Ranguna rangoonensis ( Rathbun, 1904 )

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 : 641-645

Ranguna rangoonensis ( Rathbun, 1904)

( Figs. 5 , 6 )

Potamon (Potamon) rangoonense Rathbun, 1904: 279 , pl. 11 fig. 2, fig. 18; Türkay & Naiyanetr, 1987: 391, figs. 1, 2.

? Potamon (Potamon) andersonianum rangoonense – Alcock, 1910b: 34, fig. 41.

Potamiscus (Ranguna) rangoonensis – Bott, 1966: 481, fig. 15 (partim).

Ranguna (Ranguna) rangoonensis – Bott, 1970: 163, pl. 38 fig. 35, pl. 47 fig. 31 (partim).

Potamiscus rangoonensis – Türkay & Naiyanetr, 1987: 391, figs. 1, 2; Yeo & Ng, 1999: 640; Brandis, 2000: 92, pl. 20a–c; Ng et al., 2008: 165; Cumberlidge et al., 2009: supplementary table.

Material examined. Holotype: male (65.0 × 49.8 mm) ( MCZ 5562 ), “ Rangoon ”, Burma, coll. W. Theobald, June 1865. Others: 2 males (larger 31.3 × 24.8 mm) ( MBA 952 a), “Unto Assam”, coll. and date unknown.

Diagnosis. Carapace with regions behind epigastric and postorbital cristae tuberculate to strongly rugose; frontal and postorbital regions very narrow ( Fig. 5A, B ); external orbital angle acutely triangular; epibranchial tooth sharp, distinct, separated by V-shaped cleft ( Fig. 5A ); anterolateral margins cristate, serrated, convex; branchial regions prominently tuberculate, rugose ( Fig. 5A ). Male pleon narrowly triangular, appearing subrectangular in shape ( Fig. 6A ). G1 subterminal segment relatively slender, terminal segment gently 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 ). Females not known.

Description of holotype male. Carapace broader than long, not high, dorsal surface almost flat in frontal view; epigastric cristae well developed, separated by distinct groove opening up into inverted V-shape posteriorly, 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; frontal and postorbital areas compressed, with these regions very narrow in dorsal view ( Fig. 5A, B ); regions behind epigastric and postorbital cristae very rough, tuberculate to strongly rugose; pterygostomial, sub-hepatic, suborbital and sub-branchial regions covered with low tubercles; cervical groove not distinct, obscured by tubercles; H-shaped gastric grooves distinct ( Fig. 5A, B ). Front deflexed downwards; frontal margin granulated, bilobed, with shallow wide median concavity ( Fig. 5A, B ). External orbital angle acutely triangular, sharp, outer margin slightly longer than inner margin; epibranchial tooth sharp, distinct, separated by strong V-shaped cleft; anterolateral carapace margins cristate, serrated, convex, not confluent with posterolateral carapace margins; posterolateral carapace margins converging posteriorly; posterior carapace margin gently convex ( Fig. 5A ); branchial regions prominently tuberculate, rugose; metabranchial region with distinct oblique striae ( Fig. 5A ). Orbits large, slightly oblique in position; supraorbital margin almost straight, cristate; eyes large, almost occupying orbit, peduncle short, stout; cornea large ( Fig. 5B ). Antennular fossae longitudinally narrow, slit-like ( Fig. 5B, C ); antenna small, flagellum not extending beyond orbit ( Fig. 5C ). Epistome posterior margin with triangular median tooth, outer part gently sinuous ( Fig. 5B, C ). Endostome smooth, no clear ridge visible.

Third maxilliped with surface mostly glabrous, inner margin lined with dense, short setae; merus subquadrate, subequal to half of ischium length, anteroexternal angle rounded; median part gently depressed; ischium relatively short, broadly rectangular, with shallow suboblique median sulcus; exopod slender, long, distal part exceeding distal edge of ischium, reaching one third length of merus, flagellum vestigial or absent ( Fig. 5D–G ).

Chelipeds stout, not elongate, right chela larger ( Fig. 5H, I ); outer surfaces with numerous tubercles and granules; margins of merus lined with rows of large rounded tubercles, without obvious subterminal spine, outer surface with tubercles and granules ( Fig. 5H ); carpus margins appear gently serrated, inner distal angle with large, subdistal spine and basal tubercle ( Fig. 5I ); fingers shorter than stout palm, dorsal and ventral margins appear serrated due to sharp tubercles; cutting edges of fingers with strong cutting teeth ( Fig. 5H, I ).

Ambulatory legs not elongate, stout, glabrous; merus subdistal spine weak, absent, upper margin weakly serrated; propodus relatively long; dactylus long, slender ( Fig. 5J–M ; Rathbun, 1904: pl. 11 fig. 2; Brandis, 2000: pl. 20a).

Thoracic sternum and associated characters not known (structures crushed in holotype).

Male pleon narrowly triangular, appearing subrectangular in shape, all somites and telson free; somites 3 and 4 trapezoidal, lateral margins almost straight to gently concave; somite 5 subtrapezoidal, appearing almost rectangular; somite 6 long, ca. 0.54 times maximum width, lateral margins gently convex; telson triangular with strongly concave lateral margins, as long as somite 6 ( Fig. 6A ).

G1 sinuous, slender; terminal and subterminal segments clearly demarcated; subterminal segment relatively slender, not neck-like distally, without shelf on upper part of outer margin; terminal segment gently curved outwards, subconical, distally tapered, relatively short, less than half length of subterminal segment, with very low dorsal flap, without longitudinal torque, without swelling on inner margin, groove for G2 marginal ( Fig. 6B, C ; Türkay & Naiyanetr, 1987: fig. 2a, b; Brandis, 2000: pl. 20b, c). G2 longer than G1; distal segment longer than half length of basal segment; basal segment relatively wide, with outer margin convex ( Fig. 6D ).

Variation. Females not known. The two MBA males examined here agree best with the holotype male and we refer them to this species for the time being. The left third maxilliped of the holotype male is deformed, and appears to have regrown after damage ( Fig. 5E, F ). The exopods of both sides are intact; the left one has no trace of flagellum (may have broken off) ( Fig. 5F ) and the right one has a vestigial flagellum, reaching only a quarter of the width of the merus ( Fig. 5G ). In the MBA male, the flagellum is only vestigial ( Fig. 6F ). The right G2 of the holotype male is somewhat deformed and is shorter than the left one, and appears to have been previously damaged ( Fig. 6E ). The male sternum of the intact MBA specimens can be described as follows: suture between anterior thoracic sternites 2 and 3 distinct, complete, straight; groove or suture between anterior thoracic sternites 3 and 4 not discernible; posterior thoracic sternites 5 and 6 medially interrupted; thoracic sternite 7 completely separated by longitudinal median line; thoracic sternite 8 completely separated by longitudinal median line, lacking transverse ridge; sternopleonal cavity not reaching or barely reaching imaginary line joining posterior edge of cheliped bases.

Remarks. Türkay & Naiyanetr (1987) commented only on the condition of the G1 of Ranguna rangoonensis , omitting a detailed redescription of the holotype, including the condition of the taxonomically important third maxilliped exopod flagellum, which was also not reported on by Rathbun (1904). The present diagnosis is based primarily on the re-examined holotype. The non-type specimens examined in the present study (two males, larger 31.3 × 24.8 mm, MBA 952a) were labelled as “ Potamon (Potamiscus) decourcyi ” but more closely match the Ranguna rangoonensis holotype in both external and G1 morphology and have a vestigial flagellum on the third maxilliped exopod ( Fig. 5D–G ); hence they are referred instead to Ranguna rangoonensis , but only provisionally, because the locality of these specimens, “Unto Assam ”, is questionable as it is far from the type locality, “ Rangoon ”. It is important to note here that neither of these MBA specimens (MBA 952a from Unto Assam) is the one that Bott (1966, 1970) had examined (MBA 951a from Naga Hills). All that being said, we also have doubts about the actual type locality of “ Rangoon ” (today Yangon) (see paragraph below).

The type male of P. rangoonensis was obtained by M. W. Theobald in June 1865, a well-known geologist who made many important botanical and zoological collections from many parts of Myanmar. In 1865, he was tasked to conduct surveys in the Pegu mountain range in central Myanmar (cf. Oldham, 1865), and it was likely the specimen was collected from there at that time rather than from Rangoon .

Alcock (1910b) examined two lots of non-type material in the ZSIK: one from Tonghoo (= Taungoo or Toungoo), Burma (coll. W. Theobald); and another without any location (coll. Captain Butler); and concluded that R. rangoonensis was merely a subspecies of Potamon andersonianum ( Wood-Mason, 1871) , noting only three minor differences in carapace morphology between his specimens and the type specimen of P. andersonianum . Alcock (1910b) listed five male and three female specimens, but none were illustrated, and although a figure number “41” was stated, no such figure can be found in Alcock (1910b). The holotype of R. rangoonensis , however, is clearly distinguished externally from P. andersonianum , as figured by Alcock (1910b: fig. 40) by its more acutely triangular external orbital angle (versus broadly triangular); distinct cleft between external orbital angle and epibranchial tooth (versus indistinct cleft); and relatively more strongly developed epibranchial tooth (versus weak epibranchial tooth) (see Rathbun, 1904: pl. 11 fig. 2; Türkay & Naiyanetr, 1987: fig. 1). In addition, the groove for the G 2 in the G1 terminal segment of R. rangoonensis is approximately marginal, whereas it is distinctly ventral in P. andersonianum and its allies. Potamon andersonianum is currently in Indochinamon Yeo & Ng, 2007 .

As discussed in the remarks for the genus, the records of “ Potamiscus (Ranguna) rangoonensis ” and “ Ranguna (Ranguna) rangoonensis ” by Bott (1966, 1970) are confusing and probably a mixture of taxa, including Potamon beieri s. str. and the type.

Distribution. “ Rangoon ” ( Rathbun, 1904);? Assam (present study).

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. 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.