Retusa abyssicola, Valdés, 2008

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 737

Retusa abyssicola n. sp.

Figs 75C, D, 76

TYPE MATERIAL. — Holotype MNHN 20405 and 1 paratype MNHN 20406, 1 paratype LACM 3004 .

TYPE LOCALITY . — Loyalty Ridge, 20°59’S, 167°00’E, 1639 m [BIOCAL: stn KG 85 ] .

MATERIAL EXAMINED. — Loyalty Ridge. BIOCAL: stn KG 85, 1935 m, 1 dd, paratype (MNHN 20406); stn CP 317, 20°48’S, 20°59’S, 167°00’E, 1639 m, 1 dd, holotype (MNHN 20405; 166°53’E, 1620-1630 m, 1 dd, paratype ( LACM 3004 ).

Figs 75C, D). — BIOGEOCAL: stn KG 267, 21°02’S, 166°59’E,

DISTRIBUTION. — Known only from New Caledonia (Fig. 76), in 1630-1935 m .

DESCRIPTION. — Shell morphology. Length 3 mm, width 1 mm (holotype). Shell fragile, elongate, with nearly parallel sides (Fig. 75C). Only 1 whorl visible, forming the entire shell. At its apical end, the aperture bends over the apex of the shell and the parietal wall forms a blunt prolongation that completely covers the apex. There is a narrow apical umbilicus (Fig. 75D). Anterior end of the shell rounded. Umbilicus closed. Aperture long, wide anteriorly, narrowing gradually at about 1/2 of its length. Columellar margin thickened near the anterior end of the shell. Sculpture of numerous, faint and densely concentrated spiral grooves crossed by fine growth lines all over the shell surface. Colour uniformly shiny whitish.

Anatomy. Unknown. All shells collected lacked soft parts.

REMARKS. — Retusa abyssicola differs from the other species reviewed in this paper in having a blunt apical prolongation instead of a flat or umbilicate apex.

Cylichna sulcata Watson, 1886 , described from the Torres Strait in 6-20 m, has a similar morphology, but the shell is more elongate and the spiral grooves are more conspicuous and only present near the anterior and posterior ends of the shell.

The generic placement of this species is problematic. The presence of a blunt prolongation of the apex resembles members of Volvulella , but R. abyssicola has a narrow apical umbilicus that is absent in that genus. This species is placed in Retusa pending complete specimens becoming available for study.

ETYMOLOGY. — From the Ancient Greek abyssos (deep) and the Latin cola (dweller), in reference to the habitat of this species, used as a noun in apposition.