Thalassomonhystera oxycephalata Tchesunov et Miljutina, sp. n.

Thalassomonhystera oxycephalata Tchesunov et Miljutina sp. n.

Table 1, Figs 3, 4

Material. One holotype male, four paratype males and two paratype females. The slides are deposited in the nematode collection of the Shirshov Institute of Oceanology of Russian Academy of Sciences, Moscow.

Type locality. Arctic Ocean, area between Greenland and Svalbard, 79°8.2'N & 02°53.6'E, depth 5569 m (Molloy Deep), silt, 16 August, 2000.

Etymology. The name oxycephalata reflects a narrowing cephalic end.

Description. Body slender, elongate spindle­shaped. Cuticle smooth. Cephalic end narrowed.

Labial region set off slightly. Mouth opening surrounded with six small lips. Labial papillae not discernible. The outer labial setae and cephalic setae are jointed in one circle of ten equal tiny setae approximately 1.5 µm long. Amphid at a considerable distance from the head apex, well posterior to the stoma ending. Amphid large, circular to slightly longitudinally oval, with fine but distinct cuticular edging, very fine concentric striation and more or less certain central spot. No other sensilla visible posterior to the amphid along the body. Buccal cavity elongate tubular and very narrow, its walls hardly differ from internal cuticular lining of the esophagus. Esophagus slender and weak. Cardia small, surrounded with intestinal tissue. No indications of a renette cell. Vulva not sclerotised. Female gonad singular, outstretched and situated entirely to the right of the midgut. Uterus can contain a single egg 30–47 µm long and 8.5–11 µm wide, with coarsely granulated cytoplasm. No postvulvar sac. There are a few round cells with light cytoplasm and small dense nuclei (spermatozoa or spermatides?) in the posterior prevaginal portion of the gonad. Male gonad singular, outstretched and situated entirely to the right of the midgut. Spermatocytes relatively large and arranged in one row in the anterior part of the gonad. Ripe spermatozoa (or spermatides?) look like tiny light­refracted crumbs in the posterior part of the male gonad. Spicules short, arcuate, distally pointed and proximally ending with slightly differentiated knobs. Gubernaculum with a dorso­caudal apophysis. No supplementary organs. Tail distinctly consists of two parts roughly equal in length, proximal conical and distal slender flagelliform portions. Tail tip slightly swollen, with minute spinneret therein. No pre­ or postanal setae visible. Two caudal gland cell bodies visible within the proximal tail portion in some specimens.

Discussion. There are no similar species to Thalassomonhystera oxycephalata among current valid Thalassomonhystera species. However, T. oxycephalata sp. n. is related to several Thalassomonhystera species reported by Bussau (1993) from> 4000 m depth in the southern west Pacific Ocean, namely T. mortalis, T. praetenuis, T. subtilis, T. tristis, and especially T. peruensis. The new species shares with these five species such characters as narrow cephalic end, very wide amphid, lacking evident renette cell, short arcuate spicules and gubernaculum with small dorso­caudal apophysis, tail with distinct slender cylindrical portion. However T. oxycephalata sp. n. differs from them all by having elongate cylindrical weakly sclerotised buccal cavity instead of non­sclerotised funnelshaped or hardly developed stoma and esophagus gradually widening posteriorly instead of distinct pear­shaped posterior widening (except T. praetenuis with its cylindrical esophagus, from whom T. oxycephalata differs with smaller amphid and absence of serial preanal pores in male).