Spermatogenesis and sperm structure of the normally parthenogenetic freshwater snail melanoides tuberculata

The freshwater snail Melarwides tuberculata ranges throughout Asia and Africa, and throughout much of its range it reproduces parthenogenetically. In Israel, however, males have been found. These males produce two types of spermatozoa, euspermatozoa and paraspermatozoa. The euspermatozoa are fillform (about 230 μm in length) with a head comprising a nucleus and conical acrosome, a mid-piece (98 μm long) of four equalsized mitochondria, and a tail. Unlike in other cerithiaceans, the nucleus is unusually elongate (38 μm long × 1 μ.m diameter) and the axoneme is housed within an intranuclear canal. The paraspermatozoa consists of a head of electron-dense blocks (up to 25 μm long) which surround up to 15 axonemes, a mid-piece of elongate mitochondria and a tail tuft, as in other cerithiaceans. Structural changes during spermatogenesis resemble those described for other gastropods with dimorphic spermatozoa.