The fine structure of the polymorphic spermatozoa of Fusitriton oregonensis (Mollusca: Gastropoda), with notes on the cytochemistry of the internal secretions

Abstract

The prosobranch Fusitriton oregonensis exhibits an unusual form of sperm polymorphism. The viable, eupyrene sperm are attached in groups of about fifty to worm-shaped, apyrene, carrier sperm. There is a second apyrene sperm, which is lancet-shaped and has a different internal organization than the carrier, but does not transport eupyrene sperm. The eupyrene sperm are filiform (185 μm long), with a conical acrosome, elongate nucleus and midpiece. They contain large stores of glycogen in the principal piece, together with an unusually high proportion of protein. The latter is due to a complex interconnecting system of fibres that supports the tail internally. A distinct annulus is located, characteristically, at the junction between midpiece and principal piece. The carrier sperm has a core of about 112 axonemes that arise from basal bodies in the anterior end and extend through its entire length of 36 μm. The basal bodies have unstriated rootlets that are embedded in a granular cap. Large membrane-bound “yolk bodies” are arranged along the length of the carrier sperm, on either side of the median axonemal core. Dense bodies, which may be indigestible residues formed from the degeneration of the nucleus, are excreted by exocytosis. Individual carrier sperm are capable of “corkscrew” propulsion, resembling that of spirochaetes. The lancet sperm is three times as long as the carrier. The sixteen or so axonemes, which are arranged peripherally like a cage enclosing the cytoplasm, originate from a dense centriolar plate in the anterior end. The cytoplasm is filled with secretions including small yolk granules, dense bodies (also excreted), clear vesicles, and a membranated granular secretion that resembles mucus. The possible functions of the lancet and carrier sperm are discussed.