The ultrastructure of the tardigrade spermatozoon: a comparison between Paramacrobiotus and Macrobiotus species (Eutardigrada, Macrobiotidae).

The spermatozoan ultrastructure was investigated in three semiterrestrial eutardigrade species belonging to two different genera of Macrobiotidae (Paramacrobiotus and Macrobiotus). The spermatozoa of P. areolatus and P. richtersi are very similar and are made by three regions, namely a long head, a short kidney-shaped middle piece and a short tail with its terminal tuft. In both species the spermatozoa are particularly long (up to 100 µm) and very thin. The peculiar length is due to the remarkably developed head consisting of a cylindrical acrosome and a weakly coiled nucleus increasing in width caudally. The presence of a long nucleus, an electron-dense core of fibrils running parallel to the nucleus, as well as nine outer electron-dense fibers around the proximal part of the axoneme represents novelties in the ultrastructure of the tardigrade spermatozoa. These structures, never described before for a tardigrade spermatozoon, could be related to the movement of those extraordinary long male gametes of Paramacrobiotus. The spermatozoon of M. harmsworthi too is made up of three regions: the head, including the acrosome and nuclear region, the middle piece and the terminally tufted tail. Nevertheless it is only 46-50 µm in length and the head, including a slightly tight helical nuclear region, is similar in length to the tail. In all macrobiotid species here examined, including those from literature, the spermatozoa within the was deferens always appear folded, with the hinge located between the end of the head and the beginning of the middle piece, thus resembling a long nutcracker. The use of spermatozoan characters as phylogenetic information in tardigrades is also discussed.