Indawmon, Ng & Mar & Shih, 2020

Ng, Peter K. L., Mar, Win & Shih, Hsi-Te, 2020, Indawmon thanayei, a new genus and new species of freshwater crab (Crustacea: Brachyura: Potamidae) from Indawgyi Lake, Kachin State, Myanmar, Raffles Bulletin of Zoology 68, pp. 529-538 : 530

Indawmon , new genus

Type species. Indawmon thanayei , new species, by present designation and monotypy.

Diagnosis. Adult carapace transversely ovate; epigastric cristae anterior to postorbital cristae; postorbital cristae not reaching anterolateral margin; anterolateral margin cristate, lined with low granules; third maxilliped with slender exopod with long flagellum; male pleon broadly triangular; male thoracic sternites 3 and 4 longitudinally elongate, with sternopleonal cavity reaching only to imaginary line connecting proximal edge of coxae of chelipeds; press-button tubercle of male sternopleonal locking mechanism laterally flattened, distal part distinctly chitinised; G1 subterminal segment tapering gradually from broad base, distal part not prominently narrow; G1 terminal and subterminal segments not clearly demarcated; without visible suture or any feature separating segments in ventral view; barely visible area between segments slightly less chitinised; terminal segment subcylindrical, gradually tapering to sharp tip, bent at angle of about 45–90° from longitudinal axis; G2 longer than G1; vulvae large with chitinised dorsal rim.

Etymology. The name is derived from the name of the lake, Indawgyi, in arbitrary combination with the ending of “ Potamon ”. Gender neuter.

Remarks. The general carapace features of Indawmon , new genus, are typical of many potamid genera and have few diagnostic features. In Myanmar, the carapace shape, surface features, structure of the epigastric and postorbital cristae, and armature anterolateral margin of Indawmon resemble those of genera like Indochinamon Yeo & Ng, 2007 , and Shanphusa Yeo & Ng, 2007 . Their G1 structures, however, differ markedly.

One of the diagnostic features of Indawmon is that the terminal and subterminal segments are not well demarcated. In almost all potamiscines (and many potamines), the terminal and subterminal segments are distinctly separated by a suture, clearly visible in ventral view and indicated by sutures or less chitinised sections in dorsal view (see Yeo & Ng, 2007). In Indawmon , there is no clear suture separating the terminal and subterminal segments in ventral view ( Fig. 6B, F ). In dorsal view, there is an area between the two segments that appear to be less chitinised but this is only barely visible ( Fig. 6C, G ).

The G1 of Indawmon superficially resembles that of Inlethelphusa Yeo & Ng, 2007 , in that the terminal segment is elongate. Inlethelphusa is endemic to Lake Inle and adjacent areas (see Kemp, 1918; Yeo & Ng, 2007; Ng et al., 2020). Indawmon , however, differs from Inlethelphusa in having the anterolateral margin lined with low granules ( Figs. 2 , 3A–D ) (versus anterolateral margin lined with many sharp teeth in Inlethelphusa ; cf. Ng et al., 2020: fig. 6A, B); the G1 subterminal segment tapering gradually from the broad base, the distal part not being prominently narrow and not clearly demarcated from the subcylindrical terminal segment which ends in a sharp tip ( Fig. 6A–C, E–G ) (versus distal part of G1 subterminal segment narrows abruptly from broad base, narrow and clearly demarcated from cylindrical terminal segment which ends in a truncate tip in Inlethelphusa ; cf. Ng et al., 2020: fig. 6D, E); and male thoracic sternites 3 and 4 being longitudinally elongate, with the sternopleonal cavity reaching only to an imaginary line connecting the proximal edge of the coxae of the chelipeds ( Fig. 3E, G, H ) (versus male thoracic sternites 3 and 4 proportionately longitudinally shorter with sternopleonal cavity reaching to imaginary line connecting middle part of coxae of chelipeds in Inlethelphusa ; cf. Ng et al., 2020: fig. 6C). Also noteworthy is that the press-button tubercle in Indawmon is unusual in that it is laterally flattened, while in most potamiscines, it is rounded or peg-like (e.g., in Inlethelphusa, Ng et al., 2020 : fig. 4J). Compared to Inlethelphusa , the vulva of Indawmon is directed obliquely and anteriorly with a distinct posterior rim ( Fig. 5D ) (versus vulva directed more laterally with a large lateral rim; cf. Ng et al., 2020: fig. 7B). The genetic data also supports their separation (see Discussion below).

Figures

Fig. 6. Indawmon thanayei, new genus, new species. A–D, holotype male (27.8 × 22.9 mm) (ZRC 2019.1032); E–G, paratype male (27.6 × 21.9 mm) (ZRC 2019.1033); H, paratype male (19.7 × 16.5 mm) (ZRC 2019.1033). A, E, H, left G1 (ventral view); B, F, distal part of left G1 (ventral view); C, G, left G1 (dorsal view); D, left G2. Scales: A, D, E, H = 1.0 mm; B, C, F, G = 0.5 mm.

Fig. 2. Indawmon thanayei, new genus, new species. Dorsal habitus. A, holotype male (27.8 × 22.9 mm) (ZRC 2019.1032); B, paratype male (27.6 × 21.9 mm) (ZRC 2019.1033); C, paratype male (19.7 × 16.5 mm) (ZRC 2019.1033); D, paratype female (32.2 × 24.6 mm) (ZRC 2019.1033).

Fig. 3. Indawmon thanayei, new genus, new species. A, E, F, holotype male (27.8 × 22.9 mm) (ZRC 2019.1032); B, G, paratype male (27.6 × 21.9 mm) (ZRC 2019.1033); C, H, paratype male (19.7 × 16.5 mm) (ZRC 2019.1033). A–D, dorsal view of carapace; E, G, H, anterior thoracic sternum and pleon; F, anterior thoracic sternum and sternopleonal cavity.

Fig. 5. Indawmon thanayei, new genus, new species. A, B, holotype male (27.8 × 22.9 mm) (ZRC 2019.1032); C, D, paratype female (32.2 × 24.6 mm) (ZRC 2019.1033). A, male sternopleonal cavity and G1; B, penis; C, female pleon and thoracic sternum; D, female sternopleonal cavity and vulvae. Abbreviations: cx4 and cx5, coxae of third and fourth ambulatory legs, respectively; pe, penis; pt, pressbutton tubercle of pleonal locking mechanism; st4–st8, sternites 4–8, respectively; vu, vulvae.