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Summary
The spermatozoa from testes and uteri of the free-living marine nematode Paracanthonchus macrodon (Cyatholaimidae, Chromadorida) were studied with an electron microscope. The spermatozoa from the testes are unpolarized cells covered by numerous filopodia. The sperm nuclei have oval- or bean-shaped outlines and are surrounded by mitochondria. The peripheral cytoplasm is devoid of organelles and contains evenly distributed filamentous material. The filopodium content resembles that of the sperm cytoplasm. A submembrane layer of 13–16 nm thick, longitudinally oriented, microtubule-like fibres (MLF) strengthens the filopodia. The spermatozoa from the uterus are amoeboid cells with the discrete nucleus surrounded by mitochondria. The bulk of the sperm cytoplasm lacks organelles and is filled with MLF, which also underlie the sperm plasmalemma. The spermatozoa develop large pseudopodia which form hemidesmosome-like junctions with the uterus wall. A layer of a subsurface osmiophilic material concentrated at the junction point is associated with MLF, which run out into the cytoplasm as parallel arrays. The unique cytoplasmic components of the nematode spermatozoa, the membranous organelles and fibrous bodies, were not found in the spermatozoa of P. macrodon; this pattern of the spermatozoon structure is interpreted as a highly reduced condition.