Potamiscus annandali

Potamiscus annandali (Alcock, 1909)

Material examined

Lectotype INDIA – Assam State • ♂, CW 33.0 mm, CL 25.0 mm; Cachar District: Nemotha; [25.029° N, 92.948° E]; J. Wood-Mason leg.; ZSIK 6602-3/9.

Remarks

Gurumon gen. nov. certainely belongs to Potamiscinae (sensu Yeo & Ng 2004) because the transverse ridge on s7/s8 is absent (Fig. 2C–D). The medial portion of the s8, however, is conspicuously narrow so that the longitudinal medial groove is indiscernible (Fig. 2C–D). The indistinct, low external orbital angle (Figs 1A, 2A, E, H, 3A), the relatively stouter exopod of the third maxilliped (Figs 1C, 3B), and the relatively broader male pleonal somite 6 (proximal width ca 3 × the medial length) (Figs 1C, 3C–D) are characteristic to Gurumon gen. nov. by which it can be distinguished from the morphologically related Potamiscus loshingensis, Potamiscus rongjingensis, Abormon, and Pararanguna. The external orbital angle is distinct and triangular, the exopod of the third maxilliped is relatively slenderer, and the male pleonal somite 6 is relatively narrower (proximal width ca 2.0–2.5 × the medial length) in Potamiscus loshingensis, Potamiscus rongjingensis, Abormon, and Pararanguna (see Wu 1934: fig. 1; Dai et al. 1990: pl. 1 fig. 2, fig. 2 (1, 8); Dai 1999: pl. 25 fig. 1, fig. 200 (1–2); Naruse et al. 2018: figs 20a, 21; Mitra et al. 2021: figs 2a–b, e, 6a–b, e).

In carapace morphology, Gurumon gen. nov. most resembles Abormon in that both genera have a transversely ovate carapace, the low epigastric cristae, the indiscernible postorbital cristae, a very low epibranchial tooth, barely visible cervical grooves, the deep s2/s3, a broad male pleon, and a tongueshaped male telson with the lateral margins concave (Figs 1A–C, 2A, C, E, H, 3A, C–D; see Mitra et al. 2021: figs 1a–c, 2a, d–e, 4a, 5a–c, 6a, d–e, 8a). In addition to the character states of the external orbital angle, third maxilliped exopod and male pleonal somite 6, Gurumon gen. nov. can be separated from Abormon by the relatively small flexible zone of the G1 (Figs 3E, 4A) (vs G1 flexible zone relatively large; see Mitra et al. 2021: figs 3a–b, 7a–b), the cylindrical G1 terminal segment with the dorsal flap absent (Figs 3E, 4A–B) (vs G1 terminal segment conical with the dorsal flap distinct but low; see Mitra et al. 2021: figs 3a–c, 7a–c), and the mesially open, subovate and relatively large vulvae (Fig. 2G) (vs vulvae anteriorly open, transversely ovate and relatively small; see Mitra et al. 2021: figs 4c, 8c). Although both genera are known from the mountains of the Arunachal Pradesh State (Fig. 5), Gurumon gen. nov. seems to be restricted to the elevated areas (2473–2513 m altitude), while Abormon dwells at relatively lower altitude (406–1240 m) (Mitra et al. 2021).

In G1 structure, Gurumon gen. nov. is quite similar to Potamiscus loshingensis and P. rongjingensis because all possess a stout G1 with the terminal segment being cylindrical, relatively long (ca 0.5–0.6 × the combined length of the flexible zone and the subterminal segment) and lacking a dorsal flap (Figs 3E, 4A–B; see Wu 1934: fig. 1; Dai et al. 1990: fig. 2(5); Dai 1999: figs 103 (5–6), 105 (4–5)). Despite their similar G1s, Gurumon gen. nov. differs from Potamiscus loshingensis and P. rongjingensis by the relatively small flexible zone of the G1 (Figs 3E, 4A) (vs G1 flexible zone relatively large; see Dai et al. 1990: fig. 2(5); Dai 1999: fig. 103 (5–6)), and the relatively less stout G1 terminal segment (Figs 3E, 4A, B) (vs G1 terminal segment relatively stouter; see Wu 1934: fig. 1; Dai et al. 1990: fig. 2(5); Dai 1999: figs 103 (5–6), 105 (4–5)). Other features of carapace, including those of the external orbital angle, third maxilliped exopod and male pleonal somite 6, however, confirm their separation. For instance, the epigastric cristae are low in Gurumon gen. nov. (Figs 1A, 2A, E, H, 3A) (vs epigastric cristae well-developed in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: pl. 1 fig. 2); the postorbital cristae are indiscernible in Gurumon gen. nov. (Figs 1A, 2A, E, H, 3A) (vs postorbital cristae relatively distinct in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: pl. 1 fig. 2); the external orbital angle is indistinct, low in Gurumon gen. nov. (Figs 1A, 2A, E, H, 3A) (vs external orbital angle distinct, triangular in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: pl. 1 fig. 2); the epibranchial tooth is very low in Gurumon gen. nov. (Figs 1A, 2A, E, H, 3A) (vs epibranchial tooth relatively distinct in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: pl. 1 fig. 2); the exopod of the third maxilliped is relatively stouter in Gurumon gen. nov. (Figs 1C, 3B) (vs third maxilliped exopod relatively slenderer in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: fig. 2 (1)); and the male pleon is relatively stouter, with the pleonal somite 6 broad, proximal width ca 3 × the medial length in Gurumon gen. nov. (Figs 1C, 3C–D) (vs male pleon relatively slenderer, with the pleonal somite 6 narrow, proximal width ca 2.0–2.1 × the medial length in Potamiscus loshingensis and P. rongjingensis; see Wu 1934: fig. 1; Dai et al. 1990: fig. 2 (8)). While the structure of the vulvae is not known in Potamiscus rongjingensis, P. loshingensis possesses the transversely ovate vulvae, which are close to each other (VD/SW = ca 0.1) (see Dai 1999: fig. 103 (9)) against the subovate and relatively widely located vulvae (VD/SW = ca 0.25) of Gurumon gen. nov. (Fig. 2G). Gurumon gen. nov. is found in the Arunachal Pradesh State of northeastern India, while Potamiscus loshingensis and P. rongjingensis are known only from Guangxi and/or Sichuan provinces of China (Wu 1934; Dai et al. 1990; Dai 1999) (Fig. 5). The disjunct geographical distributions with several mountain barriers further corroborate their separation.

Gurumon gen. nov. need not to be confused with Potamiscus s. str. (represented by the type species) because the new genus possesses the following characters in contrast to those of Potamiscus s. str.: the low epigastric cristae (Figs 1A, 2A, E, H, 3A) (vs epigastric cristae well-developed; see Yeo & Ng 2007: fig. 11a); the indiscernible postorbital cristae (Figs 1A, 2A, E, H, 3A) (vs postorbital cristae distinct, reaching each epibranchial tooth; see Yeo & Ng 2007: fig. 11a); the indistinct, low external orbital angle (Figs 1A, 2A, E, H, 3A) (vs external orbital angle distinct, triangular; see Yeo & Ng 2007: fig. 11a); the very low epibranchial tooth (Figs 1A, 2A, E, H, 3A) (vs epibranchial tooth distinct; see Yeo & Ng 2007: fig. 11a); the relatively stouter exopod of the third maxilliped (Figs 1C, 3B) (vs third maxilliped exopod relatively slenderer; see Alcock 1910: pl. III fig. 10b); the relatively stouter male pleon, with a relatively broad pleonal somite 6, proximal width ca 3 × the medial length (Figs 1C, 3C–D) (vs male pleon relatively slenderer, with a relatively narrow pleonal somite 6, proximal width ca 2.5 × the medial length; see Bott 1970: pl. 46 fig. 26); the tongue-shaped male telson, with the lateral margins concave (Figs 1C, 3C–D) (vs male telson triangular, with the straight lateral margins; see Bott 1970: pl. 46 fig. 26); and the relatively stouter and longer G1 (tip reaching up to s4/s 5 in situ), with a cylindrical, less strongly curved and long terminal segment, measuring ca 0.6 × the combined length of the flexible zone and the subterminal segment (Figs 2C, 3E, 4A–B) (vs G1 relatively slenderer and shorter (tip reaching slightly beyond s5/s6 up to the proximal third of s 5 in situ), with a conical, strongly bent and short terminal segment, ca 0.2 × the combined length of the flexible zone and the subterminal segment; see Bott 1970: pl. 38 fig. 28; unpublished data). Moreover, Potamiscus s. str. was originated from the hills of lower Assam (Alcock 1909), which is some 460 km away from the known range of Gurumon gen. nov. (Fig. 5).

As mentioned earlier, Gurumon gen. nov. is immediately distinguished from Pararanguna by the shapes of external orbital angle, third maxilliped exopod and male pleonal somite 6. The additional differences include the very low epibranchial tooth (Figs 1A, 2A, E, H, 3A), the distinctly concave lateral margins of the male telson (Figs 1C, 3C–D), the relatively small flexible zone of the G1 (Figs 3E, 4A), and the cylindrical G1 terminal segment that lacks a dorsal flap (Figs 3E, 4A–B) in Gurumon gen. nov. against the distinct epibranchial tooth (see Dai 1999: pl. 25 fig. 1; Naruse et al. 2018: fig. 20a), the almost straight lateral margins of the male telson (see Dai 1999: fig. 200 (2); Naruse et al. 2018: fig. 21), the relatively large flexible zone of the G1 (see Dai 1999: fig. 200 (5); Naruse et al. 2018: fig. 22b), and the conical G1 terminal segment with a distinct dorsal flap (see Dai 1999: fig. 200 (4–5); Naruse et al. 2018: fig. 22a–b) in Pararanguna. Pararanguna is known only from the Yunnan Province of southwestern China (Dai 1999; Naruse et al. 2018) (Fig. 5). The elevated mountains with deep valleys between Yunnan and Arunachal Pradesh justify the separation of Gurumon gen. nov. from Pararanguna.

Geographical distribution

Gurumon gen. nov. is currently known only from the Mehao Wildlife Sanctuary in the Lower Dibang Valley District of Arunachal Pradesh State, northeastern India (Fig. 5).