Spermatozoa ultrastructure, spermatogenesis and reproductive system of Acanthodasys australis (Gastrotricha, Macrodasyida).

Spermatozoon morphology, spermatogenesis and reproductive system of Acanthodasys australis (Thaumastodermatidae) were studied at ultrastructural level. The spermatozoon is a filiform cell formed by a cork-screw-shaped acrosome, a helical nuclear–mitochondrial complex and a long flagellum. The acrosome contains an axial tubular structure composed of a pile of electron-dense rings. The helical nucleus is thin and wraps a single, giant mitochondrion. A large and thick perinuclear helix surrounds the nuclear–mitochondrial complex extending for its whole length, except the two most basal coils. The flagellum with typical 9 × 2 + 2 axoneme is surrounded by a monolayered obliquely striated cylinder, in the terminal part of which the axoneme disappears. The general architecture of the spermatozoon of Acanthodasys species agrees with the general sperm model described for the family Thaumastodermatidae, and perfectly matches that observed in Acanthodasys aculeatus and Diplodasys ankeli, so confirming the great uniformity of the sperm ultrastructure morphology within the subfamily Diplodasyinae. Spermatogenesis includes the early and simultaneous development of acrosome and flagellum, the elongation of the nucleus and the fusion of mitochondria into a single giant mitochondrion, which later sinks into the nucleus, and the final appearance of a perinuclear helix and a peraxonemal striated cylinder. The process takes place through the same main steps already observed in all Macrodasyida species studied so far. The reproductive system consists of paired testes arrangement laterally to the intestine, a single ovary located dorsally to the posterior part of the intestine, a caudal organ located ventrolaterally to the intestine end, and, a dorsal frontal organ. The latter is in contact with mature oocyte posteriorly and with a compact cellular band connecting to the ‘rosette’ organ anteriorly. The general layout of the reproductive apparatus of A. australis perfectly matches that of the members of Diplodasyinae.