Ranguna Bott, 1966

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 : 640-641

Genus Ranguna Bott, 1966

Potamiscus (Ranguna) Bott, 1966: 481 (partim).

Ranguna – Bott, 1970: 162 (partim); Chuensri, 1974: 23 (partim).

Potamiscus – Brandis, 2000: 60 (partim); Ng et al., 2008: 165 (partim).

Diagnosis. Carapace broader than long, not high, dorsal surface almost flat in frontal view; epigastric cristae well developed, anterior to postorbital cristae, more or less confluent with postorbital cristae, hardly separated by short, indistinct groove; postorbital cristae strong, not clearly confluent with epibranchial tooth; regions behind epigastric and postorbital cristae tuberculate to strongly rugose; frontal and postorbital regions narrow to very narrow ( Figs. 5A, B , 7A ); external orbital angle acutely triangular, outer margin slightly longer than inner margin; epibranchial tooth sharp, distinct, separated by V-shaped cleft ( Figs. 5A , 7A ); anterolateral margins cristate, serrated, convex; posterolateral margins converging posteriorly; branchial regions tuberculate, rugose ( Figs. 5A , 7A ). Antennular fossae longitudinally narrow, slit-like ( Figs. 5B, C , 7B ). Epistome posterior margin with triangular median tooth, outer parts gently sinuous ( Figs. 5B, C , 7B ). Ischium of third maxilliped relatively short, broadly rectangular, with shallow suboblique median sulcus; exopod long, exceeding distal edge of ischium, with flagellum vestigial or absent ( Figs. 5D–G , 6F , 7B, C ). Ambulatory legs not elongate, stout, with long, slender dactyli ( Figs. 5J–M , 7A ; Brandis, 2000: pl. 20a). Anterior male thoracic sternites relatively narrow transversely; suture between anterior male thoracic sternites 2 and 3 distinct, complete; no obvious suture between anterior male thoracic sternites 3 and 4; male sternopleonal cavity just reaching imaginary line joining posterior edge of cheliped bases; tubercle of male press-button locking mechanism small, on posterior third of thoracic sternite 5 ( Fig. 7C, D ). Male pleon narrowly triangular, appearing subrectangular in shape ( Fig. 6A ). G1 subterminal segment relatively slender, terminal segment relatively less curved outwards, subconical, distally tapered, with very low dorsal flap, groove for G2 marginal ( Fig. 6B, C, G–K ). G2 longer than G1; distal segment longer than half length of basal segment; basal segment relatively wide, with outer margin convex ( Fig. 6D, L ). Females not known.

Remarks. Bott (1966) established Ranguna (type species Potamon (Potamon) rangoonense Rathbun, 1904 ), as a subgenus of Potamiscus . The problem with this action, however, was that Bott (1966) designated Potamon (Potamon) rangoonense Rathbun, 1904 , as the type species of Ranguna without examining the type specimen of the species. Specimens Bott (1966) had with him and which he identified to this species originated from “Oberburma” (SMF 2807) and “Naga Hills an der Grenze von Burma und Assam” (MBA 951a), which he later changed to “Assam” and “Naga Hills”, respectively (see Bott, 1970). It was only when Türkay & Naiyanetr (1987) finally re-examined the male holotype of P. (P.) rangoonense (65.0 × 49.8 mm) (MCZ 5562) that they found its G1 to be completely different from those of Bott’s SMF and MBA material, and that it actually more closely resembled Potamon beieri Pretzmann, 1966a . Türkay & Naiyanetr (1987: 391) showed that P. (P.) rangoonense s. str. did not possess the characters for Ranguna stated by Bott (1966, 1970). Nevertheless, since Rathbun’s species is the type for Ranguna , they noted that “[t]aking into account a number of other pleopod characters it is clear that the present species [ Potamon rangoonense ] belongs to Potamiscus … ” and Ranguna is just a junior subjective synonym of Potamiscus . Türkay & Naiyanetr (1989) proposed to the International Commission for Zoological Nomenclature (ICZN) that the type species for Ranguna Bott, 1966 , be changed to Thelphusa longipes A. Milne-Edwards, 1869 , to retain the concept Bott (1966, 1970) had envisaged, but this was not approved ( ICZN, 1991) (see Holthuis, 1990; Ng, 1990). Naiyanetr (1992) subsequently proposed a new subgenus name, Demanietta (Dromothelphusa) , for Thelphusa longipes .

As for the identity of Bott’s (1966, 1970) “ Ranguna rangoonensis ” specimen from “Oberburma” (SMF 2807), Türkay & Naiyanetr (1987) noted that the only difference between the G1s of the P. beieri holotype (30.0 × 23.5 mm) [Sukli, Dawane Hills] and the SMF 2807 specimen (52.0 × 43.0 mm) figured by Bott (1970: pl. 38 fig. 35) was the “… larger angle between the stem and endpiece of beieri ”, which they attributed to size-related variation. The holotype of Potamon beieri in the NHM was examined by the first author and it is quite different from the SMF specimen illustrated by Bott (1970) as well as the MBA specimens on hand, specifically in the shape of the G1 terminal segment with its abruptly narrowed tip. It is important to note here that the original gonopod figured by Pretzmann (1966b: fig. 12) for Potamon beieri is actually wrong and belongs to another specimen from another location, the actual G1 terminal segment of the species being almost straight, tubular, with a sharp gently curved tip without any trace of a dorsal flap. The holotype of Potamon beieri also differs externally from Bott’s specimen in its relatively less broadly triangular external orbital angle, with straight outer margin (versus external orbital angle distinctly more broadly triangular, with distinctly convex outer margin); less distinct cleft separating external orbital angle from epibranchial tooth (versus cleft more distinct); relatively less strongly convex anterolateral margin (versus anterolateral margin relatively more strongly convex); and narrower pleon (versus pleon relatively less narrow). It is clear therefore that the actual Potamon beieri is a different taxon altogether; the taxonomy of this and allied species is currently under study by S.K. Pati and the authors.

Our re-examination of the type male of Potamon rangoonense (type species of Ranguna ) and a pair of MBA specimens (not specimen MBA 951a examined by Bott (1966, 1970)), which we believe to be conspecific, shows that there are enough morphological differences with Potamiscus annandalii (type species of Potamiscus ) to treat them as separate genera. A revised definition is therefore provided here based on a suite of external (carapace and male pleon) and G1 characters. Ranguna can be separated from Potamiscus s. str. by the following characters: carapace relatively lower (versus carapace relatively higher); carapace dorsal surface relatively flat, distinctly tuberculate and granulose (versus carapace dorsal surface gently convex, almost smooth); epigastric and postorbital cristae rugose (versus epigastric and postorbital cristae sharp); epibranchial tooth prominent (versus epibranchial tooth low); male pleon narrowly triangular (versus male pleon broadly triangular); G1 terminal segment relatively longer, more slender (versus G1 terminal segment relatively shorter, stouter); and G1 subterminal segment relatively more slender (versus G1 subterminal segment relatively stouter) (see Rathbun, 1904: pl. 11 fig. 2, fig. 18; Türkay & Naiyanetr, 1987: figs. 1, 2; Dai, 1999: figs. 99, 100, pl. 12 figs. 2, 3; Dai & Cai, 1998: figs. 1, 3; unpublished data).

As rediagnosed here, Ranguna contains only two species for now: R. rangoonensis ( Rathbun, 1904) , and R. decourcyi ( Kemp, 1913) , new combination.

Distribution. Probably northern Myanmar, and northeastern India.

Figures

Fig. 5. Ranguna rangoonensis (Rathbun, 1904), holotype male (65.0 × 49.8 mm) (MCZ 5562), Burma. A, dorsal view of carapace; B, frontal view of cephalothorax; C, epistome and antennules; D, right third maxilliped (outer view); E, left third maxilliped (deformed, outer view); F, left third maxilliped (deformed, inner view); G, right third maxilliped (inner view); H, left chela (outer view); I, right chela (outer view); J–M, left first to fourth ambulatory legs, respectively. Photographs courtesy of MCZ. Scales: A–C, H, I = 10.0 mm; D–G = 5.0 mm; J–M = 2.0 mm.

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.

Fig. 6. Ranguna rangoonensis (Rathbun, 1904). A–E, holotype male (65.0 × 49.8 mm) (MCZ 5562), “Rangoon”; F–L, male (25.9 × 20.8 mm) (MBA 952a), “Unto Assam”. A, pleon and telson; B, G, right G1 (ventral view); C, left G1 (ventral view); D, left G2; E, L, right G2 (deformed?); F, left third maxilliped; H, right G1 (dorsal view); I, distal part of right G1 (ventral view); J, K, distal part of right G1 (dorsal views in two different angles). A–D courtesy of MCZ. Scales: A = 10.0 mm; B–E = 5.0 mm; F–H, L = 2.0 mm; I–K = 0.5 mm.