Spermiogenesis and ultrastructure of spermatozoa in Saxipendium coronatum (Hemichordata, Enteropneusta), with consideration of their relation to reproduction and dispersal

Abstract

Sperm ultrastructure and spermiogenesis of the enteropneust hemichordate Saxipendium coronatum conforms to the general pattern of the prototype spermatozoon found in many phyla. The sperm is about 29 μm long, including head, middle piece, and tail. The Saxipendium spermatozoon has some unique features. The head is pyramidal in shape and the nucleus has four frontal ridges radiating from the base of the acrosomal region. The acrosome is composed of a large acrosomal vesicle surrounded by periacrosomal material. The acrosomal region projects about 1 μm in front of the nucleus and has a width at the base of 1.5 μm. The middle piece is dish-shaped and contains a large mitochondrial mass surrounding the centriolar region. The centriolar region is partially located in a centriolar fossa at the basal part of the nucleus. In spermatids, an anchoring fiber apparatus is observed surrounding the centriolar region. The distal ends of the fibers are attached to the plasmalemma by electron-dense thickenings. The tail is a simple flagellum. The sperm of Saxipendium and the small eggs found in the female suggest non-specialized external fertilization and embryogeny leading to a planktotrophic larva. The main results of the fine structure of the spermatozoon in Saxipendium are summarized in Fig. 12.