Litinium abyssorum Tchesunov, Nguyen Dinh Tu, Nguyen Vu Thanh

Tchesunov, Alexei V., Thanh, Nguyen Vu & Tu, Nguyen Dinh, 2014, A review of the genus Litinium Cobb, 1920 (Nematoda: Enoplida: Oxystominidae) with descriptions of four new species from two contrasting habitats, Zootaxa 3872 (1), pp. 57-74 : 66-69

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

Fig. 4 , Table 3

Material. One holotype male and one allotype female. The specimens are deposited in the Senckenberg Natural History Museum (Frankfurt am Main, Germany) with slide numbers SMF 16981 (holotype male) and SMF 16982 (allotype female).

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 slim, cylindrical. Cuticle smooth and evenly thick (about 2 µ m) over entire body.

Anterior end shaped as truncated cone; ratio of body diameter at the level of inner labial setae to that of the cephalic setae is 2 in the male and 1.8 in the female. There are two circles of six setae (inner labial setae and outer labial setae), consecutive but close to each other. The setae of both circles slender and cylindrical, nearly equal in length. Distance from the apex to the inner labial setae 2 µ m in the male and 7 µ m in the female. Circle of four cephalic setae situated posterior to the amphideal fovea, far posterior to the circle of outer labial setae; cephalic setae slender cylindrical and slightly shorter than setae of the two anterior circles. Amphideal fovea in male large, rounded in outer outline, with deep sinuous invagination on the surface at the anterior edge. Amphideal nerve emerges as broad funnel from beneath the posterior amphideal fovea. Amphideal fovea in female shaped in outline as an upside-down pear, with a small groove on its anterior edge; posterior smaller lobe of the fovea set off with a constriction. A number of minute somatic sensilla distributed sublaterally along the; each inserted in the middle of a short longitudinal ‘slit’ in the cuticle.

Mouth opening tiny. Somatic cuticle thickened slightly around the mouth opening. Buccal cavity not developed as such (internal lumen of the anterior pharynx not enlarged), but internal lining of the anterior pharynx thickened and cuticularised forming a narrow buccal capsule 13 µ m long in the male and 15 µ m long in the female. Pharynx gradually widening to the cardia; with clear internal cuticular lining; no muscular cross striation apparent in pharynx.

Nerve ring not seen. Ventral gland not visible.

Testes not discernible. A midventral seta 2 µ m long 2 µ m anterior to the cloacal opening. No precloacal midventral supplementary papillae. Right spicule short, weak, slightly curved, with slightly curved anterior handle. Left spicule reduced to a short straight rod-like structure with its pointed tip directed dorsally. Gubernaculum a short bar parallel to the spicule at its posterior part.

One posterior ovary antidromously reflexed and situated on the right of the intestine. Vulva depressed, surrounded by thickened cuticle.

Tail cylindrical, with rounded tip. There is a terminal pore, thin canal and coiling ducts of caudal glands with light transparent content. Caudal gland cell bodies with nuclei not visible. There is a minute lateroventral subterminal papilla on either side of the tail.

Diagnosis. Litinium . Body length 710–897 µ m; a 50.7–69; c 13.3–15.7; c' 3.61–5.76. Inner labial, outer labial, cephalic setae 2.5–4 µ m in length. Amphideal fovea large, with deep anterior sinuous invagination of the external contour. Cephalic setae posterior to the amphideal fovea. One precloacal midventral supplementary seta just anterior to cloacal vent. Spicules unequal, the left spicule vestigial.

Differential diagnosis. Litinium abyssorum sp. n. differs from all other Litinium species with known males by having unequal spicules. Further, L. abyssorum sp. n. has a peculiar amphideal fovea with deep anterior invagination similar to those of L. bananum , L. volutum , L. obtusilobus and L. profundorum . Litinium abyssorum differs from L. bananum by smaller body length (710–897 µ m vs 2140–3225 µ m, shorter anterior setae (2.5–4 µ m vs 10–14 µ m), presence vs apparent absence of precloacal midventral seta; from L. volutum by simpler shape of the amphideal fovea without curls, one vs four preanal papillae in male and longer tail (c ' 5.76 vs 1.5, in females), from L. profundorum by longer tail (c 15.7 vs 40.6 and c' 3.6 vs 1.2) and two vs one precloacal midventral setae, from L. obtusilobus by short cephalic setae (2.5 µ m vs 5 µ m) and relatively longer tail (c' 5.8 vs 1.5).

Etymology. Species name is derived from Hellenic "abyssus" (abyss, chasm).

Figures

FIGURE 4. Litinium abyssorum sp. n. A: holotype male, entire; B: allotype female, entire; C: holotype male, anterior end; D: anterior end of the allotype female; E: anterior end of the allotype female, optical section; F: holotype male, posterior body; G: allotype female, tail. Arrows indicate somatic papillae in longitudinal slits in the cuticle (lo. sl.), gubernaculum (gub.), left vestigial spicule (l. sp.). Scale bars: A, B 300 µ m; C – G 10 µ m.

Tables

TABLE 3. Morphometry of Litinium abyssorum sp. n. Measurements in µ m.

Character	Specimens
	Holotype male	Allotype female
L	710	897
a	50.7	69
b	4	4.31
c	15.7	13.3
c’	3.61	5.76
V, %	-	35.4
diam.c.s.	11	9
diam.am.	11	7.5
diam.n.r.	14	12
diam.ca.	15	12
diam.midb.	14	13
diam.ani	12	9
i.l.s.	3.5	3
o.l.s.	4	3
c.s.	2.5	2.5
dis.c.s.	23	14.5
dis.am.	7.5	6
am.l.	10	6
am.w.	8	5
spic.arc (right)	12.5	-
spic.arc (left)	8	-