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- Plectostoma Adam 1865: 177. Type species: Plectostoma DeCrespignii (by original designation)
- Geothauma Crosse 1892: 282. Type species (by original designation): Plectostoma grandispinosum (Godwin-Austen, 1889).
Generic classification dispute
The genus Opisthostoma was described by Blanford and Blanford (1860) based on one species–Opisthostoma nilgiricum from India. Adam (1865b) described a second species of Opisthostoma, namely, Opisthostoma decrespignyi, which he previously described under the new genus Plectostoma (Adam 1865a). Nevertheless, Blanford (1867) concluded that the conchological differences between these two taxa were not enough to create a different genus. Instead, he suggested these could be two different subgenera. Next, another two subgenera–Gyrostropha Ancey, 1887 and Geothauma Crosse, 1892, were proposed for different forms of Opisthostoma and Plectostoma. However, Smith (1893a) suggested that this subgeneric classification was not necessary until more data other than shell morphology were available. Since then, a classification into three subgenera within the genus Opisthostoma, namely, Geothauma, Opisthostoma, and Plectostoma has generally been accepted (e.g. von Martens and Thiele 1908, van Benthem Jutting 1932, van Benthem Jutting 1952), until, in a recent review of the genus Opisthostoma, Vermeulen (1991, 1994) followed a classification into only two subgenera, namely, Opisthostoma and Plectostoma.
Despite the distinct ecological niche differences (see below–Distribution and habitat) between Opisthostoma and Plectostoma, it is not feasible to use this criterion in the genus identification, because information about the ecology is usually not available as most collections are made by soil sampling. After 150 years of work on Opisthostoma, it is still difficult to identify reliable apomorphic character states that can be used to distinguish between Opisthostoma and Plectostoma (Vermeulen 1991, 1994). Both share the character state of the constriction, which is a slight shrinkage in the whorl towards the end of the spire. When the animal retracts into its shell, its operculum rests at the constriction (Vermeulen 1991). It is, however, possible to make a morphological distinction between Opisthostoma and Plectostoma on the basic of the shell colouration in a fully grown adult, which is orange or pinkish in Plectostoma and white or pale yellowish in Opisthostoma. The colour differences between these two genera are very clear when comparing the living snails or freshly dead shell material (Figure 17, and Appendix 18). Some Plectostoma species have a regularly coiled tuba, and a shell form that is similar to Arinia. However, Plectostoma and Arinia can be easily distinguished by shell colour differences. The shell colour in Arinia is similar to that in Opisthostoma.
Apex. Protoconch is either slightly, moderately or distinctly convex (Figure 3).
Spire. Height: 1.0 mm–3.7 mm. Width: 0.85 mm–2.60 mm (Figure 11). Number of whorls between 2 3/4–71/4. Apical spire shape: oblong or depressed conical (Figure 4). Basal spire shape: conical, ovoid or ellipsoid (Figure 5). Whorl periphery: flat, slightly, moderately or distinctly convex. Umbilicus: open, partially closed, or totally closed.
Constriction. Parietal teeth: parietal side of inner constriction whorl (Figure 2) with two long lamellae (Figure 6A), two ridges with a knob at each end (Figure 6B), one ridge with a knob at one end (Figure 6C), or no teeth (Figure 6D). Basal teeth: basal side of inner constriction whorl (Figure 2) with no teeth (Figure 7B), one ridge running parallel with the whorl growing direction, one ridge with a knob at one end running perpendicular to the whorl growing direction, or a combination of the latter two types (Figure 7A).
Tuba. Coiling direction: regular coiling (type 1, Figure 8A) or distorted (Type 2, or 3) (Figure 8B and C). Tuba whorl length in proportion to spire last whorl: ca. 3/8–1 1/2. Proportion of tuba that attaches to spire: whole to none.
Aperture and peristome. Peristome: simple aperture without outer peristome (Figure 10B), or double peristome (Figure 10C and 10D). Shape of outer peristome (Figure 10A): same as inner peristome and uniformly round, or highly projected or slightly projected at either a particular side or at a several sides of anterior, poteriorior, left and right laterial (Figure 9 and 10A).
Spiral lines. Either thick or thin, or only thin lines present (Figure 12).
Radial ribs. Rib density: 4–32 per mm on the spire’s last whorl in right lateral view (Figure 13A). Intensity: thick or thin (Figure 13B). Shape: straight, slightly curved, distinctly curved, single humped, single looped or double looped and the shape remaining the same or changing between between the spire and the tuba (Figure 13C, but single-looped, and double-humped not shown). Inclination: from orthoclin to prosoclin.
Distribution and habitat
The distribution range of Plectostoma is about 4.6 million square kilometres within the extent limited by 11°N, 97°E and 5°S, 120°E. However probably less than 5% of this large area is covered by limestone outcrops where suitable habitat may exist for obligate karst taxa like Plectostoma. The genus counts 79 species and occurs in Vietnam (1 species), Thailand (1), Peninsular Malaysia (28), Sumatra (1), and Borneo (48). Peninsular Malaysia, Sumatra and Borneo are part of the biogeographical region called Sundaland (Johnson 1964). Plectostoma is found on most limestone hills. However, the genus is conspicuously absent on the limestone hills to the west of the central mountain ranges, such as the hills in the States of Perak and Kedah in Peninsular Malaysia, and in the northwestern half of Sumatra (Figure 18). No species have been recorded from the east coast of Sumatra, where hardly any limestone outcrops exist. Based on collection data and our field experience, there is a distinct ecological divergence between Plectostoma and Opisthostoma. This was already observed in the 19th century (Blanford and Blanford 1860, de Crespigny 1865, Blanford 1866), and also by Berry (1961). Plectostoma can only be found in limestone outcrops, where the rock face is its major habitat, although a few individuals can occasionally be found on vegetation debris below the limestone rock face. Opisthostoma, on the other hand, is a soil dweller, living in leaf litter on the forest floor. They are mostly but not exclusively found in forest over limestone bedrock (Schilthuizen et al. 2003b).
Our molecular phylogenetic analysis reveals that Plectostoma, Opisthostoma, and Arinia are phylogenetically closely related (Figure 16). It is important to point out that the phylogenetic relationships among Plectostoma, Opisthostoma (except Opisthostoma vermiculum), Opisthostoma vermiculum, and Arinia are unresolved. Figure 16 shows representative shell morphologies of the taxa that were included in the phylogenetic analysis, and it is clear that it is rather difficult to find shared derived characteristics (synapomorphies) in size, spire shape, or tuba coiling regime, for either Opisthostoma or Plectostoma.
Nonetheless, we treat Plectostoma and Opisthostoma as two separate genera based on their ecological divergence and differences in adult shell colouration. Similarly, we propose that Opisthostoma vermiculum and Arinia should be considered as two separate genera. However, this hypothesis needs further testing with more genetic data from Opisthostoma vermiculum Clements & Vermeulen, 2008 (in Clements et al. 2008), the conchologically similar Opisthostoma gittenbergeri Vermeulen & Clements, 2008 and further Arinia species.
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