Stelomon pruinosum ( Alcock, 1909 )

Yeo, D. C. J. & Naiyanetr, P., 2000, A new genus of freshwater crab (Crustacea: Decapoda: Brachyura: Potamidae) from Thailand, with a description of a new species, Journal of Natural History 34 (8), pp. 1625-1637 : 1630-1634

Stelomon pruinosum ( Alcock, 1909)

(®gures 3±5)

Potamon pruinosum Alcock, 1909: 246 ; Alcock, 1910: 50, ®gure 8.

Ranguna (Ranguna) rangoonensis: Bott, 1970: 163 (part) (not Potamon (Potamon) rangoonensis Rathbun, 1904 ).

Material examined

LECTOTYPE: male (27.4 Ö 21.2 mm) (ZSI 5531/10), Hills between Burma and Siam, coll. and date unknown.

PARALECTOTYPE: female (31.2 Ö 23.4 mm) (ZSI 5531/10), same data as lectotype .

Other. Male (40.5 Ö 30.4 mm) ( ZRC 1998.1145 ), Ban Krang, Huai Mae Phraeng, Kaeng Krachan National Park , Phetchaburi Province, Thailand, coll. T. Bundhitwongrut, 28 September 1998 .

Diagnosis

Third maxilliped exopod with well-developed ¯agellum, slightly longer than merus width. Male abdomen triangular; telson triangular, with distinctively concave lateral margins, tip rounded. G1 terminal segment relatively long, c. 0.6 times length of subterminal segment, stout, c. 3.9 times longer than broad, twisted along longitudinal axis, with groove for G2 clearly visible from ventral view, with distinct swelling on median part of inner margin, narrowed distal part straight, gently tapered, tip rounded with more or less ventral distal opening. G2 distal segment c. 0.6 times length of basal segment.

Remarks

In the original description of this species, Alcock (1909) did not provide any other information about the material examined, except for the locality, which was stated only as`Hills between Burma and Siam’( Alcock, 1909: 246). Alcock (1910) subsequently listed three lots of specimens in the Indian Museum under this species including one from the above-mentione d location consisting of an immature male and female specimen (ZSI 5531/10). These two specimens, which were examined, are the de facto syntypes of the species. The male specimen (27.4 Ö 21.2 mm) (ZSI 5531/10) is hereby designated as the lectotype of Stelomon pruinosum ( Alcock, 1909) . The remaining two lots of specimens mentioned by Alcock (1910), which were collected from`Tavoy’, are not types as they were not mentioned in the original publication and could not be located for examination in the ZSI. Another specimen from Tavoy, a juvenile male (NHM 1909.9.2.1), presented to the Natural History Museum, London, by the ZSI, was previously examined by the second author but this proved inconclusive as the G1 was not fully developed (TuÈrkay and Naiyanetr, 1987: 391).

It is a policy of the ZSI not to allow G1s of crab specimens to be detached for examination, therefore, we can only provide a photograph of the lectotype’s G1s in situ (®gures 3C, D), while the present drawings and diagnosis of the G1 of S. pruinosum are based on a non-type adult male specimen instead (®gures 5B±E). This specimen (40.5 Ö 30.4 mm) ( ZRC 1998.1145 ) was recently collected from Kaeng Krachan National Park in Phetchaburi Province, Thailand, and matches the lectotype very well in external morphology and most aspects of the G1 structure. The only diOEerence between the smaller lectotype and the non-type specimen is the G1 being slightly more slender in the former (®gures 3C, D, 5B±E). This is likely to be due to size-related variation and does not have any interspeci®c signi®cance .

Bott (1970) synonymized Potamon pruinosum under Potamon (Potamon) rangoonensis Rathbun, 1904 (as a Ranguna ), without explanation and without indicating whether he examined the types of either species. However, the specimen collected from Assam and deposited in the Forschungsinstitut Senckenberg (SMF 2807), which was ®gured by Bott (1970: pl. 38 ®gure 35, pl. 47 ®gure 31), is clearly not P. (P.) rangoonensi s sensu stricto. Bott’s specimen is not even congeneric with P. (P.) rangoonensis as its carapace is distinctly less rugose; the external orbital angle is broadly triangular (versus acutely triangular); and the G1 is very diOEerent in form (cf. Rathbun, 1904: pl. 11 ®gure 2; TuÈrkay and Naiyanetr, 1987: 391, ®gures 1, 2). In any case, Bott’s (1970) synonymy was not valid, as Stelomon pruinosum can be immediately separated from both Bott’ s specimen as well as P. (P.) rangoonensis , by its more broadly triangular male abdomen (®gures 3B, 5H; cf. Bott, 1970: 1970: pl. 47 ®gure 31; Rathbun, 1904: ®gure 18) and very diOEerent G1 structure (®gures 3C, D, 5B±E; cf. Bott, 1970: pl. 38 ®gure 35; TuÈrkay and Naiyanetr, 1987: 391, ®gure 2).

Externally, Stelomon pruinosum can be separated from S. kanchanaburiens e and S. tharnlod sp. nov. by its relatively broader triangular male abdomen (®gures 3B, 5H, 7H; cf. Ng and Naiyanetr, 1993: ®gure 1C). It is also easily separated from S. kanchanaburiens e by the distal part of its G1 terminal segment being upright (versus distally hooked terminal segment) and the ventral facing G1 distal opening (versus lateral facing distal opening) (®gures 2B±H, 5B±E). A less obvious diOEerence is seen in the ¯agellum of the third maxilliped exopod of S. pruinosum , which is slightly longer than the merus width, while that of S. kanchanaburiens e never exceeds the merus width (®gures 2A, 5A). DiOEerences in the G1 form between S. pruinosum and S. tharnlod are discussed under the Remarks for the latter species (see later). The G1 terminal segment (especially the gently tapered distal part) of S. pruinosum resembles that ®gured by Bott (1970: pl. 39 ®gure 38) for Potamon (Potamon) turgidulimana Alcock, 1910 , which may prove to be a fourth Stelomon species (see Remarks under genus). However, there are two other notable diOEerences in the G1 subterminal segment of S. pruinosum (when viewed from all possible orientations) and the species ®gured by Bott (1970), namely: (i) the inner margin lacking a distal hump (versus the inner margin with a distinct distal hump present); and (ii) the outer margin distinctly sloping outwards and being gently convex (versus the outer margin not sloping outwards and being gently concave) (®gure 5B; cf. Bott 1970: pl. 39 ®gure 38).

In the live specimen, the dorsal carapace was very dark purple with whitish margins (frontal and anterolateral), cristae (epibranchial and postorbital) and rugae. The chelipeds were light purple with yellowish ®nger tips, and were covered with whitish granules and rugae. The ambulatory legs were light purple with slightly yellowish joints and bright yellow dactyli.