Anatoma laevapex Hoffman & Kniesz & Arbizu & Kihara 2022, sp. nov.

Anatoma laevapex sp. nov.

urn:lsid:zoobank.org:act: FCBC4FC9-05DD-4472-9D3F-336EE8A17002

Figs 8–10

Etymology

The species name ‘ laevapex ’ refers to the smooth initial teleoconch.

Type material

Holotype INDIAN OCEAN • SEIR; 27.65° S, 73.88° E; depth 2469 m; 11 Dec. 2019; stn INDEX19_127RO; spec. I19 _Ma_74, SEM, Fig. 8A–C; SMF 358989.

Paratypes INDIAN OCEAN • 1 specimen (paratype 1); same location data as for holotype; spec. I19 _Ma_75, SEM, Fig. 8D–E; SMF 358990 • 1 specimen (paratype 2); same location data as for holotype; spec. I19 _Ma_76, SEM, Fig. 8F–G; SMF 358991.

Other material examined

INDIAN OCEAN • 24 specimens; CIR; 23.87° S, 69.62° E; depth 2980 m; 12 Nov. 2019; stn INDEX19_033RO; specs I19 _Ma_8– 26, I19 _Ma_95–99 • 1 specimen; CIR; 23.78° S, 69.55° E; depth 3049 m; 2 Dec. 2015; INDEX15_49R; spec. I15 _Ma_2 • 4 specimens; SEIR; 27.63° S, 73.87° E; depth 2532 m; 5 Dec. 2019; stn INDEX19_102RO; specs I19 _Ma_ 62, I19 _Ma_ 65, I19 _Ma_69–70 • 2 specimens; SEIR; 27.65° S, 73.88° E; depth 2469–2471 m; 11 Dec. 2019; stn INDEX19_127RO; specs I19 _Ma_72–73.

Shell description

PROTOCONCH (Fig. 8C). One slightly elevated whorl, pointed nucleus, coarsely pitted sculpture; sharp lip with flexuous margin at the transition to teleoconch: transition clear by change in sculpture; diameter 0.19 mm.

TELEOCONCH (Fig. 8A–B). Flattened spire, disk-shaped outline with sloping weakly convex shoulders, smooth apex, protruding margins of selenizone; flexuous axial ribs; pear-shaped aperture with slit; suture deep. Height 1.0 mm, width 1.5 mm, height of aperture 0.67 mm (67% of total height); colour opaque grayish white. 2¼ regular whorls with convex shoulder area and rounded base, selenizone above periphery; distance between base of selenizone and suture small but increasing regularly. Teleoconch I from protoconch to start of selenizone: one convex whorl, decending below protoconch; smooth, flexuous growth lines; no spiral cord. Teleoconch II with selenizone: 44 regular weak axial ribs on first whorl, last ¼ whorl of body whorl with 17 axial ribs; ribs at regular spacing. Smooth margins of selenizone sharply protruding outwards, axial ribs absent on margins; inside margins smooth. Frequency of axial ribs below selenizone similar to that above; rough growth lines between ribs. Umbilicus open, tortuous and deep; steep spiral keel to base of columellar callus (Fig. 8A of holotype and 8D, F–G of paratypes).

APERTURE (Fig. 8B). Rounded base, columella and parietal area; flattened shoulder, funnel-shaped selenizone, pointed union with penultimate whorl. Lip sharp, strongly reclining at union with penultimate whorl, protruding at upper shoulder and columella, thin reclining callus at parietal area; disk-shaped lower lip; slit above periphery, slit ¼ whorl deep with flattened sharp margins above and below. Callus thin; inside aperture smooth.

VARIATION. Little morphological variation of the shell. 40–50 axial ribs on body whorl. Adult height range 0.9–1.0 mm; width range 1.3–1.5 mm; H/W ratio range 0.65–0.70.

Anatomical description

Body is translucent white anteriorly with orange-brown internal organs in the visceral mass; white radula visible inside translucent neck area. Snout smooth, broad, flattened with two symmetrical lobes in front covered by one flat convex margin on the antero-dorsal part of the snout (Fig. 9D–E). No eyes observed. Pair of broad cephalic tentacles, tapered, with six rounded lobes, deeply folded between lobes, tip blunt, non-papillate. Foot small in contracted form, with irregular surface. Epipodium with five pairs of appendages (Fig. 9D–E). Anteriorly, three pairs of epipodial tentacles or possibly small ESOs,

marginally papillate. Posteriorly, pair of large ESOs and pair of small epipodial tentacles below the operculum. Mantle smooth with rounded margin, non-papillate.

OPERCULUM (Fig. 9). Yellowish transparent, thin, corneous, multispiral with central nucleus, number of whorls uncertain.

RADULA (Fig. 10). With central, lateral and marginal teeth arrangement: 21–4–1–4–21; width 0.15 mm. Central cusped tooth with 5 curved denticles (Fig. 10A), largest central denticle, smaller towards margin; 4 lateral teeth on either side with curved tips and 2 denticles on each tooth, proximal denticle larger than distal denticle; 21 marginal teeth (Fig. 10C), proximal marginal teeth with 1–3 hooked (curved) denticles (Fig. 10E), distal marginal teeth ending in strongly curved cusp with 6–10 symmetricallyplaced, curved denticles (Fig. 10D).

Distribution

Central and SE Indian Ridge, 23– 28° S, 69– 74° E, depth 2469–3049 m.

Habitat

Living on rocky surface with bacterial mats near active and inactive vents. Shells frequently covered by red bacterial deposit.

Remarks

Shells, anterior soft parts and radulae of Anatoma laevapex sp. nov. are similar to those from Anatoma declivis sp. nov. However, the shell of the latter species has fewer and coarser axial ribs including on teleoconch I; its protoconch is sunken. The shell differences with the similar Anatoma finlayi (Powell, 1937) and other known species in the Indian Ocean are mentioned under the remarks of Anatoma declivis sp. nov.