Stelomon kanchanaburiens

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 : 1627-1628

Stelomon kanchanaburiens e (Naiyanetr, 1992)

(®gures 1, 2)

Ranguna kanchanaburiensi s: Naiyanetr, 1978a: 84 (nomen nudum); Naiyanetr, 1978b: 7 (nomen nudum); Naiyanetr, 1978c: 32, ®gure 3 (nomen nudum); Naiyanetr, 1980: 51 (nomen nudum); Ng, 1988a: 25 (nomen nudum).

Potamon kanchanaburiensi s: Naiyanetr, 1985: 260 (nomen nudum); Naiyanetr, 1988: 9, pl. 6 ®gure 5 (nomen nudum); Naiyanetr, 1992a: 2, ®gures 1A, B; Naiyanetr, 1992b: 49; Naiyanetr, 1998: 111; Ng and Naiyanetr, 1993: 7, ®gures 1, 34.

Material examined

HOLOTYPE: male (62.4 Ö 47.7 mm) ( RMNH D 42352), Sai Yok Noi waterfall, Sai Yok District , Kanchanaburi Province, Thailand, coll. P. Naiyanetr, 7 March 1976.

PARATYPES: one female (44.2 Ö 34.0 mm) ( ZRC 1991.1835 ), same data as holotype . one male, one female (male 55.1 Ö 41.8 mm) ( RMNH D 41616), same locality and collector as holotype, 19 July 1981 .

Others. Eight males, four females (largest male 59.5 Ö 44.3 mm) ( ZRC 1998.114 2), one male, one female (male 52.7 Ö 37.9 mm) ( ZRC 1995.43 7), 25 males, 35 females (largest male 61.7 Ö 44.2 mm) ( CUMZ), Thong Pha Phum District , Kanchanabur i Province, Thailand, coll. S. Panha and C. Ekavibhathai, 17 November 1991 . Two males (larger 42.8 Ö 33.0 mm) ( ZRC 1998.1143 ), two males (larger 51.9 Ö 39.3 mm) ( CUMZ), Ban Huay Ka Yeng, Thong Pha Phum District , Kanchanaburi Province, Thailand, coll. S. Phitsaksindhorn, October 1997 . Four males (largest 45.9 Ö 35.0 mm) ( ZRC 1998.114 4), Nam Chon waterfall, Si Sawat District , Kanchanaburi Province, Thailand, coll. P. Naiyanetr, 24 November 1990 .

Remarks

Stelomon kanchanaburiens e has been diagnosed and illustrated by Ng and Naiyanetr (1993: 7, ®gures 1, 34). We have only to add that in males, thoracic sternites 5 and 6 are medially interrupted, while sternites 7 and 8 are separated by distinct longitudinal median suture (®gure 2I). Part of the thoracic sternum of a female paratype (RMNH D 41616) (44.7 Ö 33.1 mm) is illustrated in ®gure 2J. The nomenclature of this species was clari®ed by Naiyanetr (1992a).

In the present study, we re-examined part of the type series, including the holotype, as well as a large series of specimens from other parts of Kanchanaburi Province, to document infra-speci®c variation. Externally, all the specimens are very similar except for noticeable variation in the third maxilliped exopod ¯agellum length. Ng and Naiyanetr (1993: 7, ®gure 34A) noted that the left third maxilliped ¯agellum was distinctly shorter than the merus width. The holotype appears to be unusual in this respect, as other specimens examined had ¯agella ranging from slightly shorter than to subequal to the width of the merus (e.g. ®gure 2A). Unfortunately, the ¯agellum on the right third maxilliped of the holotype is also broken.

The present specimens also show variation in the narrowed distal part of the G1 terminal segment. The left G1 of the holotype has a distinctly outwardly hooked distal part with a relatively sharp tip although its right G1 ®gured by Ng and Naiyanetr (1993: ®gures 34B±E) is not hooked but slightly bent distally with a rounded tip. This diOEerence is probably due to the distal part of the right G1 being worn down by abrasion possibly during mating. All the other specimens examined, including paratypes, have G1s with the distal parts hooked outwards to varying degrees and tips being sharp or rounded (®gures 2B±H). The degree of twist of the terminal segment also varies slightly among individuals but the distal opening remains more or less lateral-facing in position (®gures 2E±H). The distinctive hooked structure is present even in the smallest adult male specimen (35.2 Ö 26.4 mm) (ZRC 1998.114 2) and is, therefore, very diagnostic for S. kanchanaburiens e.