One new genus and two new deep-sea nematode species (Desmodoridae, Stilbonematinae) from phosphorite nodule deposits on Chatham Rise, Southwest Pacific Ocean

Abstract

One new genus and two new nematode species of the subfamily Stilbonematinae are described from phosphorite nodule deposits on Chatham Rise, Southwest Pacific Ocean (350 m water depth). Centonema n. gen. is characterised by finely striated cuticle, absence of cephalic capsule, loop-shaped amphid, pharynx differentiated into swollen anterior procorpus, slender median portion, and round posterior bulb, and the male by sub-ventral and sub-dorsal pairs of thick conical setae (porids) on the tail. Centonema n. gen. most closely resembles Eubostrichus, but differs from the latter in the presence of a tripartite pharynx with swollen procorpus, pairs of sub-dorsal porids on the tail of males, and in the size and shape of its bacterial symbionts. The type and only species of the genus, Centonema renamphidum n. gen et n. sp., is characterised by six sub-cephalic setae with one sub-cephalic seta situated directly posterior to each amphid, spicules with capitulum, gubernaculum with broad dorso-caudal apophyses, and tail of males with three pairs of sub-dorsal and four pairs of sub-ventral porids. Eubostrichus hortulanus n. sp. is characterised by cryptospiral amphideal fovea and spiral amphideal aperture, eight short sub-cephalic setae with two sub-cephalic setae situated directly posterior to each amphid, spicules with capitulum, and a gubernaculum with broad dorso-caudal apophyses, the male by one pair of sub-ventral porids in the pre-cloacal region and three pairs of sub-ventral porids on the tail, and sexual dimorphism in the shape of the tail. A key to all valid species of Eubostrichus is provided. Both species were most common in subsurface sediments (3–5 cm depth), and rare in surface sediments (0–1 cm). This is only the second record for this subfamily in a deep-sea environment; the presence of this taxon on Chatham Rise is likely due to the proximity of the highly productive Subtropical Front and the relatively coarse nature of the sediments.