Euspermatozoa, Paraspermatozoa and Spermatozeugmata of Littoraria (Palustorina) articulata (Prosobranchia: Caenogastropoda) with Special Reference to the Pseudotrich

Abstract

Abstract Paraspermatozoa and euspermatozoa of the littorinid gastropod Littoraria (Palustorina) articulata are examined using transmission electron microscopy and light microscopy. In the seminal vesicle, both sperm types occur, either as free cells or organized into spermatozeugmata. It is shown that the elongate (120–140 μm), flagellum‐like component of the paraspermatozoon is in fact a tubular extension of the plasma membrane which encloses granular material but no axonemes or microtubules. This structure, here termed the pseudotrich, shows no evidence of motility and its function remains obscure. The main body region of the paraspermatozoon (length 32–36 μm) contains numerous spherical vesicles, scattered mitochondria, one or two large, rod‐shaped bodies (length 20–24 μm) and a fusiform, granular body (containing DNA; probably a modified nucleus). The rod‐shaped bodies, granular body and surrounding matrix are contained by a common membrane, and are therefore separated from other contents of the paraspermatozoon. In each spermatozeugma, euspermatozoa are attached via the tips of their acrosomes to the paraspermatozoan body at the opposite end to the pseudotrich. Euspermatozoa exhibit a conical acrosomal complex (with axial rod and basal plate), a tubular nucleus sheathing the axoneme, a midpiece (5–6 helical mitochondrial elements sheathing the axoneme), an annulus (with two rings), a glycogen piece and an end piece (total sperm length 268–272 μm). The euspermatozoa of L. articulata are similar to those of most littorinids and many other caenogastropods. The presence of a pseudotrich in the paraspermatozoon appears to be restricted to the subgenus Palustorina.