Georissa niahensis Godwin-Austen, 1889

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

Georissa niahensis Godwin-Austen, 1889

Georissa niahensis Godwin-Austen, 1889: 353; Thompson and Dance 1983: 119.

Type locality.

Niah Hills, Borneo. (Unspecified)

Type material.

Lectotype (Designated by Thompson and Dance 1983). Niah Hills, Borneo: NHMUK 1889.12.7.69 (glued on paper) (seen). Paralectotype. Niah Hills, Borneo: NHMUK 1889.12.7.70 (glued on paper) (seen).

Other material.

Painted Cave, Niah National Park, Niah, Sarawak (03°48.68'N, 113°47.25'E): MZU/MOL 17.25.

Description.

Protoconch. Color: red. Sculpture pattern: smooth and meshed - ellipsoid to irregular mesh shape. Mesh width: 12-19 µm. Teleoconch. Color: orange to red. First whorl: curved above the shoulder, flat and cylindrical below the shoulder. Subsequent whorls: convex, angular at the periphery. SH: 1.81-2.53 mm, SW: 1.51-1.99 mm, SI: 1.10-1.29. Total number of whorls: 3-3 ¼. Shell sculpture. Radial sculpture: absent, only strong and unevenly layered growth lines. Spiral sculpture: present, strongly sculpted, continuous to discontinuous, well defined from the first whorl all the way to the peristome. Scales: a single spiral series of low and minute acute scales, regularly spaced at the first whorl, but weaker, grading to imperceptible on the body whorl. Aperture. Shape: rounded. Basal side: rounded, angular at the columellar region. Parietal side: straight to curved. AH: 0.85-1.24 mm, AW: 0.92-1.27 mm, AI: 0.83-1.02.

Cross diagnosis.

Georissa niahensis has a distinctive single series of small scales on the whorl shoulder, close to the suture. G. niahensis is one of the largest Bornean Georissa , in shell size only matched by G. hadra (which, however, is more slender, angular at the shoulder and has a flat to slightly rounded whorls). In general shell shape, G. niahensis is closest to G. kobelti , but the latter species is more rounded, while G. niahensis has a distinctly convex periphery.

Distribution.

Known to occur only at Niah, Sarawak.

Molecular analysis.

ML and Bayesian analyses of G. niahensis (16S: n = 8; CO1: n = 7) showed that all G. niahensis specimens form one clade with 100% BS and 100% PP. The sister group is the G. kobelti population from Baram ( G. kobelti is paraphyletic).

Discussion.

Both Godwin-Austen (1889) and Thompson and Dance (1983) did not mention anything about the small scale-like nodules close to the suture of G. niahensis . Godwin-Austen (1889): "Shell elongately conoid, solid, imperforate; sculpture a very in-distinct, ill-defined spiral liration, about 20 on the penultimate whorl, upon a rough surface crossed by transverse lines of growth; color ruddy ochre; spire high; apex pointed, finely papillated, minutely lirate; suture impressed; whorls 4 ½ convex; aperture oval, oblique; peristome simple, acute below; columellar margin straight". Thompson and Dance (1983): " G. niahensis is similar in sculpture to G. williamsi but is much larger. G. niahensis also shows similarities to the hosei group in the depth of the suture and the relatively rapid expanding whorls, but it lacks the node-like sculpture found among species of that group." The scales are relatively small which are not very conspicuous among the strong growth lines, and this is the reason why in the previous description of the species the scale characters were lacking. Thompson and Dance (1983) compared G. niahensis with what they call the hosei group, based on the size and the deeply impressed suture.