Euspermatozoa and paraspermatozoa in the trochoid gastropodZalipais laseroni (Trochoidea: Skeneidae)

Pronounced sperm dimorphism is reported for the first time in the prosobranch order Vetigastropoda. Using transmission electron microscopy, it is demonstrated thatZalipais laseroni Kershaw (Trochoidea: Skeneidae) produces uniflagellate euspermatozoa (eupyrene, fertile sperm) and multiflagellate paraspermatozoa (oligopyrene, infertile sperm). Euspermatozoa show the following features: (1) a conical acrosomal vesicle; (2) a long tubular, helically coiled nucleus; (3) a short midpiece (mitochondrial sleeve surrounding a 3µm-long electron-dense rod); (4) a chambered body (? fused centrioles) continuous with the dense rod of the midpiece; (5) a flagellum (characterized by an electron-dense sheath surrounding and partly obscuring the central pair of tubules). Paraspermatozoa are composed of an elongate head (lacking an acrosomal complex), a short midpiece (centriolar rods interspersed with mitochondria), and a posterior tuft of flagella. The head consists of a rodshaped anterior body and a condensed nuclear remnant — the latter lodged in a shallow invagination of the anterior body. Multiple flagella are attached via centriolar rods to a layer of dense material lining the nuclear remnant membrane. During paraspermatozoan development, the nucleus partially degenerates, then condenses, while the endoplasmic reticulum (ER), apparently assisted by the Golgi complex, is responsible for production of numerous, electron-dense secretory vesicles. These vesicles subsequently fuse to form the elongate, anterior body of the head region. The ability of at least one line of trochoid gastropods to produce an oligopyrene, multiaxonemal paraspermatozoon, suggests that the Caenogastropoda (with this feature) might have been derived from the Vetigastropoda rather than from any other archaeogastropod source.