Georissa pyrrhoderma Thompson & Dance, 1983

Khalik, Mohd Zacaery, Hendriks, Kasper, Vermeulen, Jaap J. & Schilthuizen, Menno, 2018, A molecular and conchological dissection of the " scaly " Georissa of Malaysian Borneo (Gastropoda, Neritimorpha, Hydrocenidae), ZooKeys 773, pp. 1-55: 1

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Georissa pyrrhoderma Thompson & Dance, 1983


Georissa pyrrhoderma Thompson & Dance, 1983 

Georissa pyrrhoderma  Thompson & Dance, 1983: 123, fig. 64.

Georissa pyrrhoderma  van Benthem-Jutting, in Beron 2015: 181.

Type locality.

Borneo, Sarawak, First Division, western side of Gunong Selabor, Semabang entrance to Lobang Batu Cave (00°55'N, 110°25'E).

Type material.

Holotype. Borneo, Sarawak, First Division, western side of Gunong Selabor, Semabang entrance to Lobang Batu Cave: UF36183 (not seen). Paratypes. Borneo, Sarawak, First Division, western side of Gunong Selabor, Semabang entrance to Lobang Batu Cave: UF 36184, UF 36185 (not seen).

Other materials.

Gunong Silabur, Serian, Sarawak, Malaysia (00°57.45'N, 110°30.20'E): MZU/MOL 17.09, MZU/MOL 17.10, MZU/MOL 17.11, MZU/MOL 17.12, MZU/MOL 17.13, MZU/MOL 17.14, MZU/MOL 17.15, MZU/MOL 17.16, MZU/MOL 17.17, MZU/MOL 17.18, MZU/MOL 17.19, MZU/MOL 17.20, MZU/MOL 17.21, MZU/MOL 17.22, MZU/MOL 17.23, MZU/MOL 17.24.


Protoconch. Color: red to brown. Sculpture pattern: smooth to meshed, with ellipsoid mesh shape. Mesh width: 11-26 µm. Teleoconch. Color: brown to red. First whorl: shouldered, slightly curved above the shoulder, flat, cylindrical below the shoulder. Subsequent whorls: initially shouldered, but soon grading into uniformly rounded and quickly expanding whorls, with a deeply impressed suture; number of whorls: 2 ¼ -2 ½. SH: 1.16-1.31 mm, SW: 1.12-1.20 mm, SI: 1.03-1.09. Shell sculpture. Radial sculpture: absent, only weak to strong growth lines are visible. Spiral sculpture: present, strong spiral sculpture. Scales: a single series of low, small and acute, unevenly spaced scales above the periphery, occasionally, in the vicinity of the aperture, subordinate series of minute scales accompany the major series. Aperture. Shape: rounded, tilted below the palatal side. Basal side: rounded, strongly angular at the columellar region. Parietal side: straight, palatal edge attached to the body whorl. AH: 0.58-0.63 mm, AW: 0.75-0.85 mm, AI: 0.73-0.81.

Cross diagnosis.

Georissa pyrrhoderma  has a shell habitus that is similar to G. kobelti  , G. hosei  , and G. sepulutensis  . The latter two species are high variable and are morphologically, especially in sculpture, closely related to G. pyrrhoderma  . Therefore, G. pyrrhoderma  is nearly indistinguishable from certain forms of these other species.


Only known from the type locality, Gunung Silabur, Serian, Sarawak, Malaysia.

Molecular analysis.

In the ML and Bayesian analyses, all G. pyrrhoderma  (16S: n = 28; CO1: n = 26) individuals group together in one clade with 99% BS and 100% PP. Its sister clade is G. scalinella  + G. kinabatanganensis  .


In the original description, Thompson and Dance (1983) did not compare G. pyrrhoderma  with members of their hosei  -group (which our molecular analyses show it belongs in). Instead, they considered it allied to G. borneensis  . Possibly this misalignment was caused by the fact that the type specimens appear to lack the series of scales that is present on most of the specimens we collected. Nonetheless, given the restricted range of collection localities at Gunung Silabur and the degree of variability in our material, we consider our and Thompson and Dance’s material as conspecific. The paratypes specimen NHMUK 1984005 (Semabang entrance to Lobang Batu Cave, W. side of Gunong Selabor, 1st. Div., Sarawak, Malaysia: seen) is G. silaburensis  .