The spermatozoa of the Holothuroidea (Echinodermata): An ultrastructural review with data on two Australian species and phylogenetic discussion

Holothuroidea, like the Crinoidea, Asteroidea and Ophiuroidea have aquatic sperm with a subspherical nucleus in which is embedded a subspherical acrosome and its surrounding periacrosomal material. A ring-shaped mitochondrion behind the nucleus surrounds two centrioles from the distal of which a 9 + 2 axoneme arises. The term "echinosperm" is proposed for this sperm type. Holothuroid echinosperm are characterized by anterior constriction of the periacrosomal fossa; a rounded posterior limit to the acrosome; absence of a distinct subacrosomal depression; outgrowth of a flagellar rootlet from the proximal centriole, of which vestigial homologues are known only in one crinoid and one echinoid. Modified sperm, though not greatly divergent from the echinosperm, are known in two holothuroids and one crinoid. The sperm of echinoids, which are not referable to the echinosperm, are considered plesiomorphic in failure of the acrosome to become embedded in the nucleus (a feature retained from an ancestry shared with enteropneusts), but apomorphic in the conical form of the nucleus and development of spine-like structures around the anterior rim of the nucleus. Spermatozoal data suggest that the holothuroid-crinozoan-asterozoan assemblage is the sister group of the Echinoidea. Inclusion of holothuroids in the Echinozoa is not supported