Ultrastructure of spermatozoa in two species of prolecithophorans (Plathelminthes, Prolecithophora)

Prolecithophorans (Plathelminthes, Prolecithophora) stand out among the free-living flatworms (“turbellarians”) for a number of unique features of spermatozoa. The ultrastructure of male germ cells of two species of prolecithophorans from the Plagiostomidae family – Plagiostomum vittatum (Frey et Leuckart, 1847) from the White Sea and Vorticeros ijima Tozawa, 1918 from Inner Japanese Sea – was described. Spermatozoa of both species are characterized by an elongated fusiform shape, a spiral shape of the nucleus with an electron-dense fibrillar chromatin, the presence of numerous mitochondria that tend to merge, the cortical position of microtubules, and the absence of flagella. “Folded membrane derivatives”, specific for prolecithophorans, are present in mature spermatozoa of both species, but their morphology differs. They were not found in spermatids of Pl. vittatum. The nucleus of spermatozoon of Pl. vittatum is distinguished by the presence of numerous thin outgrowths. It has been suggested that the ultrastructural features of prolecithophoran spermatozoa (number and location of mitochondria, morphology and origin of “folded membrane derivatives”) can be applied in the phylogenetics of this group.