Ultrastructure and Development of Dimorphic Sperm in the Abyssal Echinoid Phrissocystis multispina (Echinodermata: Echinoidea): Implications for Deep Sea Reproductive Biology.

Mature males of the abyssal echinoid Phrissocystis multispina produce two types of sperm including a euspermatozoon typical of echinoids, and a paraspermatozoon, which is bipolar-tailed. The structure of the testis and most features of spermatogenesis are similar to that of other echinoids. Development of both sperm types is identical until the spermatid stage when the nucleus of the paraspermatozoon undergoes chromatin reduction. Both sperm types have acrosomes typical of other echinoid sperm. However, we never observed a Golgi complex during any stage of sperm differentiation so the origin of the acrosome is unclear. Both the distal and proximal centrioles are involved in the formation of an anteriorly and posteriorly directed flagellum in the paraspermatozoon. Mixtures of both sperm types tend to clump due to the entanglement of sperm axonemes in the paraspermatozoon flagellum. Although the function of the paraspermatozoa is unknown, they may play a role in facilitating fertilization through the reduction of euspermatozoon diffusion during spawning. This study reports only one of several recently discovered reproductive adaptations associated with deep-sea habitats.