evolved in the m uscle of some tunas to enhance the locomotory performance of the fish by increasing muscle power output, Using the work loop technique, we h ave determined the relationship between cycle frequency and power outp ut, over a range of temperatures, in isolated bundles of slow muscle f ibres from the endothermic yellowfin tuna (Thunnus albacares) and its ectothermic relative the bonito (Sai da chiliensis), Power output in a ll preparations was highly temperature-dependent. A counter-current he at exchanger which could maintain a 10 degrees C temperature different ial would typically double maximum muscle power output and the frequen cy at which maximum power is generated (f(opt)) The deep slow muscle o f the tuna was able to operate at higher temperatures than slow muscle from the bonito, but was more sensitive to temperature change than mo re superficially located slow fibres from both tuna and bonito, This s uggests that it has undergone some evolutionary specialisation for ope ration at higher, but relatively stable, temperatures, f(opt) of slow muscle was higher than the tailbeat frequency of undisturbed cruising tuna and, together with the high intrinsic power output of the slow mu scle mass, suggests that cruising fish have a substantial slow muscle power reserve, This reserve should be sufficient to power significantl y higher sustainable swimming speeds presumably at lower energetic cos t than if intrinsically less efficient fast fibres were recruited.