Lamyronema horizonensis Leduc, 2015, n. gen.

Lamyronema horizonensis n. gen. n. sp.

(Figs 2–4)

Measurements. See Table 1.

Type specimens. Holotype male (99741), collected 10 October 2013 (JAMSTEC cruise YK13-10, deployment 2-1, core 1). One paratype male and one paratype female (99742), collected 12 October 2013 (JAMSTEC cruise YK13-10, deployment 2-3, core 2). One paratype female (99742), collected 12 October 2013 (JAMSTEC cruise YK13-10, deployment 2-3, core 1).

Species Lamyronema horizonensis n. gen. n. sp. Daptonema amphorum n. sp.

Males Females Male Females

Holotype Paratype Paratypes Paratype Holotype Paratype

n - 1 2 1 - 1 L 1423 1387 1243, 1377 1032 955 956 Type habitat. Horizon Deep, Tonga Trench (174.74971º W, 23.27382º S and 174.75224º W, 23.27514º S), water depth: 10817 and 10811 m, sediment depth: 3–10 cm.

Etymology. The species name is derived from the type locality.

Description. Males. Body cylindrical, tapering slightly towards anterior and posterior extremities. Cuticle annulated, ~1.2–1.4 µm thick, with 10 annulations every ~16 µm over entire body. Sixteen to twenty two subcephalic setae situated between anterior edge of amphideal fovea and cephalic setae, 9–11 µm long, not arranged in row; somatic setae sparse, 8–10 µm long.

Head blunt or slightly rounded, with slight indentation at level of amphideal fovea; non-annulated labial region conspicuous. Six small inner labial papillae; six long outer labial setae and four cephalic setae in one circle, 0.45– 0.75 cbd long, cephalic setae of same length or shorter than outer labial setae. Amphideal fovea large, 0.70 cbd wide, circular, outline not cuticularised, 1.0–1.2 cbd from anterior. Buccal cavity large, always filled with detrital particles (Fig. 4), heavily cuticularised, without teeth, tripartite; anterior-most portion (cheilostom) is a cuticularised ring, 2–3 µm deep, middle portion (gymnostom) is wide, barrel to cup-shaped, 15–18 µm deep, and posterior portion (stegostom) narrow or funnel-shaped, 7–12 µm deep. Pharynx muscular, surrounding almost all of buccal cavity to junction of anterior and middle portions, sometimes forming an anterior bulb (Fig. 4), often with irregular outline; no posterior bulb. Cardia well developed, 12–14 µm long. Nerve ring situated at ~42% of pharynx length from anterior. Secretory-excretory system not observed.

Reproductive system diorchic with two outstretched and opposed testes; anterior testis to the left and posterior testis to the right of intestine. Sperm cells nucleated, globular, 4–5 × 5–7 µm. Spicules short, strongly cuticularised, straight to slightly arcuate, with pointed distal end and slightly bent proximally; two small glands apparently connected to proximal end of each spicule. Gubernaculum absent. Ejaculatory glands not observed. Three to four sub-ventral setae situated anteriorly to cloaca. Tail conico-cylindrical, with a few somatic setae situated near half tail length and three terminal setae, 6–8 µm long. Three caudal glands and spinneret present.

Females. Similar to males but with slightly lower a values and markedly smaller amphideal fovea 0.33–0.37 cbd wide. Reproductive system, monodelphic, prodelphic, with ovary to the left of intestine. No spermatheca observed. Vulva transverse, situated at mid-body; small vaginal glands present on either side of vagina.

Diagnosis. Lamyronema horizonensis n. gen. n. sp. is characterised by the presence of 16–22 sub-cephalic setae anterior to the amphideal fovea, pronounced sexual dimorphism in the size of the amphideal fovea, large, strongly cuticularised, tripartite buccal cavity, unarmed, filled with detrital particles, and short, straight spicules 29–36 µm long.

Remarks. Nematode spicules were observed in the intestine of one female paratype, indicating that this species is a predator as well as a deposit feeder.