Plectostoma ikanensis, Liew, Thor-Seng, Vermeulen, Jaap Jan, Marzuki, Mohammad Effendi bin & Schilthuizen, Menno, 2014

Liew, Thor-Seng, Vermeulen, Jaap Jan, Marzuki, Mohammad Effendi bin & Schilthuizen, Menno, 2014, A cybertaxonomic revision of the micro-landsnail genus Plectostoma Adam (Mollusca, Caenogastropoda, Diplommatinidae), from Peninsular Malaysia, Sumatra and Indochina, ZooKeys 393, pp. 1-107 : 50-52

Plectostoma ikanensis sp. n. Figures 17J, K, 34, 35, Appendix 11, 14

Type material.

Holotype: BOR 5645(1)

Paratypes: BOR 5507(6), BOR 5622(>50), BOR 5504(>10), V 9446(>100), V 9320(6).

Etymology.

This species is named after its type locality–Gua Ikan.

Diagnosis.

Shares with Plectostoma kayiani the general shell form, in terms of spire and tuba shape, but differs by having both thick and thin spiral lines.

Description for shell form 5504

(Figs 17K and 34, and Appendix 14). Apex. Shape: moderately convex.

Spire. Height: 1.7 mm. Width: 1.3 mm. Number of whorls: 4 1/8-4 1/4. Apical spire shape: depressed conical. Basal spire shape: conical. Whorl periphery: distinctly convex. Umbilicus: open.

Constriction. Parietal teeth: none. Basal teeth: none

Tuba. Coiling direction: type 2 and aperture visible in right lateral view. Tuba whorl length in proportion to spire last whorl: ca.3/4-7/8. Proportion of tuba that attaches to spire: whole.

Aperture and peristome. Peristome: double peristomes. Outer peristome shape: similar to inner peristome, projected all around, except the posterior part, where left lateral sides are slightly more projected than the anterior and right lateral side.

Spiral lines. Thick lines: present. Thin lines: present.

Radial ribs. Rib density: 5 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Description for shell form 5507

(Figs 17J and 35, and Appendix 11). Apex. Shape: moderately convex.

Spire. Height: 1.8-1.9 mm. Width: 1.3 mm. Number of whorls: 3 7/8-4. Apical spire shape: depressed conical. Basal spire shape: conical. Whorl periphery: distinctly convex. Umbilicus: open.

Constriction. Parietal teeth: none. Basal teeth: none.

Tuba. Coiling direction: type 2 and aperture visible in left lateral view. Tuba whorl length in proportion to spire last whorl: ca. 5/8. Proportion of tuba that attaches to spire: whole.

Aperture and peristome. Aperture with double peristome. Peristome: double peristomes. Outer peristome shape: similar to inner peristome, projected all around, except the posterior part, where left lateral sides are slightly more projected than the anterior and right lateral side.

Spiral lines. Thick lines: present. Thin lines: present.

Radial ribs. Rib density: 7 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. Gua Ikan in the State of Kelantan (5°21'9"N, 102°1'34"E).

Distribution range. In addition to the type location, this species also occurs at nearby limestone hills as far as 30 km away (Figure 18C).

Conservation status.

Least concern. Living populations of Plectostoma ikanensis were recorded at the type locality in 2011. The type locality is gazetted as recreation forest.

Discussion.

The two forms of Plectostoma ikanensis were found in the type locality at two different parts of the hill that within a distance of 100 m. The overall shell appearances of these two forms are very different, especially in terms of tuba coiling direction and spire shape. Interestingly, one of the Plectostoma ikanensis forms (i.e. BOR 5507) is very similar to Plectostoma davisoni on the basis of these two shell characters. In most of the other Plectostoma species examined in this study, tuba coiling direction and spire shape are rather stable characters within a species. Nevertheless, both Plectostoma ikanensis forms lack a constriction which unite them and distinguish them from other similar species (see Diagnosis). In addition to the morphological evidence, we found that the genetic divergence of these two forms is smaller than 1% (Table 4), and our preliminary phylogenetic analysis shows that the two forms are reciprocally monophyletic. We therefore conclude that they be classified as the same species.