Plectostoma retrovertens (Tomlin, 1938)

Plectostoma retrovertens (Tomlin, 1938)

General description:

to be added

Conservation status:

Vulnerable D2 ver 3.1
http://www.iucnredlist.org/details/168188/0

Diagnostic description:

to be added

Taxon biology:

Plectostoma retrovertens and Plectostoma palinhelix are very distinct from other Plectostomaby 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%.

Distribution:

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

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

Wed, 2014-09-03 19:37 -- thorseng
Plectostoma retrovertens (Tomlin, 1938)

General 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.

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 byBerry (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).

Diagnostic description:

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

Wed, 2014-09-03 19:37 -- thorseng
Plectostoma retrovertens (Tomlin, 1938)

Diagnostic description:

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

Wed, 2014-09-03 19:37 -- thorseng
Plectostoma retrovertens (Tomlin, 1938)

Diagnostic description:

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

Wed, 2014-09-03 19:37 -- thorseng
Plectostoma retrovertens (Tomlin, 1938)

Morphology:

Tomlin (1938) original descriptions on Opisthostoma retrovertens – “Shell dextral, regularly conical until the last whorl, which mounts one side of the cone until in the adult the upper margin of the peristome is exactly level with the apex of the shell; number of whorls six, apex rather blunt; aperture almost circular, facing backwards vertically in a position the reverse of a normal aperture; peristome duplex; there is no spiral sculpture, but the whorls other than the smooth protoconch have prominent, but the whorls other than the smooth protoconch have prominent, thin, rather distant, retractive axial costulations; the protoconch consists apparently of one whorl, and the costulations though few on the second whorl are distinctly traceable; suture deep; umbilicus oval in shape and rather deep, not covered by the deviating whorl. The specimens are all dead and no operculum is obtainable.”

Size:

Opisthostoma retrovertens – “Diam. max., 2.4 mm; alt., 2.5 mm.” (Tomlin, 1938)

Distribution:

Type locality – “Bukit Chintamani, Pahang” leg. M. W. F. Tweedie (Tomlin, 1938)

Wed, 2014-09-03 19:36 -- See citation fo...

Opisthostoma and Plectostoma (Family Diplommatinidae)

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Plectostoma retrovertens (Tomlin, 1938)

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