Hyalorisia melanesica Fassio, Bouchet & Oliverio new species

MOTU-8 (Figs 3H, 5D, 7G, P)

Type material: Holotype: Bellona Reefs, New Caledonian part of the Coral Sea, EBISCO, Stn CP2552, 21°05 S, 158°33 E, 723–741 m, MNHN-IM-2007-33873. One paratype: same locality as holotype, MNHN-IM-2007-33951.

Material examined: Type material and other material (MNHN-IM-2007-32453, MNHN-IM-2007-33839, MNHN-IM-2007-33889, MNHN-IM-2007-34205, MNHN-IM-2007-35554, MNHN-IM-2013-45965, MNHN-IM-2013-58395 and MNHN-IM-2013-66983).

Etymology: After Melanesia, the subregion of Oceania that includes the general area of occurrence of the species.

ZooBank registration: urn:lsid:zoobank.org:act: BDF85F1B-32C3-47A0-B7CE-3D524B483EDF.

COI-based diagnosis: Six diagnostic (autapomorphic) positions in the alignment: 22(T), 175(G), 205(C), 266(C), 478(C) and 580(C).

Shell (data for holotype shown within parentheses): Size small to medium for the genus: length 4.5–8 (6.6) mm, width 2.5–5.04 (4.5) mm, height 1.5–3 (2.4) mm; thin, ellipsoidal, depressed; white shell surface with smooth yellowish periostracum; anterior margin rounded or slightly angled; protoconch of 1.1–1.2 (1.1) whorls, diameter of nucleus 0.09–0.15 (0.09) mm; teleoconch smooth except for distinct commarginal growth ridges; apertural margin thin, interior glossy white; lamella from wide to narrow, width 0.34–1.03 (0.68) mm.

Soft parts (ethanol preserved): Head and cephalic tentacles range from completely whitish to dark grey at the base; pseudoproboscis and foot whitish; mantle completely whitish or grey with whitish border.

Radula: Taenioglossate, 2-1-R-1-2; marginal teeth sickle shaped; lateral teeth triangular, denticulated only on inner edge or on both edges; rachidian tooth triangular, denticulated on both edges.

Host: As indicated in field notes, the holotype and paratype were attached to two individuals of P. investigatoris (MNHN-IM-2007-33874 and MNHN-IM-2007-32441, respectively). One specimen (MNHN-IM-2007-33951) was also found attached to P. sibogai (MNHN-IM-2007-33954); another specimen (MNHN-IM-2007-33839) from the same station and some specimens of P. investigatoris (MNHN-IM-2007-33838) bear consecutive registration numbers (the scallop also showed a mark on the shell, which corresponds to the contour of the Hyalorisia specimen). Two other specimens (MNHN-IM-2013-45965 and MNHN-IM-2013-58395) were found at stations where the only propeamussiid collected was P. investigatoris .

Distribution: So far known from New Caledonia, the Solomon Islands and Papua New Guinea, at depths of 447– 741 m.

Remarks: Hyalorisia melanesica n. sp. was found sympatrically with three other species of Hyalorisia: with H. tosaensis (MOTU-3) in New Ireland and Papua New Guinea; with H. nupta n. sp. (MOTU-4) in the Coral Sea; and with H. solomonensis n. sp. (MOTU-11) in Ainto Bay, New Britain, Papua New Guinea (Solomon Sea).