Stelomon, Yeo & Naiyanetr, 2000

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 : 1626-1627

Stelomon gen. nov.

Potamon: Ng and Naiyanetr, 1993: 6 (part) (not Potamon Savigny, 1816 ).

Type species. Potamon kanchanaburiens e Naiyanetr, 1992, by present designation.

Diagnosis

Carapace distinctly broader than long, dorsal surface relatively ¯at. Epigastric cristae well-developed, rugose, almost level with postorbital cristae, separated from postorbital cristae by short, narrow groove; postorbital cristae rugose. External orbital angle triangular; epibranchial tooth distinct, triangular; anterolateral margins strongly convex; posterolateral margins strongly convergent posteriorly; branchial region granulose to rugose; metabranchial region with oblique striae. Ischium of third maxilliped rectangular; exopod exceeding distal margin of ischium, with welldeveloped ¯agellum. Ambulatory legs normal, not elongate. Suture between thoracic sternites 3 and 4 not visible. Male abdomen triangular. G1 stout, slightly sinuous; terminal segment long, stout, twisted along longitudinal axis, with groove for G2 clearly visible from ventral view, subdistal part appearing pectinated, surrounded by short, stiOE setae. G2 distal segment greater than half length of basal segment.

Remarks

Stelomon kanchanaburiens e (Naiyanetr, 1992), belonged to one of the four groups recognized by Ng and Naiyanetr (1993: 6) among the Thai species of Potamon . While retaining it in the genus Potamon, Ng and Naiyanetr (1993: 8) also commented on the unusual nature of its G1, adding that,`... it might perhaps be necessary in the future to erect a new genus for P. kanchanaburiens e ’. Indeed the form of its G1 is so unique among potamids that its exclusion from Potamon and placement into a separate genus, Stelomon , is necessary. This is reinforced by the discovery of two other species, one undescribed, sharing the same genus diagnostic characters as S. kanchanaburiens e (see later).

Externally, members of Stelomon gen. nov. resemble a number of Thai Potamon species. They can, however, be immediately separated from Potamon sensu stricto as well as almost all other Thai potamid genera by the very stout and relatively long G1 terminal segment, which constricts distally and is twisted along the longitudinal axis (versus slender and relatively shorter G1 terminal segment, which usually gently tapers distally and is not longitudinally twisted) (cf. Naiyanetr and Ng, 1990; Ng and Naiyanetr, 1993). Tomaculamon Yeo and Ng, 1997 [north-western Thailand] also shows longitudinal torque in the terminal segment of the G1. Stelomon , however, can easily be diOEerentiated from Tomaculamon by the G2 distal segment being slender and tapering, lacking any distal projection (versus G2 distal segment cylindrical and stout in appearance with an acute conical distal projection) (cf. Yeo and Ng, 1997).

The G1 form of Stelomon also appears super®cially similar to that of certain Malayopotamon species [Java, Sumatra], e.g. M. brevimarginatum (De Man, 1892) , but the two genera can still be diOEerentiated by the position of the groove for the G2 on the terminal segment of the G1 (ventral in Stelomon versus dorsal or marginal in Malayopotamon ) ( Ng and Naiyanetr, 1993: 8). The G1 of Stelomon species also resembles that of Stoliczia chaseni ( Roux, 1934) [Peninsular Malaysia] but the latter species does not have a ¯agellum on the third maxilliped exopod, and their carapace features diOEer considerably ( Ng and Naiyanetr, 1993: 8; Ng, 1988b: 64).

Stelomon currently consists of three species, namely, S. kanchanaburiens e (Naiyanetr, 1992), S. pruinosum ( Alcock, 1909) , and S. tharnlod sp. nov. Potamon (Potamon) turgidulimana Alcock, 1910 , reported by Bott (1970: pl. 39 ®gure 38, pl. 48 ®gure 38) [as a species of Ranguna ] is probably a fourth species of Stelomon . The G1 ®gured by Bott (1970: pl. 39 ®gure 38) appears to be of the Stelomon type; however, we cannot be sure as the photograph is not very clear. Bott (1970) stated that the specimen (collected from`Upper Tenasserim’) was deposited in the Natural History Museum, London, but we have not been able to ®nd it there and therefore cannot con®rm its generic placement as yet.

Etymology

The genus name is derived from the Latin word, stela, meaning pillar or column, in arbitrary combination with the genus name Potamon , alluding to the proportionatel y long and stocky G1 terminal segment characteristic of this genus. Gender neuter.

Distribution

Stelomon gen. nov. is so far known only from Tavoy, Myanmar (5 Burma) and`hills between Burma and Siam’ ( Alcock, 1909, 1910); Kanchanaburi and Phetchaburi provinces in western Thailand.