This research is directed toward understanding the evolution of swimming locomotion in bony fish, the most diverse group of vertebrates. It will demonstrate how muscle structure and function change during development in each of four families of fish, and how those changes affect the "fast- start" ability of the fish. The research will show what changes in muscle anatomy or physiology underlie superior locomotor performance in fish, and show how such changes are correlated with evolutionary success. The results will lead to increased understalding of the constraints that muscle development place on the ecology of larval fish, with possible application in managing populations of particular species.