Spermatogenesis of deep-sea nematode Paramesacanthion sp. (Enoplida: Thoracostomopsidae)

The ultrastructure of spermatocytes, spermatids, immature and mature spermatozoa of the deep-sea nematode Paramesacanthion sp. was studied using a transmission electron microscope. Spermatocytes are large polygonal cells with a central nucleus with a nuclear envelope. The cytoplasm of cells is filled with typical structures, such as the endoplasmic reticulum, Golgi apparatus and mitochondria, as well as special structures, membranous organelles (MO). In spermatids, cell polarisation occurs: the nucleus, surrounded by mitochondria and MO, occupies the central part of the cell, while all elements of the synthetic apparatus are displaced to the periphery of the cell. Immature sperm are round cells with a central nucleus with a nuclear envelope. The sperm cytoplasm is filled with mitochondria and MO. Mature sperm are bipolar amoeboid cells with an anterior pseudopodium and a posterior main cell body containing a nucleus with a nuclear envelope, mitochondria and free MO not attached to the cell membrane. In general, spermatozoa of Paramesacanthion sp. are characterised by the main characteristics of enoplid spermatozoa: the presence of a nuclear envelope and MO. In this case, fibrous bodies, characteristic of most nematode spermatozoa, do not appear at any stage of spermatogenesis.