Plectostoma retrovertens (Tomlin, 1938)

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 : 48-50

Plectostoma retrovertens (Tomlin, 1938) Figures 17R, 33, Appendix 13

Opisthostoma retrovertens Tomlin, 1938: 73, Plate 2 - figure 3 (original description).

Opisthostoma retrovertens Tomlin, van Benthem Jutting (1952: 39).

Opisthostoma retrovertens Tomlin, Berry (1961).

Opisthostoma retrovertens Tomlin, Berry (1962).

Opisthostoma retrovertens Tomlin, Berry (1963).

Opisthostoma retrovertens Tomlin, Berry (1964).

Opisthostoma retrovertens Tomlin, Berry (1966).

Opisthostoma retrovertens Tomlin, Solem (1966).

Opisthostoma retrovertens Tomlin, Illert (1987: 800, figure 2a).

Opisthostoma retrovertens Tomlin, Solem and Solem (1976: 31).

Opisthostoma retrovertens Tomlin, Heller (2001: 426).

Opisthostoma retrovertens Tomlin, Clements (2007: 74).

Opisthostoma retrovertens Tomlin, Clements et al. (2008: 2760).

Type material.

Holotype: BMNH 1938.10.25.2(1) (Seen). Paratype: ZMA 136044(>10) (Seen).

Other examined materials.

RMNH 244699(1), RMNH 44725(8), ZMA 162133(>10), ZMA 162148(8), BOR 5559(3), BOR 5621(9), V 5124(9).

Diagnosis.

Shares with Plectostoma palinhelix the spire and tuba form, but differs by having spire height between 2.3-2.6 mm.

Description.

Apex. Shape: moderately convex.

Spire. Height: 2.3-2.6 mm. Width: 1.5-1.7 mm. Number of whorls: 4 5/8-5. Apical spire shape: depressed conical. Basal spire shape: ovoid. Whorl periphery: distinctly convex. Umbilicus: partially or completely closed by tuba.

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

Tuba. Coiling direction: type 2 and aperture visible between right lateral and back view; the tuba coils upward until the first teleconch whorl of the spire. Tuba whorl length similar to the length of the last whorl of the spire. Proportion of tuba that attaches to spire: whole.

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

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

Radial ribs. Rib density: 7-9 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. Bukit Chintamanis, Pahang (3°26'45"N, 102°0'51"E).

Distribution range. Endemic to the type locality (Figure 18C).

Conservation status.

Critically Endangered (B2ab(iii)+C2a(i) ver. 10.1). This species is endemic to a single limestone hill–Bukit Chintamanis. Most of the western part of this hill is gone due to the quarry activity in the past and this part is now covered by secondary vegetation. The rest of the outcrop is surrounded by plantation, which is subjected to periodic clearing and replanting. According to Berry (1961), live individuals can only be found at the rock surface (Site A in Berry 1961). Berry (1962) reports finding several thousand individuals at Site A.

A recent intensive survey for Plectostoma retrovertens was done at the type locality in August 2010 and May 2011. Site A is a rock surface smaller than 20 m2. It appears that the vegetation cover and habitat structure of Site A has not changed as compared to the assessment done by Berry (1962, 1964). However, only three live individuals were found during the survey in Aug. 2010. Furthermore, the surrounding of Bukit Chintamanis has been cleared in 2011. Judging by the population trends and habitat conditions together, this species is at the brink of extinction. The status of this species in a previous assessment (IUCN redlist) was: vulnerable D2 ver. 3.1 ( Clements 2009c).

Discussion.

Plectostoma retrovertens and Plectostoma palinhelix are very distinct from other Plectostoma by having a very long tuba in relation to their spire. In fact, a preliminary phylogenetic analysis suggests that these two are basal species for all Peninsular Malaysia’s Plectostoma . These two species are located more 40 km apart on outcrops that belong to the same limestone facies (Figure 18C). The two species are very similar in their shell shape, but Plectostoma retrovertens is about one-third larger than Plectostoma palinhelix . Furthermore, the genetic divergence between the two species is 10%.