Potamiscus chizami, Pati, 2021

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 : 3-10

Potamiscus chizami sp. nov.

( Figs. 2A–F , 3A–D , 4A–C )

Zoobank: urn:lsid:zoobank.org:act:0EB4C575-8D19-4025-B80F-94973E782B48

Type material. Holotype: adult male ( CW 31.22 mm, CL 24.19mm,CH 17.97 mm, FW 8.63 mm), Chizami, Phek District , Nagaland, India (25.600°N 94.394°E), altitude 1424 m a.s.l., 8 July 2017, coll. Rukuto ( ZSI-WRC C.1950) . Paratypes: adult male ( CW 29.10 mm, CL 22.67 mm, CH 16.82 mm, FW 8.96 mm) and adult female ( CW 27.71 mm, CL 21.26 mm, CH 16.01 mm,FW 8.21 mm),same data as holotype ( ZSI-WRC C.1951); adult male ( CW 27.71 mm, CL 21.35 mm, CH 15.68 mm, FW 8.00 mm) and adult female ( CW 27.81 mm, CL 21.27 mm, CH 16.29 mm, FW 8.20 mm), same data as holotype ( ZSI-WRC C.1952) .

Additional material. 2 males ( CW 27.89–28.35 mm, CL 21.45–21.93 mm, CH 15.92–15.97 mm, FW 8.37–8.57 mm) and 3 females ( CW 24.75–25.90 mm, CL 18.96–19.82 mm, CH 14.10–14.52 mm, FW 7.61– 7.84 mm), same data as holotype ( ZSI-WRC C.1953) .

Diagnosis. Carapace small ( CW <32 mm), subovate, broad ( CW /CL = 1.3), deep ( CH / CW = 0.6); dorsal surface convex in frontal view; anterolateral margins cristate; frontal margin broad (FW/CW = 0.3); epigastric cristae distinct, anterior to postorbital cristae, separated from latter by long, distinct groove; postorbital cristae distinct, sharp, not confluent with epibranchial tooth; external orbital angle triangular, with relatively short outer margin, approximately 1.5 times length of inner margin; epibranchial tooth distinct, with distinct cleft; cervical grooves indiscernible; suborbital margin confluent with supraorbital margin ( Figs. 2A–C , 4A, B ). Third maxilliped exopod with vestigial f lagellum ( Fig. 2D ). Chelipeds generally smooth, asymmetrical, with broad inner distal major tooth on carpus ( Figs. 2A–C , 4A, B ). Ambulatory legs relatively stout (P3 merus approximately 3.0 times long as broad), short, generally glabrous ( Figs. 2A–C , 4A, B ). Suture between male S2/S3 distinct as moderately deep, narrow groove, reaching lateral margins; suture between male S3/ S4 indiscernible ( Fig. 2C, E ). Male sternopleonal cavity relatively long, reaching to imaginary line joining medial part of cheliped coxae ( Fig. 2C, E ). Male pleon broad, triangular, with strongly concave lateral margins; somites 1, 2 equally broad; somite 6 trapezoidal, distinctly broad, with convex lateral margins ( Fig. 2C, F ). Male telson as long as pleonal somite 6, with strongly concave lateral margins ( Fig. 2C, F ). G1 relatively slender, with terminal segment bent outwards at angle of about 30° from longitudinal axis, tip narrow, reaching beyond pleonal locking tubercle, not touching suture S4/S 5 in situ; flexible zone small; terminal segment sinuous, slender, subconical, long, approximately 0.4 times combined length of flexible zone and subterminal segment, distal third narrow, gently curved, dorsal flap distinct but low, broadly rounded; subterminal segment gently sinuous ( Figs. 2E , 3A–C ). G2 distinctly longer than G1, approximately 1.3 times length of G1; distal segment long, approximately 0.7 times length of basal segment ( Fig. 3C, D ). Vulvae on S6 relatively closely located, suborbicular, large, occupying two-thirds length of S6, anterior margin touching suture S5/ S6 with mesial end clearly away from suture S4/S5 ( Fig. 4C ).

Description of male holotype. Carapace transversely subovate, broader than long ( CW /CL = 1.3), deep ( CH /CW = 0.6); dorsal surface convex in frontal view, glabrous, smooth except for epigastric cristae, postorbital cristae and epibranchial region; anterolateral surface gently inflated in frontal view; anterolateral margins convex, granular, distinctly emarginated at distal quarter, raised, separated from anterolateral surface by shallow groove; posterolateral margins converging posteriorly, joining with gently concave posterior margin; front trapezoidal, anterior portion strongly def lexed; frontal margin gently concave medially, broad (FW/CW = 0.3); epigastric cristae distinct, generally smooth, anterior to postorbital cristae, separated from latter by long, distinct groove; postorbital cristae distinct, sharp, not conf luent with epibranchial tooth; external orbital angle triangular, with gently convex, short outer margin, approximately 1.5 times length of inner margin; epibranchial tooth low, blunt, positioned above level of postorbital cristae, cleft distinct; postorbital region concave; branchial regions inflated, smooth except for strongly granular epibranchial region; cervical grooves barely visible; mesogastric groove deep, narrow, long, extending into frontal region, bifurcated posteriorly; H-shaped groove visible; subhepatic region rugose; suborbital region generally smooth, glabrous; suborbital margin concave, cristate, smooth, joining with supraorbital margin; pterygostomial region smooth except for anteriorly located low granules; frontal medial triangle incomplete, with dorsal margin only, lateral margins indiscernible; epistome posterior margin with distinct, triangular medial tooth and strongly sinuous lateral margins ( Fig. 2A–C ).

Eyes smaller than orbital space; eye stalk short, narrow; cornea large, pigmented ( Fig. 2B ).

Antennules long, folded in longitudinally broad fossae; antennae very short, reaching slightly beyond base of eye stalk ( Fig. 2B ). Mandibular palp 3-segmented; terminal segment simple, undivided. First, second maxillipeds each with long flagellum on exopod. Third maxillipeds cover most of buccal cavity when closed; ischium subrectangular, longer than broad, with narrow, deep, oblique medial groove; merus subpentagonal, broader than long, depressed; exopod slender, distally narrow, longer than ischium, reaching proximal third of merus, with vestigial f lagellum, approximately 0.25 times merus width ( Fig. 2B–D ).

Chelipeds generally smooth, asymmetrical, right chela larger ( Fig. 2A–C ). Major chela with 3 or 4 large, blunt teeth on each finger, small gape when fingers closed; dactylus smooth, gently curved, slender, longer than upper margin of palm, proximally with 4 distinct to low dorsal granules; palm slightly longer than high, with widely spaced, distinct but low granules on upper- and lower surfaces; carpus slightly rugose, gently inflated, with stout, sharp, triangular inner distal major tooth and low, moderately sharp sub-basal tooth; merus rugose on outer surface, lacking subterminal spine ( Fig. 2A–C ).

Ambulatory legs moderately stout (P3 merus approximately 3 times long as broad), short, P3 longest, generally glabrous; merus (P2–P5) long, lacking subdistal spine; propodus (P2–P5)moderately stout; dactylus (P2–P5) gently recurved, slightly longer than propodus, with short, sharp chitinous spines on margins ( Fig. 2A–C ).

Thoracic sternites punctate, glabrous; S1 and S2 completely fused; suture S2/S3 distinct as moderately deep, narrow groove, reaching lateral margins; suture S3/S4 indiscernible; suture S4/S5, S5/S6, S6/S7 shallow, narrow, medially indiscernible; suture S7/S8 laterally indiscernible, only visible towards sternopleonal cavity but interrupted by longitudinal groove between S7 and S8, lacking transverse ridge ( Fig. 2C, E ). Pleonal locking mechanism with prominent tubercle on submedial part of S5 ( Fig. 2E ). Sternopleonal cavity deep, long, reaching to imaginary line joining medial part of cheliped coxae ( Fig. 2C, E ).

Pleon broad, triangular, with strongly concave lateral margins; somites 1, 2 almost rectangular, narrower than somite 3; somites 3–5 trapezoidal, with distally converging lateral margins; somite 6 trapezoidal, broader than long (proximal width approximately 1.8 times medial length), distinctly longer than preceding somites, equal in length to telson, with convex lateral margins ( Fig. 2C, F ). Telson triangular, broader than long (proximal width approximately 1.4 times medial length), with strongly concave lateral margins, apex narrow, round ( Fig. 2C, F ).

G1 moderately stout, with terminal segment bent outwards at angle of about 30° from longitudinal axis, tip narrow, reaching beyond pleonal locking tubercle, not touching suture S4/S 5 in situ; f lexible zone small; terminal segment sinuous, slender, subconical, long, approximately 0.4 times combined length of flexible zone and subterminal segment, distal third narrow, gently curved, dorsal f lap distinct but low, broadly rounded; subterminal segment gently sinuous, distal third narrow, broad at base ( Figs. 2E , 3A–C ). G2 distinctly longer than G1, approximately 1.3 times length of G1; distal segment sinuous, subcylindrical, long, approximately0.7 times length of basal segment; basal segment stout at proximal third,appearing ovate ( Fig. 3C, D ).

Paratypes. The male paratypes ( ZSI-WRC C.1951, 1952) resemble the holotype in all the carapace and gonopod features. The female paratypes ( ZSI-WRC C.1951, 1952) also share most of the non-sexual character states with the holotype. The pleon of the female paratypes is narrowly ovate, which covers the thoracic sternum except for S1, S2, and lateral edges when closed ( Fig. 4B ). In the female paratypes, pleonal somite 1 is the shortest; pleonal somites 2–5 are progressively longer; and pleonal somite 6 is the longest, much broader than long, almost equal in length to the telson, with gently convex lateral margins ( Fig. 4B ). The female telson is broadly ovate, much broader than long, with gently convex lateral margins and round apex ( Fig. 4B ). The vulvae on S6 are close to each other ( VD /SW = approximately 0.1), suborbicular, open laterally, large, occupying two-thirds the length of S6, deep, anterior margin touching the suture S5/S6 with the mesial end clearly away from the suture S4/S5, and laterally partially covered by a protruding sternal cover ( Fig. 4C ).

Etymology. The species is named after the type locality, Chizami, a large village in the Phek District of Nagaland State, India. Used as a noun in apposition.

Remarks. Potamiscus chizami sp. nov. most resembles three Indian species ( Po. mima sp. nov., Po. palelensis , and Po. tumidulus ) and a species from Myanmar ( Po. whitteni ) than any other congeners due to its slender, subconical, sinuous, gently outwardly curved, and long G1 terminal segment (approximately 0.4 times the combined length of the flexible zone and subterminal segment), with a distinct but low dorsal f lap ( Fig. 3A–C, E–G ; cf. Mitra and Waikhom, 2019: figs. 2A, B, E, F, 4A–C for Po. palelensis ; Ng et al., 2020: fig. 3D for Po. whitteni ; personal observation for Po. tumidulus ). Although these five species have a similar looking G1 terminal segment, Po. chizami sp. nov., Po. mima sp. nov., Po. palelensis , and Po. whitteni can still be differentiated from Po. tumidulus due to their relatively slender G1 ( Fig. 3A, C, E, G ; cf. Mitra and Waikhom, 2019: figs. 2A, E, 4A, B; Ng et al., 2020: fig. 3D) against the relatively stout G1 of Po. tumidulus (personal observation). Furthermore, the G1 tip reaches the pleonal locking tubercle in Po. chizami sp. nov. and Po. mima sp. nov. ( Fig. 2E, K ), whereas it never reaches the pleonal locking tubercle in Po. tumidulus (personal observation).

In carapace morphology, Po. chizami sp. nov., Po. mima sp. nov., Po. palelensis , and Po. whitteni are very distinct from Po.tumidulus in the following characters: the postorbital crista is relatively sharp ( Figs. 2A, G , 4A, D ; cf. Mitra and Waikhom, 2019: fig. 1A; Ng et al., 2020: fig. 1A–C) (vs. rugose postorbital crista in Po. tumidulus ; personal observation); the external orbital angle is triangular, with short outer margin, approximately 1–2 times the length of the inner margin ( Figs. 2A, G , 4A, D ; cf. Mitra and Waikhom, 2019: fig. 1A; Ng et al., 2020: fig. 1A–C) (vs. broadly triangular external orbital angle, with long outer margin, approximately 3 times the length of the inner margin in Po. tumidulus ; personal observation); the epibranchial tooth is distinct, with the cleft separating it from the external orbital angle being distinct ( Figs. 2A, G , 4A, D ; cf. Mitra and Waikhom, 2019: fig. 1A; Ng et al., 2020: fig. 1A–C) (vs. indistinct epibranchial tooth and cleft in Po.tumidulus ; personal observation); the suture between male S3/S4 is either indiscernible or only visible as a shallow depression ( Fig. 2C, E, I, K ; cf. Mitra and Waikhom, 2019: fig. 1C; Ng et al., 2020: fig. 2A, B) (vs. distinct and deep suture between male S3/S 4 in Po. tumidulus ; personal observation); and the male sternopleonal cavity is relatively long, reaching to the imaginary line joining the medial part of the cheliped coxae( Fig.2C, E,I, K ; cf. Mitra and Waikhom, 2019: fig. 1C; Ng et al., 2020: fig. 2A, B) (vs. relatively short male sternopleonal cavity, reaching only to the imaginary line joining the posterior part of the cheliped coxae in Po.tumidulus ; personal observation).

Although Po. chizami sp. nov. is similar to Po. palelensis in carapace morphology and G1 structure, it is nevertheless distinguished from Po. palelensis mainly by its indiscernible cervical grooves ( Figs. 2A , 4A ) (vs. relatively distinct cervical grooves; cf. Mitra and Waikhom, 2019: fig. 1A), the relatively stout ambulatory legs, P3 merus approximately 3.0 times long as broad ( Figs. 2A , 4A ) (vs. relatively slender ambulatory legs, P3 merus approximately 3.7 times long as broad; cf. Mitra and Waikhom, 2019: fig. 1A), the gently curved distal third of the G1 terminal segment ( Fig. 3A, B ) (vs. distinctly curved distal third of the G1 terminal segment; cf. Mitra and Waikhom, 2019: figs. 2A, B, 4A, C), and the relatively stouter G1 subterminal segment ( Fig. 3A ) (vs. relatively slender G1 subterminal segment; cf. Mitra and Waikhom, 2019: figs. 2A, 4A).

The similarities in carapace and G1 structures notwithstanding, Po. chizami sp. nov. is distinguished from Po. mima sp. nov. by its distinctly convex dorsal surface of the carapace ( Fig. 2B ) [vs. gently convex dorsal surface ( Fig. 2H )], the distinct groove between epigastric- and postorbital cristae ( Figs. 2A , 4A ) [vs. barely visible groove between epigastric- and postorbital cristae ( Figs. 2G , 4D )], the confluent suborbital- and supraorbital margins ( Fig. 2B ) [vs. separation of the suborbital margin from the supraorbital margin by the external orbital angle ( Fig. 2H )], the relatively broad male pleon with the strongly concave lateral margins ( Fig. 2C, F ) [vs. relatively narrow male pleon with the gently concave lateral margins ( Fig. 2I, L )], equally broad male pleonal somites 1, 2 ( Fig. 2F ) [vs. broader male pleonal somite 2 than somite 1 ( Fig. 2L )], the convex lateral margins of the male pleonal somite 6 ( Fig. 2C, F ) [vs. straight lateral margins ( Fig. 2I, L )], the smaller f lexible zone on G1 ( Fig. 3A, B ) [vs. larger f lexible zone on G1 ( Fig. 3E, F )], the relatively more stout G1 terminal segment ( Fig. 3A–C ) [vs. relatively slender G1 terminal segment ( Fig. 3E–G )], the relatively longer G2, approximately 1.3 times the G1 length ( Fig. 3C, D ) [vs. relatively shorter G2, approximately 1.1 times the G1 length ( Fig. 3G, H )], and the suborbicular vulvae, with the mesial end of the vulval anterior margin clearly away from the suture S4/S5 ( Fig. 4C ) [vs. ovate vulvae, with the mesial end of the vulval anterior margin reaching close to the suture S4/S5 ( Fig. 4F )]. Moreover, on the basis of the material available, adult Po. chizami sp. nov. appears to be a smaller species ( CW <32 mm) than Po. mima sp. nov. ( CW <53 mm).

Although Po. chizami sp. nov. shares many features of the carapace and G1 with the congener from Myanmar, Po. whitteni , the new species is easily separated from Po. whitteni by its indiscernible cervical grooves ( Figs. 2A , 4A ) (vs. relatively distinct cervical grooves; cf. Ng et al., 2020: fig. 1A–C), the strongly convex lateral margins of the male pleonal somite 6 ( Fig. 2C, F ) (vs. gently convex lateral margins of the male pleonal somite 6; cf. Ng et al., 2020: figs. 2A, B, 3A), the strongly concave lateral margins of the male telson ( Fig. 2C, F ) (vs. gently concave to almost straight lateral margins of the male telson; cf. Ng et al., 2020: figs. 2A, B, 3A), and the relatively stouter G1 terminal segment, with straight tip ( Fig. 3A–C ) (vs. relatively slender G1terminal segment, with upcurved tip; cf. Ng et al., 2020: fig. 3C–F).

Geographical distribution. Potamiscus chizami sp. nov. is known only from Chizami village in Phek District of Nagaland, northeastern India.

Figures

Figure 2. Potamiscus chizami sp. nov., holotype male (31.22 × 24.19 mm) (ZSI-WRC C.1950) (A–F); Potamiscus mima sp. nov., holotype male (42.65 × 33.82 mm) (ZSI-WRC C.1954) (G–L). A, G, overall dorsal view; B, H, overall frontal view; C, I, overall ventral view; D, J, left third maxilliped; E, K, thoracic sternites with G1 in situ (left G1 removed); F, L, pleonal somites 1–6 and telson. Scale bars = 10 mm (A–C, G–I), 5 mm (E, F, K, L), 2 mm (D,J).

Figure 3. Potamiscus chizami sp. nov., holotype male (31.22 × 24.19 mm) (ZSI-WRC C.1950) (A–D); Potamiscus mima sp. nov., holotype male (42.65 × 33.82 mm) (ZSI-WRC C.1954) (E–H). A, E, dorsal view of left G1; B, F, dorsal view of left G1 terminal segment; C, G, ventral view of left G1; D, H, left G2. Scale bars = 2 mm (A, C–E, G, H), 1 mm (B, F).

Figure 4. Potamiscus chizami sp. nov., paratype female (27.71 × 21.26 mm) (ZSI-WRC C.1951) (A–C); Potamiscus mima sp. nov., paratype female (43.04 × 33.27 mm) (ZSI-WRC C.1955) (D–F). A, D, overall dorsal view; B, E, overall ventralview; C, F, thoracic sternites showing vulvae. Scale bars = 10 mm (A, B, D–F), 5 mm (C).