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.