Amphimonhystrella bullacauda Tchesunov et Miljutina, sp. n.

Amphimonhystrella bullacauda Tchesunov et Miljutina sp. n.

Table 1, Figs 1, 2

Material. One holotype male, four paratype males and five 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, August 16, 2000.

Etymology. The name bullacauda means “knob­terminated tail”.

Description. Body slender, elongate spindle­shaped. Cuticle distinctly annulated with approximately ten annules per 10 µm. Cephalic end slightly narrowed, in shape of a truncated cone. Cephalic cuticle smooth from the apex to the stoma ending. There are very minute, obscure inner labial papillae in the labial region. Outer labial and cephalic setae united in almost one circle. Outer labial setae obscurely two­jointed and longer than smooth cephalic setae at about one third. Amphid relatively large, round or slightly longitudinally oval, with discontinuous fine but distinct edging, sometimes with central spot. There are no somatic setae visible along the body. Buccal cavity from elongateconical to nearly cylindroid, with thin but neat sclerotised walls, surrounded with an esophageal cuff at two thirds of its length. Esophagus slender but distinct, with weak radial muscular striation, slightly widened at the posterior end. Cardia cordate, vacuolised and surrounded with the intestinal tissue. There is no differentiated progaster or two­rowed arrangement of intestinal cells in the midgut. No renette cell visible. Single anterior female gonad situated to the left of the midgut. Female genital branch outstretched, poorly differentiated. The largest and ripest oocyte differs sharply from preceding oocytes by its greater size and dense coarse granulation. The ripest oocyte may reach 57–60 µm in length and 9–13 µm in width. Uterus plainly continues into a postvulvar sac narrowing to the blind posterior end situated ventrally of the intestine. A few spherical spermatozoa with minute central nuclei are present in the postvulvar sac.

Single anterior male gonad outstretched and situated to the left of the midgut. Spermatocytes in the testis are very large and arranged in one row. There are no ripe spermatozoa found in the male gonad. No supplementary organs are present. Spicules very short, slightly arched, shaped as a curved dagger, distally acute and proximally slightly cephalated. Gubernaculum with a short dorso­caudal apophysis.

Tail consists of a proximal conical portion sharply differentiated by a deep narrowing from terminal portion strongly enlarged in drop­like or clavate fashion. There are two or three very short terminal setae (1.5 µm) on the tail tip. No other setae on the tail. There are two caudal glands barely visible inside the tail and usually three separate caudal gland ducts with pores located at the terminal widening.

Diagnosis. Amphimonhystrella bullacauda is similar to A. unita Lorenzen 1977, being of similar minute size (in A. unita, body length 485 µm in both male and female specimens), clearly annulated cuticle, absence of additional setae in cephalic crown, large longitudinal oval amphids, and prominent postvulvar sac with relatively large spermatozoa. However, A. bullacauda sp. n. differs from A. unita by having longer cephalic setae (1.5–3.5 versus 1.0 µm), lacking cervical setae, head set off the body, singular anterior testis, distally pointed spicules, much more protuberant terminal tail widening.