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Holotype: BOR 5649(1). Paratypes: BOR 5533(>10), BOR 5534(>10), BOR 5535(>10), V 8811(>25).
Shares with Plectostoma charasense, Plectostoma kitteli, and Plectostoma klongsangensis the general shell form, in terms of apex, spire, and tuba shape, but differs by having both thin and thick spiral lines, and the left lateral side of outer peristome projected not more than twice the width of the right lateral side of outer peristome.
This species is named after Dato’ Toh Chin Yaw, who was the Chairman of Industry, Trade and Environment Committee in the Terengganu State Government between 2008 and 2013. It is rare to find politicians working closely with the public for wildlife conservation. During his time in office, Dato’ Toh was tireless in his efforts to promote and preserve Terengganu’s natural heritage. One of his most influential decisions was getting the State to ban the hunting of threatened flying foxes (Pteropus vampyrus). Before he left office, he was working with scientists to gazette the Kenyir Wildlife Corridor as a protected area and was helping to secure funds to improve anti-poaching efforts in that area.
Apex. Shape: distinctly convex.
Spire. Height: 2.1–2.4 mm. Width: 1.3–1.5 mm. Number of whorls: 4 7/8–5 3/8. Apical spire shape: oblong conical. Basal spire shape: conical. Whorl periphery: distinctly convex. Umbilicus: open.
Constriction. Parietal teeth: two. 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. 7/8–8/8. Proportion of tuba that attaches to spire: less than 1/2.
Aperture and peristome. Peristome: double peristomes. Outer peristome shape: similar to inner peristome, projected all around, except the posterior part, where the two lateral sides are distinctly more projected than the anterior side.
Spiral lines. Thick lines: present. Thin lines: present.
Radial ribs. Rib density: 4–6 ribs per mm. Rib intensity: thick. Shape: slightly curved to single-humped. Inclination: prosoclin.
Type locality. Gua Bewah, Tasik Kenyir (4°51'3"N, 102°43'21"E).
Distribution range. To date, this species is only known to occur at two neighbouring limestone outcrops, namely, Gua Bewah and Gua Taat at the Southern part of Tasik Kenyir (Figure 18B).
Least concern. The only two limestone outcrops where this species was found are partially submerged in Southeast Asia’s largest man-made lake, Tasik Kenyir. Nonetheless, there is good forest cover around and on the limestone outcrops. Furthermore, these two hills are located in the Taman Negara Pahang, which is a prioritised protected area.
From a conchological point of view, this species is related to Plectostoma charasense. Both are thought to be affiliated with the Plectostoma species from North Borneo. Plectostoma tohchinyawi has a high conical spire, thick and projected radial ribs, and a long detached tuba. These are the typical characteristics for dozens of Plectostoma species in North Borneo. In addition, Plectostoma tohchinyawi and Plectostoma charasense live on the five lenticular limestone outcrops that are the easternmost outcrops of the Malay Peninsula, thus closest geographically to Borneo (Figure 18B).
- Liew, T; Vermeulen, J; Marzuki, M; Schilthuizen, M; 2014: A cybertaxonomic revision of the micro-landsnail genus Plectostoma Adam (Mollusca, Caenogastropoda, Diplommatinidae), from Peninsular Malaysia, Sumatra and Indochina ZooKeys, 393: 1-107. doi