An ultrastructural investigation of spermatogenesis and sperm structure in six species of South African sea urchin

Transmission electron microscopy of the spermatozoon of six species of intertidal sea urchins has shown that each species has a sperm with unique dimensions, indicating that spermatozoon morphology could be used as a taxonomic character. However, although the structure of the sperm is species-specific, all sperm have a similar appearance which is typical of the Echinoidea. The sperm head is composed of a conical nucleus capped by a small, spheroidal acrosome which is undifferentiated internally. The mid-piece consists of a single ring-shaped mitochondrion which surrounds the centrioles. An examination of spermatogenesis reveals that each spermatogonium and spermatocyte develops a flagellum, the flagellum being resorbed prior to cell division. In addition a few small vesicles which resemble proacrosomal vesicles form in primary spermatocytes, a feature which has not been reported for echinoids, although such vesicles are a common feature of pre-spermiogenic cells of ophiuroids and holothurians. Spermiogenesis in all species is similar to descriptions published by other workers.