Hyalorisia nanhaiensis Fassio, Bouchet & Oliverio new species

MOTU-6 (Figs 3F, 5B, 7E, N)

Type material: Holotype: S of Dongsha (=Pratas) Islands, South China Sea, ZHONGSHA 2015, Stn CP4162, 20°33 N, 116°40 E, 528–540 m, MNHN-IM-2013-59534 (male).

Material examined: Holotype and other material (MNHN-IM-2007-34793, MNHN-IM-2007-34796, MNHN-IM-2013-59496, MNHN-IM-2013-59697 and MNHN-IM-2013-61770).

Etymology: After the Chinese name for the South China Sea (‘Nanhai’), the general area of the type locality.

ZooBank registration: urn:lsid:zoobank.org:act: 2F50B7B5-DF14-423F-B123-34B64BCB045C.

COI-based diagnosis: Six diagnostic (autapomorphic) positions in the alignment: 75(G), 263(C), 271(C), 322(G), 400(A) and 631(C).

Shell (data for holotype shown within parentheses): Size small to medium for the genus: length 4.16–9 (8.5) mm, width 3.12–6 (6) mm, height 1.6–3 (3) mm; thin, ellipsoidal, depressed; white shell surface with smooth yellowish periostracum; anterior margin rounded or gently angled; protoconch of 1–1.2 (1.2) whorls, diameter of nucleus 0.12– 0.24 (0.15) mm; teleoconch smooth except for distinct commarginal growth ridges; apertural margin thin, interior glossy white; lamella from wide to narrow, width 0.4–1.12 (0.83) mm.

Soft parts (ethanol preserved): Head completely whitish or grey at the base; cephalic tentacles completely whitish or grey from the base to half/two-thirds of the length; pseudoproboscis whitish; mantle completely whitish to black with whitish border.

Radula: Taenioglossate, 2-1-R-1-2; marginal teeth sickle shaped; internal marginals with few denticles on the inner edge; lateral teeth triangular, highly denticulated on both edges; rachidian tooth triangular, highly denticulated on both edges.

Host: Holotype (MNHN-IM-2013-59534) was attached to P. jeffreysii (MNHN-IM-2013-59533). Of the other specimens, two (MNHN-IM-2007-34796 and MNHN-IM-2013-59496) were found attached to P. jeffreysii (MNHN-IM-2007-34797 and MNHN-IM-201359495, respectively) and two (probably a female MNHN-IM-2013-59697 and a male MNHN-IM-2013-61770) attached to the same individual of a Propeamussium sp. (MNHN-IM-2013-61769).

Distribution: So far known only from the South China Sea and the Philippines, at depths of 358– 556 m.

Remarks: Hyalorisia nanhaiensis n. sp. was found syntopically with H. profunda n. sp. (MOTU-9) on the continental slope of the South China Sea (ZHONGSHA 2015: Stn CP4137).