Plectostoma christae (Maassen, 2001)

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 : 36-38

Plectostoma christae (Maassen, 2001) Figures 17Q, 25, Appendix 6

Opisthostoma christae Maassen, 2001: 52, figures 3, 10 & 11 (original description).

Opisthostoma jensi Maassen, 2001: 56, figures 5, 14 & 15 (original description), syn. n.

Type material.

Holotype: RMNH 81805(1) (Seen). Paratypes: RMNH 81806(1) (Seen).

Other examined materials.

RMNH 81807(1), RMNH 81808(1), ZMA 138436(1), ZMA 138437(2), BOR 3496(1), BOR 5505(>25), BOR 5506(>50), BOR 5509(>25), BOR 5572(2), V 12702(1), V 8314(>25), V 8406(2), V 9153(2), V 9207(>100), V 9285(3).

Diagnosis.

Shares with Plectostoma dindingensis , Plectostoma mengaburensis , and Plectostoma panhai the general shell spire form but differs by having two parietal constriction teeth and aperture visible when shell observed in left lateral view.

Description.

Apex. Shape: moderately convex.

Spire. Height: 1.9-2.6 mm. Width: 1.4-1.8 mm. Number of whorls: 3 5/8-4 1/2. Apical spire shape: depressed conical. Basal spire shape: ovoid. Whorl periphery: distinctly convex. Umbilicus: open.

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

Tuba. Coiling direction: type 1 and aperture visible from left lateral view. Tuba whorl length in proportion to spire last whorl: ca. 3/8-5/8. Proportion of tuba that attaches to spire: whole.

Aperture and peristome. Peristome: double peristomes. Outer peristome shape: same as inner peristome and uniformly projected all around, except the posterior part.

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

Radial ribs. Rib density: 5-6 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. Limestone hills, 16 km west of Gua Musang (4°54'46"N, 102°6'22"E).

Distribution range. Limestone hills along the 50 km Northeast transect between 4°38'51"N, 101°58'58"E and 5°0'13"N, 102°11'59"E (Figure 18D).

Conservation status.

Near Threatened. Until today, this species has been recorded from at least six limestone hills. In a field survey in 2011 and 2012, living populations of Plectostoma christae could be found on four of these hills. All of these are located near the road and are surrounded by oil palm plantation, although there is no immediate threat.

Discussion.

Plectostoma christae , together with Plectostoma dindingensis , Plectostoma mengaburensis , Plectostoma sinyumensis , Plectostoma umbilicatum , Plectostoma siphonostomum , and Plectostoma panhai represent a group of Plectostoma species that have a regulary coiled tuba (type 1 tuba). The species of this group occur only in Peninsular Malaysia and are genetically highly divergent (> 10% differences in COI) from the others (Table 4). All of the seven species are distributed allopatrically (Figure 18D).

We synonymised Plectostoma jensi with Plectostoma christae , both of which were described from the same locality. Maassen (2001) distinguished between them by the slight difference in umbilicus opening and aperture tilting. In the material at our disposal, we recognised that these differences are intrapopulational variation. All individuals share the same diagnostic shell characters as mentioned above. In addition to the morphological evidence, the genetic variation between individuals with different shell forms is smaller than our species delimitation threshold of 10%.

Two species of this group, namely Plectostoma christae and Plectostoma siphonostomum , have a wider distribution range than other species in this group. The two species occur parapatrically on the limestone hills in the centre of Peninsular Malaysia (Figure 18D). On the other hand, very little is known of the distribution range of Plectostoma panhai . Although this species was reported only once and only from the type locality, it might also occur at other limestone sites near the type locality. Plectostoma panhai is very similar to Plectostoma christae , but the two are separated by more than 150 km, and the limestone hills in between are occupied by three other Plectostoma species. The disjunct distribution and its single constriction tooth support the decision that Plectostoma panhai is a distinct species from Plectostoma christae .

The remaining four species of this group, namely, Plectostoma sinyumensis , Plectostoma mengaburensis , Plectostoma dindingensis , and Plectostoma umbilicatum , are site endemics, occurring at each of the four small limestone clusters in the centre of Peninsular Malaysia (Figure 18D). These clusters are each quite isolated, with no other limestone hills within a 20 km radius.

Although these four species occur in adjacent limestone limestone clusters, and they have similar shell shapes, their taxonomic status are clear. The COI sequence divergence between these species is larger than 13% and each of them has a set of diagnostic shell characters (Tables 3 and 4). This may raise the question how each species evolved in each limestone cluster and how long these four species have been isolated. For example, a neighbouring species, Plectostoma salpidomon , has a similar distribution range as the former four species, but the morphological and genetic divergence is much smaller than in these four species. Presumably, the answer lies in the details of the geomorphological evolution of the limestone outcrops, which, however, remains largely unknown.