Аннотация: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.