Phylogenetic analysis within Monhysteridae and Thoracostomopsidae based on rDNA sequences and two new species from the Yellow Sea, China

Phylogenetic relationships based on rDNA in nematodes are complex and remain understudied. During our taxonomic study on Changdao Island, three species — Thalassomonhystera tortospicula sp. nov., Enoploides parakoreanus sp. nov., and Enoplolaimus lenunculus Wieser, 1959 — are described. Thalassomonhystera tortospicula sp. nov. is characterized by a medium-sized body, short outer labial and cephalic setae, amphidial fovea positioned at the buccal cavity, twisted spicules, gubernaculum with a dorsocaudally directed apophysis, and a conical-cylindrical tail.

Enoploides parakoreanus sp. nov. is characterized by a medium-sized body, a wide buccal cavity with three equally sized teeth, Y-shaped mandibles with claw-like distal ends, short and slightly curved spicules, a sleeve-shaped gubernaculum, a tubular-shaped precloacal supplement, and a conical-cylindrical tail. Phylogenetic analysis within Monhysteridae shows that the three morphologically similar genera — Geomonhystera, Halomonhystera, and Monhystera — fall into different clades, and ecological environment might be an important factor in their delineation.

Phylogenetic analysis within Thoracostomopsidae shows that the genera Enoploides, Epacanthion, Paramesacanthion, and Mesacanthion each form distinct monophyletic groups. Species of Enoplolaimus are shown in different clades, and the phylogenetic position at the subfamily level is not well supported with the current data.