Thalassomonhystera molloyensis Tchesunov et Miljutina, sp. n.

Thalassomonhystera molloyensis Tchesunov et Miljutina sp. n.

Table 1, Figs 5 & 6

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. Species name molloyensis is referred to the geographic area where the species was collected.

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

Labial region set off slightly with labial sensilla as tiny papillae. Outer labial and cephalic setae very short, nearly equal and jointed in one crown of ten setae 1.5–2.0 µm long (23–30% c.b.d. in males, 23–25% c.b.d. in females). Amphid circular, sometimes with slightly asymmetrical central spot, with distinct uninterrupted cuticular edging. In males, amphids are slightly larger than in females. Amphids situated well posterior to the stoma ending. There are two (males NN 1, 2, 3, 5, 6) or one (females NN 1, 4, 7) lateral or lateroventral cervical setae posterior to the amphid. There are usually one lateroventral seta just posterior to the nerve ring and a few sparse somatic setae along the body. Cervical and somatic setae 1–2 µm long. Buccal cavity cylindro­conoidal in shape and relatively voluminous. Esophastomal rhabdions slightly draw together to the posterior stoma ending or nearly parallel. Rhabdions thick but weakly sclerotised and weakly light­refracting, without an esophageal cuff. Esophagus with radial muscular striation, plainly widening to the posterior end. Cardia small, triangular, internal. Midgut with distinct peritrophic membrane and without visible intestinal cell borders. There may be a renette ampulla vaguely visible just posterior to the nerve ring and a renette cell body posterior to the cardia, to the right (males NN 2, 5, 6, females NN 4, 7, 8) or to the left (male N3) of the intestine. Female gonad single, anterior, outstretched, situated entirely to the right of the intestine. Vagina strongly sclerotised. No postvulvar sac. Uterus may contain one ripe egg 24–27 µm long and 11.5–12.5 µm wide. Male gonad single, outstretched, situated to the left (males NN 1, 3) or to the right (males NN 2, 5, 6) of the midgut. Spermatozoa (or spermatides?) as tiny light­refracting granules. Spicules short, slightly arcuate, distally pointed and proximally cephalated with oblique knobs. Gubernaculum dense­bodied, with a dorso­caudal apophysis. There is a small ventral preanal papilla (or a pair of two close papillae?) situated slightly posterior to the level of the spicular knobs. A similar small ventral postanal papilla (or a pair of two close papillae) in the middle region of the tail. There are two or three preanal setae and three to four lateroventral, lateral and laterodorsal setae postanal setae on the male tail. Tail elongate conical, with terminal spinneret and caudal gland cell bodies within. In females, the tail usually bent dorsally.

Discussion. Among Thalassomonhystera species, the new species is most similar to T. bathislandica Riemann 1995 also described from the north­eastern Atlantic abyssal (Riemann 1995). Both species are similar in minute body size and other dimensions as well as in rather voluminous stoma, position of amphids, renette cell and vulva, shape of the copulatory apparatus and tail. Also, both species show variable position of the male genital tract about the intestine, a deviation from the normal monhysterid morphology (Lorenzen 1978). T. molloyensis sp. n. differs from T. bathislandica by: 1) having a smaller body, male length 392–460 µm versus 572–684 µm, female length 376–472 µm versus 615–633 µm; 2) having a sclerotised lips of vulva; 3) the presense of two ventral papillae or pairs of papillae, first one in preanal position and second one in postanal position just behind the middle of the tail.