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1. eusperm A large amount of actin is present in tubificine (Annelida: Clitellata) parasperm tails.Roberto Marotta, Angelica Crottini, Marco FerragutiDipartimento di Biologia, Università degli Studi di Milano parasperm • Oligochaete annelids belonging to the subfamilies Tubificinae and Limnodriloidinae produce two different sperm types, eusperm and parasperm. The two sperm types differ in nearly all the characters • nuclear size (about 30 μm in the eusperm, but 3 μm in the parasperm; • the amount of nuclear DNA (about eight times more in the eusperm than in the parasperm); • the extreme reduction of the acrosome in the parasperm; • the size of the mitochondria (about double in the parasperm with respect to the eusperm); • the tail (the cell membrane is separated from the axoneme by a consistent space in the parasperm, and the space is filled by γ glycogen, whereas in the eusperm the space is reduced and has β glygogen granules). Within the spermathecae the two sperm types are grouped to form bundles, called spermatozeugmata. These are rod-shaped structures, up to 2 mm long, already known since a long time. The spermatozeugmata have an outer layer, the cortex, formed by tightly packed parasperm, and an inner core, the axial cylinder formed by paralelly arranged eusperm. The spermatozeugmata are stored in the spermathecae up to the moment of fertilization, when they go back up to the extermal opening of the spermathecal duct, where they are probably broken during the formation of the cocoon, thus delivering the fertilizing eusperm. The cocoon is a protective structure, produced by a specialized region of the epidermis, the clitellum, external to the worm’s body, where the eggs are fertilized. parasperm tail eusperm parasperm The tubificine spermatozeugmata hold together a certain number of fertilizing sperm “ready for use”. This is probably accomplished by an impressive complex of cell junctions formed mainly by septate junctions, followed, at least in Tubifex, by scalariform junctions, connecting parasperm tails The presence of cell junctions – septate and scalariform in Tubifex tubifex – connecting sperm tails for such a long tract is per se a curious or unique finding. Cell junctions are - in fact – mainly found between epithelial cells. On the other hand one could ask if the cell junctions seen in Tubifex and other tubificine species are similar to the septate junctions seen in other invertebrate epithelia. A large amount of ACTINwas localized in the parasperm tails by confocal microscopy as well as by immunogold EM. TUBULIN and DNA where also localized. The presence of actin futher confirms that the structures seen connecting parasperm tails are cell junctions. 200 nm 200 µm