Acmella nana Vermeulen, Liew & Schilthuizen

Vermeulen, Jaap J., Liew, Thor-Seng & Schilthuizen, Menno, 2015, Additions to the knowledge of the land snails of Sabah (Malaysia, Borneo), including 48 new species, ZooKeys 531, pp. 1-139 : 7-8

Taxon classification Animalia Littorinimorpha Assimineidae

Acmella nana Vermeulen, Liew & Schilthuizen sp. n. Figure 5

Acmella nana nomen nudum: Clements et al. 2008: 2761-2762.

Holotype. Malaysia, Sarawak, 4th Division, Niah Caves, South side of limestone area, West side of quarry, soil-filled crevice opened in quarry (leg J.J. Vermeulen, RMNH.5003950).

Examined material from Sabah.

Interior Province. Pinangah valley, Batu Urun (= Bukit Sinobang) (leg. J.J. Vermeulen, V 7978). Sepulut valley, Gua Pungiton (leg. J.J. Vermeulen & M. Schilthuizen, V 8075, BOR/MOL 529); Gua Sanaron (leg. J.J. Vermeulen & M. Schilthuizen, V 8063). Sandakan Province. Kinabatangan valley, Gomantong Hill 30 km South of Sandakan (leg. J.J. Vermeulen & H. Duistermaat, V 13268).

Description.

Shell minute (one of the smallest Borneo snail species), thin, translucent, white. Surface shiny. Spire conical with almost flat to distinctly convex sides, apex broadly obtuse, whorls moderately convex, sometimes slightly shouldered. Sculpture. Spiral sculpture predominant. Radial sculpture: scattered, inconspicuous growth lines only, grading into somewhat coarser riblets at irregular intervals. Spiral sculpture: rather conspicuous, densely placed and regularly spaced, fine, low and wide threads with only very narrow grooves in between. Aperture obliquely ovate in outline, with a slightly concave to slightly convex parietal side, transition from parietal to basal side rounded to obtusely angular. Umbilicus open, narrow. Dimensions. Height 0.60-0.79 mm; width 0.50-0.60 mm; h/w 1.00-1.35; number of whorls 2 7/8-3 7/8; height aperture 0.30-0.37 mm; width aperture 0.26-0.30 mm.

Habitat in Sabah and distribution.

(Disturbed) primary forest on limestone bedrock. Alt. 0-500 m. Sabah: Interior Province, South part; East coast: Kinabatangan River valley, 1 locality. Also in Sarawak and East Kalimantan. Endemic to Borneo.

Cross diagnosis.

Most similar to Acmella caelata Vermeulen & Junau, 2007, from Sarawak. This species is consistently wider (c. 0.7 mm in Acmella caelata , 0.50-0.60 mm in Acmella nana ). If specimens with the same number of whorls are compared, the whorls of Acmella caelata are less depressed (at 3 1/8-3 3/8 whorl Acmella caelata is 0.80-0.95 mm high, Acmella nana only 0.60-0.75 mm high).

Also similar to Acmella ovoidea but smaller, with a narrower spire than juvenile Acmella ovoidea of the same size. It also has coarser spiral sculpture.

Etymology.

The name refers to the size [nanus (L.) = dwarf].