Ultrastructural observations on spermiogenesis in the peanut worm, Phascolosoma esculenta (Sipuncula: Phascolosomatidea)

Abstract

Ultrastructural characteristics of spermiogenesis in the peanut worm, Phascolosoma esculenta (Phascolosomatidea), were observed by transmission electron microscopy (TEM). The spermiogenesis principally occurs in the coelom and can be divided into three stages (the early, middle and late stages) based on the chromatin morphology, acrosome development, spermatozoon midpiece and flagellum formation. Spermatids within a given spermatid mass develop synchronously. With the spermiogenesis proceeding, the spermatid masses become loosely structured, and later, adjacent spermatids are interconnected at one extremity of the cells. Gradually, condensation of the chromatin accelerates and is almost completed in late spermiogenesis, leaving the late spermatids with highly condensed homogeneous chromatin. In the spermatid head, the conical acrosome is generally composed of an acrosomal vesicle which is formed by the coalescence of small proacrosomal vesicles within the cytoplasm, a subacrosomal space that situates between the acrosomal vesicle and nucleus, and an acrosomal rod which develops from a bunch of filamentous material within the subacrosomal space. Certain mitochondria move posteriorly towards the nucleus, thus constituting the spermatozoon midpiece. The flagellum, originated from the distal centriole, appears in the early spermiogenesis. Ultimately, mature spermatidium dissociates into numerous spermatozoa, which are subsequently released as a single cell from the coelom into the nephridia. The spermatozoon has a prominent head, containing an acrosome and nucleus, a short midpiece and a slender tail. When compared with other sipunculans or invertebrates with external fertilization, the spermiogenesis of P. esculenta, presumably, is closely associated with its biological adaptations for the reproductive strategy.