Endogenous substrate for energy metabolism and ultrastructural correlates in spermatozoa of the sea urchin Diadema setosum.

Energy metabolism in spermatozoa of the sea urchin Diadema setosum of the order Diadematoida was examined. The spermatozoa contained not only several kinds of phospholipids and cholesterol, but also triglyceride (TG). Glycogen and glucose were present at extremely low levels. Following dilution of dry sperm and incubation in seawater, the TG content decreased rapidly. Other lipids, however, remained at constant levels, except for a slight increase in the level of free fatty acid. High lipase activity was demonstrated in the spermatozoa. 14C-labeled fatty acid was oxidized to 14CO2. Ultrastructural study also showed that lipid globules were present at the bottom of the midpiece. After incubation in seawater, morphological changes in the lipid globules were observed and some vacuoles appeared. Thus, the results obtained strongly suggest that D. setosum spermatozoa obtain energy through oxidation of fatty acid from TG stored in the lipid globules at the midpieces.