Ultrastructural observations on spermiogenesis in the peanut worm, Themiste pyroides (Chamberlin, 1920) (Sipuncula; Sipunculidea)

Abstract

Summary The process of spermiogenesis and the ultrastructure of the spermatozoa in the peanut worm, Themiste pyroides, from the Sea of Japan were observed with electron microscopy (SEM and TEM). The testes are composed of groups of spermatogonia and are covered by peritoneal cells. Clusters of spermatocytes are released from the testes into the coelomic fluid. Connected by intercellular bridges, the spermatocytes within a given cluster develop asynchronously. Proacrosomal vesicles and a flagellum appear in spermatocytes. Spermatids in the clusters retain the intercellular connections. During spermiogenesis, the acrosomal vesicle, formed by coalescence of small proacrosomal vesicles in the basal part of the spermatid, migrates to the apical part of the cell to form a conical-shaped acrosome. The basal concavity lying above the nucleus is filled with subacrosomal substance. The midpiece contains four mitochondria, two centrioles, and some residual cytoplasm with dark glycogen-like granules. A peculiar annulus structure develops around the base of the flagellum. The distal centriole has a pericentriolar complex consisting of radially oriented elements. Before the spawning process, the spermatozoa are filtered throughout the ciliary nephrostomal funnel into the excretory sac of paired nephridia where they are stored for a short time. The sperm are released into the sea water via nephridiopores. Spermatozoa remaining in the coelomic fluid after spawning are resorbed by amoebocytes. This species from Vostok Bay is characterized by a prolonged spawning period from June to early October. The reproductive strategy of T. pyroides is discussed in comparison with that of Thysanocardia nigra, the latter having a unique pattern of packaging of the spermatozoa, resulting in the formation of spermatozeugmata, as a reproductive adaptation to the very short spawning period.