The fine structure of the mature spermatozoon of Onchidella celtica (Cuvier) (Mollusca: Gastropoda) and its phylogenetic implications

The spermatozoon of Onchidella celtica shows many peculiar features. The acrosome is formed by an apical vesicle and a pedestal consisting of a principal body which is flanked by two electron-dense elliptical elements. The mitochondrial derivative consists of three different components: peripheral layers of paracrystalline material, concentric cisterns, and two cylindrical compartments. The 9 + 9 + 2 axoneme shows 9 small deposits of glycogen granules in correspondence with the axonemal “spokeheads,” and a small assembly of possibly proteic material lying close to the two central tubules. Another intraaxonemal deposit of glycogen granules is present at the level of the “glycogen piece.” Most of the observed features are in substantial agreement with those of another species of the Onchidiidae. This confirms the constancy of sperm structure among the members of a single family of the Gastropoda.