Maragnopsia hadalis Leduc 2016, n. gen.

Maragnopsia hadalis n. gen. n. sp.

(Fig. 5; Table 1)

Diagnosis. Maragnopsia hadalis n. gen. n. sp. is characterised by absence of somatic setae, cephalic setae 0.4 corresponding body diameter (cbd) long, round amphideal fovea 2 cbd from anterior extremity, relatively large and elongated sperm, short and arcuate spicules 1.1–1.3 cloacal body diameter long, and gubernaculum surrounding spicules distally and with short dorsal apophyses.

Etymology. The species name is derived from the Greek god of the nether world Haides, after which the hadal zone (> 6000 m water depth) is named.

Material examined. Holotype male (NIWA 99772), collected 4 May 2014 (WHOI cruise TN309, Nereus dive N073), axis of Kermadec Trench, Southwest Pacific (178.8790º W, 35.8396º S); water depth 7132 m, sediment depth: 3–4 cm. One paratype male and one paratype female (NIWA 99773), same data as holotype (sediment depth: 8–10 cm).

Description. Males Body cylindrical, colourless. Cuticle annulated from level of cephalic setae to near tail tip; longitudinal bars not observed. Somatic setae, porids and epidermal glands not observed. Head slightly rounded, widening slightly at level of cephalic setae. Inner and outer labial sensillae not observed; four cephalic setae, 0.4 corresponding body diameter (cbd) long. Circular amphideal fovea without cuticularised outline, situated about 2 cbd from anterior; oval amphideal aperture of same shape and size as amphideal fovea. Buccal cavity minute, not cuticularised, without teeth. Pharynx surrounding buccal cavity not swollen into a bulb; elongated posterior pharyngeal bulb present, 2.4–2.9 longer than wide. Secretory-excretory system not observed. Nerve ring situated near middle of pharynx. Cardia 5 µm long, not surrounded by intestinal tissue.

Reproductive system monorchic with single anterior outstretched testis. Sperm cells elongated, maximum dimensions 3 × 16 µm. Two equal and arcuate spicules, 1.1–1.3 cloacal body diameter long, cuticularised, pointed distally. Gubernaculum surrounding spicules distally and with short dorsal apophyses. Precloacal supplements not observed. Tail with long, gradually tapering proximal portion and cylindrical terminal portion. Caudal setae and glands not observed; spinneret present.

Female Similar to males. Reproductive system didelphic, with opposed and outstretched genital branches. Anterior branch situated to the right of intestine and posterior branch situated to the left of intestine. Vulva situated slightly posterior to mid-body, at right angle to body wall. Mature eggs not observed. Dorsal wall of uterus cuticularised proximally. Proximal portion of vagina surrounded by constrictor muscle.

Remarks. Maragnopsia hadalis n. gen. n. sp. represented 3% of all nematode specimens at the type locality (D. Leduc unpublished data). Three quarters of M. hadalis n. gen. n. sp. specimens were found in the 5–10 cm layer, and no specimens were found in the 0–1 cm layer.