Two New Species of Free-Living Nematodes (nematoda, Monhysterida) from Mekong River Mouth, Vietnam

The paper presents an illustrated description of two nematode species of order Monhysterida, found in the Mekong River delta, Vietnam. Sphaerotheristus rivalis sp. nov. is morphologically similar to S. macrostoma Timm, 1968, S. bengalensis Timm, 1968 and S. pseudodentatus Timm, 1968, however it has a longer body and specific structure of spicules. Daptonema lissum sp. nov. is close to D. donghaiensis Wang, An, Huang, 2018, D. minutum (Jurio, 1974) and D. alternum (Wieser, 1956). What makes it different from the first species is a shorter outer labial setae, shorter spicules and vulva located close to the anterior body end. The new species differs from D. minutum by a longer and thicker body, longer spicules, thicker tail and fovea amphids located closer to the anterior body end. Its difference from D. alternum lies in a shorter and thicker body, thicker tail, longer spicules, shorter outerlabial setae and fovea amphids located closer to the anterior body end.