The fine structure of the spermatozoa of Siphonaria algesirae (Gastropoda, Pulmonata)

Abstract

The sperm of Siphonaria algesirae (Gastropoda, Pulmonata), a species with internal fertilization, was studied by light and electron microscopy. The spermatozoon is a very long, uniflagellate cell composed of a conical head with an apical acrosome, a midpiece with a helically coiled external sheath containing a complex mitochondrial derivative with a wavelength of ∼ 5.5 μm, and an endpiece. There are no axonemal microtubules. Instead, nine homogeneous coarse fibers with transverse striations in the apical zone project toward the anterior section of the midpiece. In the posterior zone of the midpiece the coarse fibers are differentiated in a common microtubular axoneme. The complex mitochondrial derivative of the midpiece shows an organized group of 100 Å diameter spherical particles. Externally the midpiece is surrounded along its length by a cylinder formed by two membranes. A complex structure separates the transitional zone between the midpiece and the endpiece.