Summary - Two new species of free-living marine nematodes, Adoncholaimus daikokuensis sp. nov. and A. pseudofervidus sp. nov., from the coastal area of northern Japan, are described and illustrated. Adoncholaimus daikokuensis sp. nov. is similar to five congeners, A. derjugini, A. punctatus, A. oxyuroides, A. squalus comb. nov. and A. filicauda comb. nov. in the absence of a gubernaculum, but differs in the absence of a ventral swelling on tail, spicule length and buccal cavity length. Adoncholaimus pseudofervidus sp. nov. is similar to A. fervidus in having large body size, short tail, similar positions of the excretory pore and nerve ring, short spicules, arrangement of subventral setae in posterior region of body in males, and a single pair of terminal pores of the Demanian system surrounded by small gland cells in females. Adoncholaimus pseudofervidus sp. nov. differs from A. fervidus in having smaller, more anteriorly located amphids, longer buccal cavity, absence of gubernaculum in the male, and a different position of the terminal pores in the female. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene (309-337 bp) of the new species are provided for identification based on the DNA barcoding method. Metoncholaimoides is proposed as a junior synonym of Adoncholaimus. The new diagnosis of Adoncholaimus and a key to species are provided. Adoncholaimus squalus comb. nov., Adoncholaimus filicauda comb. nov., and Admirandus papillatus comb. nov. are proposed.
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