**Alexei Tchesunov 1,\*, Raehyuk Jeong <sup>2</sup> and Wonchoel Lee <sup>2</sup>**


http://zoobank.org/urn:lsid:zoobank.org:pub:4C6B07ED-52DB-42A1-80B7-2881F0559E56 Received: 19 November 2019; Accepted: 18 December 2019; Published: 1 January 2020

**Abstract:** In the context of exploration of meiofauna in a sandy intertidal zone of Jeju Island (South Korea), over 70 nematode species are identified, some which have been proven to be new for science. Two new free-living marine nematode species of the family Selachinematidae (Chromadorida, Selachinematidae, Choniolaiminae) are described from the intertidal sandy sediments of Jeju Island (South Korea). *Gammanema okhlopkovi* sp. n. is closest to *Gammanema anthostoma* (Okhlopkov, 2002) and differs by having longer cephalic setae (8.5–19 μm in *G. okhlopkovi* versus 6–7.5 μm in *G. anthostoma*) and by the presence of precloacal supplementary organs. The genus diagnosis of *Gammanema* is updated. The genus includes fourteen valid species, while three species are considered species inquirendae due to incomplete diagnoses and illustrations impeding their correct recognition. An annotated list of valid and invalid *Gammanema* species is provided. A pictorial key for valid *Gammanema* species is constructed, which consists of two components: (1) simplified images of heads, and (2) a table summarizing most of the significant measured and numeric characters between species. *Latronema obscuramphis* sp. n. differs from its related species *Latronema aberrans* (Allgén 1934), *Latronema annulatum* (Gerlach, 1953), and *Latronema spinosum* (Andrássy, 1973) by body size, number of supplementary organs, tail shape, length of spicules, and cuticle ornamentation.

**Keywords:** free-living marine nematodes; pictorial key; taxonomy
