Litinium profundorum Tchesunov, Nguyen Dinh Tu, Nguyen Vu Thanh sp. n.

Fig. 5

Material. One holotype male. The specimen is deposited in Senckenberg Natural History Museum Frankfurt with the slide number SMF 16983.

Locality. South-east Atlantic Ocean, Angola Basin, 16°17.0'S, 005°27.0'E, depth 5389 m, silt—total carbon content 0.62%, mud content 95.23%, chlorophyll–a content 1.67 µ g/g. DIVA –I expedition, st. 346/2, Multicorer 3; 27 July 2000.

Description. Body slender, nearly cylindrical. Cuticle smooth and evenly thick (1.5–2 µ m) over entire body. L 1196 µ m, a 52, b 4.54, c 40.6. Body diameter at the level of: cephalic setae 10.5 µ m, amphideal fovea 8.5 µ m, nerve ring 18 µ m, cardia 22.5 µ m, midbody 23 µ m, cloaca 21 µ m.

Anterior end rounded, conoid. Mouth opening minute. There are two circles of six inner and six outer labial setae close to one another. All setae slender and cylindrical. Inner labial setae 2 µ m long, two times shorter than the outer labial setae (4 µ m long). Inner labial setae situated at a distance 8–10 µ m from the cephalic apex. Four slender cylindrical cephalic setae situated posterior to the amphideal fovea, at a distance 14 µ m from the cephalic apex, their length 5.5 µ m.

Amphideal fovea 8 µ m long and 6 µ m wide (71% of corresponding body diameter), ovoid to cordate in outline, the anterior edge with large rounded invagination on the surface; at posterior end fovea becomes wide amphideal nerve. Distance from cephalic apex to amphideal fovea 4.5 µ m. Somatic sensilla repreresented by sparse lateral and sublateral papillae along the body.

Buccal cavity not developed as such. The anteriormost region (9 µ m long) of the internal lining of the pharynx thickened and cuticularised but not differing sharply from subsequent cuticular lining. Pharynx gradually widening to the cardia. Cross striation in the pharynx not clearly seen. Dorsal sector of pharynx contains many light transparent pharyngeal cells. Ventral pore not seen.

Testes not discernible. Spicules paired and equal, short (20 µ m long), and relatively broad, with internal stripes, posterior end slightly narrowed, anterior end knobbed. Gubernaculum a small compact body located postero-ventrally of spicules. Two equal midventral supplementary organs anterior to the cloacal opening, anterior one 54 µ m, posterior 33 µ m from the cloaca. Each comprises a small cuticular swelling with an apical crater from which a slender cylindrical seta 3–3.5 µ m long protrudes.

Tail short, rounded to conoid, 1.2 anal diameters long (c'). Terminally, internal layer of the cuticle transformed into a light-refractile cap-like structure. Spinneret or even terminal duct not evident. Caudal glands barely discernible.

Diagnosis. Litinium. Body length 1196 µ m; a 52; c 40.6; c ' 1.2. Inner labial setae 2 µ m, outer labial setae 4 µ m, cephalic setae 5.5 µ m in length. Amphideal fovea large, with very deep anterior sinuous invagination making the fovea halfmoon-shaped. Cephalic setae at a short distance posterior to the cephalic setae. Two subsequent midventral precloacal supplementary organs with short setiform processes. Spicules equal. Tail short, with terminal sclerotization of the internal cuticular layer in shape of a truncated cone.

Differential diagnosis. L. profundorum sp. n. relates to L. bananum, L. volutum, L. abyssorum and L. obtusilobus in shape of large halfmoon-formed amphideal fovea. L. profundorum differs from L. bananum by smaller body (1196 µ m vs 2140 µ m), lower index a (52 vs 98), shorter tail (c 40.6 vs 19.8, c' 1.2 vs 6–8) and shorter anterior setae (2–5.5 µ m vs 10–14 µ m); from D. volutum by higher indices a and b (52 vs 25, 4.5 vs 9, respectively), shorter anterior setae (2–5.5 µ m vs 10.5–15 µ m) and two vs four precloacal supplementary setae. Litinium profundorum is closest to L. abyssorum in general appearance and size but differs by having a shorter tail (c 40.6 vs 15.7 and c' 1.17 vs 3.61), two vs one precloacal supplementary setae, and equal spicules. Another species close to L. profundorum is L. obtusilobus, but because both species are known by only one gender (male in L. profundorum and female in L. obusilobus) proper comparison is not possible. Litinium profundorum has similar measurements to those of L. obtusilobus except for length of inner labial setae (2 µ m vs 4 µ m long). The structural character differentiating L. profundorum from L. obtusilobus is the distinct light-refractile sclerotization of the inner cuticle layer of the tail tip.

Etymology. Species name is derived from Latin "profundum" (deep sea).