Potamiscus Alcock, 1909

Pati, Sameer Kumar, 2021, Two new species of freshwater crabs of the genus Potamiscus Alcock, 1909 (Brachyura: Potamidae) from Nagaland, northeastern India, Nauplius (e 2021006) 29, pp. 1-14 : 2-3

Genus Potamiscus Alcock, 1909

Type species. Potamon (Potamiscus) annandali Alcock, 1909 , by original designation; gender masculine.

Remarks. Potamiscus is so far known by 18 species from China (Guangxi, Yunnan, and Tibet), Myanmar, and northeastern India: Potamiscus annandali (Alcock, 1909) (type species); Potamiscus cangyuanensis Dai, 1999 ; Potamiscus crassus Naruse, Chia and Zhou, 2018 ; Potamiscus decourcyi (Kemp, 1913) ; Potamiscus elaphrius Dai, Chen, Liu, Luo, Yi, Liu, Gu and Liu, 1990 ; Potamiscus fumariatus Naruse, Chia and Zhou, 2018 ; Potamiscus loshingensis (Wu,1934) ; Potamiscus montosus Tai , Song, He, Cao, Xu and Zhong, 1975; Potamiscus motuoensis Dai, 1990 ; Potamiscus palelensis Mitra and Waikhom,2019 ; Potamiscus pealianus (Wood-Mason, 1871) ; Potamiscus rangoonensis (Rathbun, 1904) ; Potamiscus rongjingensis Dai, Chen, Liu, Luo, Yi, Liu, Gu and Liu, 1990 ; Potamiscus tumidulus (Alcock, 1909) ; Potamiscus whitteni Ng, Hla Htoo and Win Mar, 2020 ; Potamiscus yiwuensis Dai and Cai, 1998 ; Potamiscus yongshengensis Dai and Chen, 1985 ; and Potamiscus yunnanensis (Kemp, 1923) ( Ng et al., 2008; 2020; Naruse et al., 2018; Mitra and Waikhom, 2019).

While the monophyly of Potamiscus was previously challenged (cf. Shih et al., 2009; Chu et al., 2017; Zhang et al., 2020), the genus is currently defined morphologically by its moderately high carapace with a rugose dorsal surface, completely absent or vestigial flagellum on the third maxilliped exopod, subconical G1 with distally tapering tip, absence of a dorsal flap on the G1 terminal segment, and marginal position of the groove for the G2 on the G1 terminal segment (cf. Naruse et al., 2018). The two new species of Potamiscus described here generally agree with the present definition of the genus (sensu Naruse et al., 2018) except for their distinct, but low, dorsal flap on the G1 terminal segment. The dorsal flap is also distinct in Po. crassus , Po. tumidulus , and Po. whitteni (cf. Naruse et al., 2018; Ng et al., 2020; personal observation), whereas it is only slightly developed in Po. palelensis , Po. pealianus , and Po. rangoonensis (cf. Bott, 1970; Türkay and Naiyanetr, 1987; Mitra and Waikhom, 2019). The remaining species do not possess a dorsal flap on the G1 terminal segment.

It is worth mentioning that existing and the present new species of Potamiscus can be separated into the following distinct groups based on their G1 terminal segment. “Group I” consists of Po. annandali (type species), Po. cangyuanensis , Po. decourcyi , Po.montosus , Po. rangoonensis , and Po. yiwuensis ; all have a slender, conical or subconical, and short G1 terminal segment, i.e., approximately 0.2–0.3 times the length of the subterminal segment (cf. Bott, 1970; Türkay and Naiyanetr,1987; Dai and Cai,1998; Dai, 1999;personal observation). “Group II” comprises Po.chizami sp.nov., Po. mima sp. nov., Po. palelensis , Po. tumidulus , and Po. whitteni ; all share a slender, conical or subconical, and relatively long G1 terminal segment, i.e., approximately 0.4–0.45 times the length of the subterminal segment (cf. Mitra and Waikhom, 2019; Ng et al., 2020; present study). Potamiscus crassus , Po. yongshengensis , and Po. yunnanensis constitute “Group III” due to their stout, conical or subconical, and relatively long terminal segment of the G1, which is about 0.5–0.6 times the length of the subterminal segment (cf. Dai, 1999; Naruse et al., 2018). The species in “Group IV” possess a stout, cylindrical or subcylindrical, and relatively long G1 terminal segment, measuring about 0.4–0.6 times the length of the subterminal segment and are represented by Po. elaphrius , Po. fumariatus , Po. loshingensis , Po. motuoensis , and Po. rongjingensis (cf. Dai, 1999; Naruse et al., 2018). Potamiscus pealianus has a slender, cylindrical, and relatively long G1 terminal segment (i.e., approximately 0.45 times the length of the subterminal segment) (cf. Bott, 1970), which seems to be a transitional state to the characters shown by Groups I–IV.

Comparative material. Potamiscus annandali : lectotype male (33.0 × 25.0 mm), Nemotha , Cachar district, Assam, India, collection date unknown, coll. J. Wood-Mason ( ZSIK 6602-3 /9). Potamiscus decourcyi : holotype male (63.0 × 49.0 mm), Sirpo Valley , near Renging, East Siang district, Arunachal Pradesh, India, March 1912, coll. M.W. R . de Courcy ( ZSIK8006 /10). Potamiscus palelensis : holotype male (29.5 × 24.2 mm), Palel , Kakching district, Manipur, India, 24 September 2017, coll. M.D. Waikhom ( ZSIK C.6466/2). Potamiscus tumidulus : syntype male (17.9 × 14.7 mm), Sikkim, India, collection date unknown, coll. J. Wood-Mason ( ZSIK 5507 /10) .