Ultrastructural Study of an Endogenous Energy Substrate in Spermatozoa of the Sea Urchin Hemicentrotus pulcherrimus. The morphology of the midpiece in spermatozoa of the sea urchin Hemicentrotus pulcherrimus was investigated ultrastructurally with particular emphasis on an endogenous substrate providing energy for motility. The midpiece was composed of a single toroidal mitochondrion surrounding the flagellum. Several lipid bodies (0.1-0.2 μm in diameter) were contained in the space between the mitochondrial outer and inner membranes. Following incubation with seawater, spermatozoa began to swim and the lipid bodies became small and finally disappeared, coincident with a decrease in the level of phosphatidylcholine (PC), an endogenous substrate for energy metabolism. In contrast, during incubation in 100 mM K+-seawater, in which spermatozoa are immotile, there was no decrease in the level of PC and the lipid bodies remained intact. These results strongly suggest that the PC available for use in energy metabolism is located in the lipid bodies within mitochondria in the midpieces of H. pulcherrimus spermatozoa.