Ranguna decourcyi ( Kemp, 1913 ) Ng & Yeo, 2023

Ng, Peter K. L. & Yeo, Darren C. J., 2023, Clarifying the identities of the Asian potamid genera Potamiscus Alcock, 1909, Ranguna Bott, 1966, and Dromothelphusa Naiyanetr, 1992 (Crustacea: Brachyura: Potamoidea), Raffles Bulletin of Zoology 71, pp. 632-657 : 645-647

Ranguna decourcyi ( Kemp, 1913) , new combination

( Fig. 7 )

Potamon (Potamiscus) decourcyi Kemp, 1913: 292 , pl. 17, figs. 1–3.

Potamiscus (Potamiscus) decourcyi – Bott, 1966: 480, fig. 13.

Potamiscus decourcyi – Bott, 1970: 159, pl. 38 fig. 29, pl. 51 fig. 27; Ghosh & Ghatak, 1999: 570; Brandis, 2000: 75, pl. 9a–c (partim); Yeo & Ng, 2007: 307; Ng et al., 2008: 165; Cumberlidge et al., 2009: supplementary table; Mitra, 2017: 24, 38, figs. 52–54, 72, 104–106 (partim).

Material examined. Holotype: male (63.0 × 48.0 mm) ( ZSIK 8006 /10), under stones in small hill stream, Sirpo Valley , near Renging, 1,000–1,500 ft asl, Arunachal Pradesh state, India, coll. W. R. de Courcy, March 1912.

Diagnosis. Carapace with regions behind epigastric and postorbital cristae tuberculate and granulate; frontal and postorbital regions relatively narrow ( Fig. 7A ); external orbital angle acutely triangular; epibranchial tooth sharp, distinct ( Fig. 7A ); anterolateral margins cristate, serrated, convex; branchial regions covered with granules, rugose ( Fig. 7A ). Male pleon narrowly triangular, appearing subrectangular in shape ( Fig. 7C ). G1 subterminal segment relatively stouter, terminal segment distinctly curved outwards, subconical, distally tapered, without dorsal flap, groove for G2 marginal ( Fig. 7D ). G2 much longer than G1; distal segment longer than half length of basal segment ( Fig. 7D ). Females not known.

Remarks. Kemp (1913) described the species from at least four specimens from and around the Sirpo Valley in Arunachal Pradesh state in India. The largest male 63.0 × 49.0 mm from Sirpo Valley was identified as the “type” of the species ( Kemp, 1913: 293, 294). This male, the holotype, was examined.

The characters of this species agree best with Ranguna as redefined here. While the combined frontal and postorbital region ( Fig. 7A ) is not as compressed as that of R. rangoonense , it is still clearly narrower than in Potamiscus . The features of the epistome and third maxilliped ischium ( Fig. 7B, C ) are also more like those of R. rangoonensis than P. annandalii . Most significantly, the shape of the male pleon ( Fig. 7C ) agrees very well with R. rangoonensis and argues for their congenericity. The G1 terminal and subterminal segments of R. decourcyi , new combination ( Fig. 7D ) are stouter than those of R. rangoonensis . Other morphological differences between these two species include: the male pleonal somite 6 is proportionately broader in R. decourcyi (less broad in R. rangoonensis ); the male telson has the lateral margins gently concave in R. decourcyi (strongly concave in R. rangoonensis ); the G1 terminal segment is more strongly curved with the structure gently curved downwards in R. decourcyi (less curved with a straight tip in R. rangoonensis ); and the flexible articulation between the G1 terminal and subterminal segments is larger in R. decourcyi (small in R. rangoonensis ) (S.K. Pati, pers. comm.). Importantly, these are all interspecific differences rather than intergeneric, and not distinct enough to exclude the species from Ranguna .

The specimens reported from Sibsagar, Assam by Brandis (2000) and from Mizoram by Mitra (2017) may not be R. decourcyi s. str. In these specimens, the epigastric and postorbital cristae appear more curved (in dorsal view) in Mitra’s specimens (2017: fig. 52), the branchial regions of both being more strongly granulated ( Brandis, 2000: fig. pl. 9a; Mitra, 2017: fig. 52), the median lobe of the posterior margin of the epistome of their material is more acutely triangular ( Mitra, 2017: fig. 53), the male pleon is somewhat more triangular ( Mitra, 2017: fig. 54), and the G1 terminal segment appears to be relatively more slender and straighter ( Brandis, 2000: pl. 9b, c; Mitra, 2017: figs. 104, 105). A re-examination of those material should be done to see if they are indeed conspecific.

Distribution. Northeastern India: Arunachal Pradesh,? Assam, and? Mizoram.

Figures

Fig. 7. Ranguna decourcyi (Kemp, 1913), holotype male (63.0 × 48.0 mm) (ZSIK 8006/10), Sirpo Valley, India. A, overall dorsal view; B, frontal view of cephalothorax; C, thoracic sternum, pleonal somites 4–6 and telson; D, sternopleonal cavity showing G1 and G2 in situ.