ABSTRACT
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.
