9507499 Rapid exchange of heat across the gills prevents most fish from being warmer than the water in which they swim. The scombroid fish (mackerels, tunas, marlins and swordfish) are a group of about 100 species in which endothermy, the ability to keep some tissues (muscles, brain, eyes, digestive systems) warmer than the surrounding sea water, has evolved several times. This research will examine the mechanisms and adaptive significance of such endothermy in these fish at levels ranging from molecular analyses to oceanographic studies at sea. The biochemistry of changes in skeletal muscle that increase its ability to generate heat will be determined. Laboratory and field measurements of free-swimming warm-bodied tunas and closely related fish that are not endothermic will test hypotheses about the advantages of endothermy. The physiological and ecological significance of having warm body muscles or a "brain heater" will be studied by examining the movements and thermal ecology of free-swimming endothermic fish. These data will show how endothermy influences muscular performance, locomotor capabilities and ecological diversity in scombroid fish. It will contribute to understanding of the advantages of increasing the temperature at which tissues and organs operate. The multidisciplinary approach integrates information from molecular, cellular, ecological and evolutionary studies. It will lead to a better understanding of the advantages of endothermy, its ecological importance, and its impact on the ecological diversity of marine fish.