Revision of the genus Metoncholaimus (Nematoda: Enoplida) with description of new species from the South China Sea

The family Oncholaimidae comprises ca 350 species of widespread nematodes. They are common in the seas and oceans and are also found in freshwater lakes and rivers. Here we provide the first description of Metoncholaimus species from the South China Sea. Metoncholaimus placatus sp. n. is characterised by the spicules 2.2-2.8 anal body diam. long, gubernaculum small, nearly parallel to the spicules, S-curved. Supplementary organ is composed of transversely elongated prominence with two sensillar structures (receptors). A pictorial key to the species level of Metoncholaimus species is provided. In this study, the D2-D3 domain of the LSU rDNA and the nearly full-length SSU rDNA were selected as targets for species identification. Phylogenetic relationships within the family Oncholaimidae remained unresolved despite of the various sequences analysed in different loci.