Comparative studies on the energy metabolism in spermatozoa of four closely related species of sea urchins (Genus Echinometra) in Okinawa

Sea urchins of the genus Echinometra are abundant on Okinawa reef flats in southern Japan. The Okinawan Echinometra is designated into four sympatric and closely related species: A, B, C, and D (Ea, Eb, Ec, and Ed). The sperm head size and shape gradually changes to become longer and more slender according to the following order: Ea, Eb, Ec, and Ed. To obtain information regarding speciation in Okinawan Echinometra, this study examined comparatively the energy production system of spermatozoa of Ea, Eb, Ec, and Ed. All spermatozoa contained cholesterol and several kinds of phospholipids. Glycogen, glucose, and triglyceride were present at extremely low levels. After incubation in seawater, a decrease in the level of phosphatidylcholine (PC) was observed in all spermatozoa concomitantly with activation of motility and respiration. The hydrolysis of PC correlated with the activity of phospholipase A(2). Interestingly, the amount of PC consumed, the respiratory rate, and the phospholipase A2 activity in spermatozoa of Ea and Eb were approximately two-fold higher than those of Ec and Ed. Ultrastructural studies showed that lipid bodies within mitochondria were present in the midpieces of all species of spermatozoa. They became small or disappeared after incubation in seawater. Thus, the results obtained strongly suggest that spermatozoa of Ea, Eb, Ec, and Ed all use PC located in the lipid bodies as a substrate for energy metabolism. Also, it seems likely that energy production activities in Ea and Eb spermatozoa are stronger than those in Ec and Ed. The properties of energy metabolism in different species of sea urchin may be related to their habitat.