Manganonema majusculum Leduc, 2015, n. sp.

Manganonema majusculum n. sp.

(Figs 11–12)

Measurements. See Table 2.

Type specimens. Holotype female (99748), collected 12 October 2013 (JAMSTEC cruise YK13-10, deployment 2-3, core 1). One paratype female (99749), collected 10 October 2013 (JAMSTEC cruise YK13-10, deployment 2-1, core 1). One paratype female and one paratype male (NIWA 100187), collected 6 May 2014 (WHOI cruise TN309, Nereus dive N074).

Type habitat. Horizon Deep, Tonga Trench (174.75224º W, 23.27514º S and 174.74971º W, 23.27382º S), water depth: 10811 and 10817 m, sediment depth: 0–4 cm. Kermadec Trench (178.17571º W, 34.34030º S), water depth: 8081 m, sediment depth 1–2 cm.

Etymology. The species name was derived from the latin word majusculus (= somewhat larger or greater), and refers to the large size of this species compared to other species of the genus.

Description. Females. Body long, cylindrical, tapering slightly towards anterior and posterior extremities. Striated cuticle, ~2.0–2.8 µm thick. Anterior extremity gradually tapering with narrow rounded head but without terminal swelling and not dummy-shaped. Somatic setae sparse, 2–3 µm long, situated latero-ventrally and laterodorsally; one pair of short, 2 µm long sub-cephalic setae, one situated latero-ventrally and the other latero-dorsally, immediately posterior to the amphideal fovea. Six minute inner labial papillae; six outer labial setae and four cephalic setae of similar length in one circle, 3–5 µm long. Large oval-shaped amphideal fovea, with cuticularised outline. Pharynx muscular, gradually widening posteriorly to form bulb; cardia small. Nerve ring situated at almost two thirds of pharynx length from anterior. Large, nucleated, clear cell visible ventrally to pharynx, but no duct or opening to exterior observed. Tail long, conico-cylindrical; anterior portion conical, 3.5–5.0 abd long, posterior portion 6.5–11.0 abd long, gradually tapering gradually along entire length or with thin, cylindrical distal end. Three caudal glands and a few 3 µm long caudal setae present.

Reproductive system monodelphic with single anterior outstretched ovary situated to the left of intestine. Vulva situated at almost two thirds of body length from anterior; pars proximalis vaginae surrounded by constrictor muscle.

Male. Similar to females but head shorter and wider, not rounded (or dummy-shaped) as in other species of the genus. Tail long, conico-cylindrical, but broken. Reproductive system diorchic with two opposed and outstretched testes; anterior testis to the left of intestine and posterior testis situated ventrally relative to intestine. Sperm cells globular, ~2 × 3 µm. Spicules relatively large, almost straight, 1.0 abd long, with rounded proximal end and slightly curved distal end. Gubernaculum and pre-cloacal supplements/setae not observed.

Diagnosis. Manganonema majusculum n. sp. is characterised by large body size (1737–2084 µm), anterior body extremity conico-cylindrical with sexual dimorphism in head shape (short and wide in male vs long and narrow in females), large amphideal fovea (0.70–0.75 cbd), male with two opposed testes and spicules 1.0 abd long with rounded proximal end, and tail 10.5–15.0 abd long.

Differential diagnosis. Manganonema majusculum n. sp. is most similar to M. kitasatoi n. sp. in body and amphideal fovea size, presence of two testes, and presence of a pair of sub-cephalic setae posterior to the amphideal fovea. Manganonema majusculum n. sp. can be distinguished from the latter, as well as all other species of the genus, by the size of the tail (10.5–15.0 vs 3.0–8.0 abd in all other species; Bussau 1993, Fonseca et al. 2006), long (1.0 abd) and conspicuous spicules with rounded proximal end (vs thin and inconspicuous with proximal end not rounded), and unusual shape of the head in males, i.e., short and wide vs long and narrow with constriction at base.

Remarks. It is unclear whether the unusual head of the male is present in all males of the species or only the single specimen that was observed. In addition, the tail of the male is broken. The female specimens, however, all possess the usual head shape for the genus and have complete tails. A female was therefore designated as holotype for this species.