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. fiUcauda comb. nov. in the absence of a gubemaculum, 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 gubemaculum 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 oí Adoncholaimus and a key to species are provided. Adoncholaimus squalus comb, nov., Adoncholaimus filicauda comb. nov., and Admirandus papillatus comb. nov. are proposed.

We report the first mermithid nematode found to be parasitic in a marine tanaidacean crustacean. Ten host tanaidaceans were collected from a depth of 52 m in Otsuchi Bay, Iwate, Japan, north-western Pacific, and identified as a species in the tanaidid genus Zeuxo Templeton, 1840. Nematodes occurred in the host's body cavity; in one case, at least two individuals inhabited a single host. We provide a brief description and illustrations of the morphology of the nematode. In a phylogenetic reconstruction based on the 18S ribosomal RNA gene, the nematode nested in a clade otherwise containing mermithids from terrestrial or freshwater hosts, showing an expansion in host utilization in Mermithidae Braun, 1883 from terrestrial/freshwater hosts to a marine organism.

A new species of free-living marine nematode, Oncholaimus langhovdensis sp. nov., is described from the intertidal zone of Langhovde (near Syowa Station), Dronning Maud Land, East Antarctica. It closely resembles 11 congeners in the conico-cylindrical tail shape present in males and amphid and excretory pore positions, short spicules, and Demanian system structure present in females. However, it mainly differs from these congeners in body size, de Man's ratios, tail length and shape, and Demanian system structure present in females. Oncholaimus langhovdensis sp. nov. also resembles four congeners known only by females, but it can be distinguished from them based on the tail length and uvette position. In addition to O. langhovdensis sp. nov., two undescribed species (Tripyloididae gen. sp. and Axonolaimidae gen. sp.) and four unidentified species (Sphaerolaimus sp., Oncholaimidae gen. sp., Comesomatidae gen. sp., and Chromadorida fam. gen. sp.) were found from the same locality.

Nectonema, the only horsehair worm (Nematomorpha) genus found in marine environments, was previously known to be parasitic only in decapod crustaceans. We report Nectonema sp.as the first record of a marine nematomorph parasitic in isopod crustaceans. This is also the third record of marine nematomorphs from the North Pacific. Six infected isopods (Natatolana japonensis) collected from 1425 m depth in the Sea of Japan each contained one to seven (mean 2.33) nematomorphs in the body cavity in the pereon. There was no correlation between the host body length and number of parasites. For Nectonema sp., we describe and illustrate morphological features of the parasitic juvenile stage and present nucleotide sequences for the cytochrome c oxidase subunit I gene (COI or cox1; 451 nt), 18S rRNA gene (1777 nt), and region spanning the internal transcribed spacer 1 (ITS1) and the 28S rRNA gene including the 5.8S rRNA gene and ITS2 (1218 nt in total). In an 18S maximum-likelihood tree that included 24 nematomorph species, Nectonema sp. grouped with N. agile from the northwestern Atlantic; the 18S gene from these two taxa was divergent by 11.8% K2P distance, suggesting they are different species. Nectonema species may have a broader range of host groups than previously suspected, but may have been previously misidentified as nematode parasites.

Three new species of firee-Iiving marine nematedes, belonging to three genera, Epacanthion Wieser, 1953, EnQplolaimus De Man, 1893 (both family Enoplidae Dujardin, 1845), and Oncholatmus Dujardin, 1845 (family Oncholaimidae Filiptev, 1916), are described from northern Japan. Eipacanthion quadridiscus sp, nov, is most similar to Elpacanthion enoploidijbrme (Gerlach, 1953), E, gorgonocephalum Warwick, 1970, and E. geongei Inglis, 1971 in spicule length and the absenee of a precloacal supplement, but diifers from them tn the shape of the gubernaculum and the arrangement of the subcephalic and cervical setae in the male. Enoplolaimus longigubernaculum sp. nov. differs from alI of its congeners in having a long and straight gubernaculum. Oncholaimus secundicoglis sp. nov.