Sexual dimorphism in Paralvinella grasslei, a polychaete annelid from deep-sea hydrothermal vents

Abstract

The Alvinellidae, like most polychaetes, are gonochoristic. In Paralvinella grasslei, the males exhibit external sexual characters in the cephalic region which are not seen in the females. Specimens of each sex have only one pair of gonoducts; their terminal parts lead into two pockets (spermathecae in females, seminal vesicles in males) which fuse and open out through a unique genital pore located at the base of the gills. The spermatozoa are very special. The head, lacking an acrosome, contains on the one side a thick layer of electron-dense granules, on the opposite side clear vesicles, and a central ovoid nucleus surrounded by mitochondria. There is no middlepiece. The flat caudal process containing electron-dense cytoplasm and numerous vesicles has no axoneme. The spermathecae harboured slightly modified sperm and degenerating sperm. These observations suggest that sperm transfer occurs by pseudocopulation and that the fecundation is internal.