Hyalorisia nupta Fassio, Bouchet & Oliverio 2020, new species

Hyalorisia nupta Fassio, Bouchet & Oliverio new species MOTU-4 (Figs 3D, 4D, 7D, M)

Type material: Holotype: Bellona Reefs, New Caledonian part of the Coral Sea, EBISCO, Stn CP2558, 21°04 S, 158°35 E, 532–573 m, MNHN-IM-2007-34010 (male). One paratype: same locality as holotype, MNHN-IM-2007-34009 (male?).

Other material examined: MNHN-IM-2007-34026, MNHN-IM-2013-68952, MNHN-IM-2007-34006, MNHN-IM-2013-68951, MNHN-IM-2007-34004, MNHN-IM-2007-34003 and MNHN-IM- 2007-34005.

Etymology: From the Latin adjective nuptus, -a, -um, meaning ‘married’, referring to the high percentage of specimens found in pairs on the same host.

ZooBank registration: urn:lsid:zoobank.org:act: 5CA3966F-5971-4C44-8CF4-B1D75DACCDFD.

COI-based diagnosis: Five diagnostic (autapomorphic) positions in the alignment: 244(C), 250(C), 286(G), 424(G) and 430(G).

Shell (data for holotype shown within parentheses): Size medium to large for the genus: length 10–23 (18.5) mm, width 7–18.5 (14.5) mm, height 3.6–7.6 (6) mm; thin, slightly asymmetrically ellipsoidal or rounded, depressed; white shell surface with a smooth yellowish periostracum; anterior margin slightly angled; protoconch of 0.7–0.9 (0.8) whorl, diameter of nucleus 0.17–0.24 (0.24) mm; teleoconch smooth except for distinct commarginal growth ridges; apertural margin thin, interior glossy white; lamella narrow, width 0.8–1.48 (1.12) mm.

Soft parts (ethanol preserved): Head, cephalic tentacles, pseudoproboscis, mantle and foot whitish.

Radula: Taenioglossate, 2-1-R-1-2; marginal teeth sickle shaped; lateral teeth triangular with several tiny denticles on the central part of the outer edge; rachidian tooth triangular, highly denticulated on both edges.

Host: Holotype (MNHN-IM-2007-34010) and paratype (MNHN-IM- 2007-34009) attached to the same specimen of P. sibogai (MNHN-IM-2007-33954) (Fig. 9A). Each of two further pairs (MNHN-IM-2007-34003 and MNHN-IM-2007-34004; MNHN-IM- 2007-34005 and MNHN-IM-2007-34006) also found attached to the same bivalve (MNHN-IM-2007-33867 and MNHN-IM-2007-33843, respectively). A fourth pair (MNHN-IM-2013-68951 and MNHN-IM-2013-68952) was found at a station where only P. sibogai was collected.

Distribution: So far known only from New Caledonia, at depths of 438– 637 m.

Remarks: Eight of the nine specimens examined were found in pairs (i.e. female + male) attached to the same bivalve specimen. Sexual dimorphism was noticed for size and shape. The shells of males are smaller (half to two-thirds female length and width) and slightly asymmetrically ellipsoidal, while females are larger and rounded. In two of the pairs, the female had an ovoidal scar on the right side of the shell; this scar corresponded in size to the male, suggesting that the male had been attached there. Hyalorisia nupta n. sp. was found sympatrically with H. melanesica n. sp. (MOTU-8) in the New Caledonian part of the Coral Sea.