ИСТИНА

Here we present the first complete description of spermatogenesis in Onuphidae at ultrastructural level. Mooreonuphis stigmatis is a tube dwelling worm inhabiting intertidal mud flats in the north-eastern Pacific. It is a dioecious species with gametes developing in the coelomic cavity of the posterior body region in both sexes. Spermatogenesis and sperm ultrastructure in M. stigmatis was studied using transmission electron microscopy techniques. In addition, a series of semithin histological sections was used for 3D reconstruction if seminal receptacles in females. Spermatogonia are formed in association with blood vessels and contain large nuclei with irregularly distributed chromatin, scattered mitochondria and extensive endoplasmic reticulum. Spermatocytes form large clusters of cells connected in the center by cytoplasmic bridges. Gametogenic stages undergo synchronous development and remain associated with a cluster until very advanced state. Elongated spermatids contain nuclei with granular chromatin, spherical mitochondria, and flattened acrosome vesicles located posteriorly in association with a Golgi complex. In the following development, the acrosome becomes conical and migrates towards the anterior part of a spermatid. Mature spermatozoa detach from a cluster and float freely in the coelomic fluid. They are about 10 μm long and have extremely long nuclei, bellshaped acrosomal complex, four elongated mitochondria in midpiece and relatively short flagellum surrounded by a cytoplasmic annulus. The shape of mature spermatozoa and the presence of sperm storage organs in females suggest the existence of sperm transfer mechanism in M. stigmatis. Following Rouse and Jamieson (1987) terminology, the sperm can be classified as ent-aquasperm.