Hyalorisia madagascarensis Fassio, Bouchet & Oliverio new species

MOTU-10 (Figs 3J, 6B, 7H, Q)

Type material: Holotype: off Mahajanga, NW Madagascar, MIRIKY, Stn CP3268, 15°31 S, 45°45 E, 640–800 m, MNHN-IM-2007-39090.

Material examined: The holotype and one other specimen (MNHN-IM- 2007-39292).

Etymology: After the general area of occurrence, Madagascar.

ZooBank registration: urn:lsid:zoobank.org:act: 5D585AFC-B8C7-4639-9535-74CA13427627.

COI-based diagnosis: Nine diagnostic (autapomorphic) positions in the alignment: 40(G), 103(C), 158(C), 283(G), 529(C), 553(C), 556(C), 605(T) and 628(G).

Shell (data for holotype shown within parentheses): Size small to medium for genus: length 6.5–11 (6.5) mm, width 4.5–9 (4.5) mm, height 2–3 (2) mm; thin, ellipsoidal or rounded, depressed; white shell surface with smooth yellowish periostracum; anterior margin rounded; protoconch of 0.9 whorl, diameter of nucleus 0.19 mm; teleoconch smooth except for distinct commarginal growth ridges; apertural margin thin, interior glossy white; lamella from wide to narrow, width 0.8–0.92 (0.8) mm.

Soft parts (ethanol preserved): Cephalic tentacles whitish with grey base; pseudoproboscis whitish.

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

Host: Holotype attached to P. caducum (MNHN-IM-2007-39094). The second specimen of this Hyalorisia species (MNHN-IM-2007-39092) has a registration number close to that of a specimen of P. sibogai (MNHN-IM-2007-39097); the latter shows a mark on the shell that corresponds to the contour of the Hyalorisia specimen.

Distribution: So far known only from Madagascar, at depths 425– 640 m.

Remarks: Off Mahajanga, Madagascar (MIRIKY: Stn CP3268), Hyalorisia madagascarensis n. sp. was found sympatrically with H. lehibe n. sp. (MOTU-5) and syntopically with H. kely n. sp. (MOTU-7).