A fresh look at Dinodasys mirabilis (Gastrotricha, Macrodasyida), with focus on the reproductive apparatus and sperm ultrastructure

Adults of Dinodasys mirabilis were studied for the first time. The specimens, collected from the west coast of Sweden, were investigated alive and with electron microscopy. Sexually mature specimens attain a total length of 450-490 µm; the adhesive apparatus is made up of anterior, lateral, ventrolateral, dorsal and posterior tubes; one pair of 'cirrata' type tubes is also present. The reproductive apparatus is hermaphroditic; paired testes extend rearward from the pharyngeo-intestinal junction to 3/4 of the trunk; sperm ducts bend anteriorly at U52 and join together into a common, midventral pore at U33. Two ovaries lie along the sides of the caudal intestine, extending anteriorly from U68. Frontal organ present on the right side of the body, centred a U70; caudal organ absent; a gland organ surrounding the terminal intestine may be present but its homology with other organs in a similar position is uncertain. The spermatozoon is a filiform cell, formed by a long acrosome, a spring-like nucleus and a flagellum. The acrosome is divided into two regions: the anterior-most is thin and corkscrew-shaped, the posterior one is rectilinear; both regions are delimited by a continuous external layer of thick, dense material, that in longitudinal section appears obliquely striated and surrounds a long pile of stout, electron-dense cylinders; the nucleus contains condensed chromatin and is coiled around an elongate mitochondrion; the flagellum possesses a 9x2 + 2 axoneme devoid of striated cylinder. Within Macrodasyida, U-bend sperm ducts and the peculiar ultrastructure of the acrosome are characteristics shared by other Turbanellidae studied so far, providing a foundation for the current systematization of Dinodasys.