Spermatozoa in the Demanian system of free-living marine nematode Admirandus multicavus (Enoplida: Oncholaimidae)

: The process of insemination and fertilization in the nematode family Oncholaimidae is of interest because females of many species have extremely unusual sperm storage organ called “Demanian system”. The mature spermatozoa from the main duct of the Demanian system of the free-living marine nematode Admirandus multicavus (Enoplida: Oncholaimidae) were studied by the transmission electron microscopy. The main duct is an epithelial tube with thick basal lamina. The dilated part of the duct is occupied by cluster of densely packed spermatozoa 7–8 µm in size, while the narrow part contains occasional strongly elongated sperm cells. The centre of spermatozoa is occupied by a nucleus with poorly condensed chromatin surrounded by a distinct nuclear envelope, the presence of which confirms isolation of enoplids from all other clades of nematodes. The prevalent components of sperm cytoplasm are ‘membranous organelles’ (MO) which resemble strongly elongated thread-like osmiophilic cisternae densely packed into bundles. Small numerous mitochondria are interspersed between bundles of MOs. Spermatozoa form pseudopods that reflect putative amoeboid cell motility. Amorphous nucleus, thread-like MOs, and numerous minute mitochondria are likely to represent preadaptation to squeezing through the narrow tubular communications of the Demanian system. Present observations of A. multicavus support basic characters of the oncholaimid sperm pattern previously described in another oncholaimid