Ultrastructural study of spermatogenesis in the free-living marine nematode Paracyatholaimus pugettensis Weiser et Hopper, 1967 (Chromadorida: Cyatholaimidae)

Abstract

Spermatogenesis and the morphology of mature sperm in the free-living chromadorid Paracyatholaimus pugettensis from the Sea of Japan were studied using transmission electron microscopy. In spermatocytes fibrous bodies (FBs) appear; in spermatids, the synthetic apparatus is located in the residual body, whereas the main cell body (MCB) houses the nucleus, mitochondria, and FBs. The nucleus of the spermatid consists of a loose fibrous chromatin that is not surrounded by a nuclear envelope; centrioles lie in the perinuclear cytoplasm. The plasma membrane of the spermatid MCB forms numerous filopodia. Immature spermatozoa from the proximal part of the testis are polygonal cells with a central nucleus. The latter is surrounded by mitochondria and FBs with poorly defined boundaries. The immature spermatozoa bear lamellipodia all along their surface. Mature spermatozoa are polarized cells with an anterior pseudopodium, which is filled with filaments that make up the cytoskeleton; the MCB houses a nucleus that is surrounded by mitochondria and osmiphilic bodies. In many ultrastructural characteristics, the spermatozoa of P. Pugettensis are similar to those of most nematode species studied so far (i.e., they are ameboid, have no acrosome, axoneme, or nuclear envelope). On the other hand, as in other chromadorids, no aberrant membrane organelles were observed during spermatogenesis of P. Pugettensis.