Fine structure of the spermatozoon of the sea cucumber,Leptosynapta clarki (Echinodermata: Holothuroidea)

Summary The spermatozoon of the holothurian Leptosynapta clarki has a small circular head measuring about 3.0 at the greatest diameter, a midpiece containing a single mitochondrion and a tail flagellum measuring between 35 and 45 in length. The acrosomal region contains a granule measuring 0.7 in diameter which consists of electron dense material arranged in concentric lamellae. Five concentric very electron dense lamellae alternate with areas of much less electron dense material in the central region of the granule. This granule rests in an anterior nuclear depression.The nucleus is circular in shape and contains one or two unbound vacuoles which frequently contain a fine granular material. Posteriorly the nucleus is bounded by a large mitochondrion and an occasional Golgi complex. The proximal centriole which contains a lateral arm of dense material lies in a deep fossa projecting into the nucleus. The distal centriole lies posteriorly in the mitochondrial mass and gives rise to nine satellite projections and their Y-shaped connective extensions.The tail contains the 9 + 2 tubule arrangement and tapers at its distal end.This investigation was supported by a National Research Council grant to F. S. Chia.