Pethia dikhuensis, Praveenraj & Knight & Moulitharan & Imchen, 2022

Pethia dikhuensis , new species

( Figures 1 View FIGURE 1 & 2 View FIGURE 2 )

Holotype. ZSI/ APRC / V /P-1961, 44.3 mm SL, male; India, Nagaland, Mokochung district, Dikhu River; 26°20’08”N, 94°23’20”E; Limaakum and team, 15 January 2022. GoogleMaps

Paratypes. ZSI/ APRC / V /P-1961, 10 ex., 38.9−45.6 mm SL ; CIARI /FF-70, 2 ex., 42.7 & 43.0 mm SL, cleared, stained and dissected for osteology, same data as holotype GoogleMaps

Diagnosis. Pethia dikhuensis is distinguished from all its congeners in the Ganges-Brahmaputra, Kaladan and the Chindwin-Irrawaddy drainage (except P. expletiforis , P. macrogramma , P. stoliczkana , P. rutila and P. tiantian ) in having the following combination of characters: lateral line complete with 22–23 pored scales up to tail-fin base; 3 rd and 4 th lateral-line scale with minute humeral spot, a large elliptical to rounded black blotch covering 16 th to 18 th (n =2) or 17 th to 19 th (n =11) lateral-line scales. Live specimens of males with reddish-green body above lateral line and reddish-orange below; dorsal, pectoral, pelvic, anal and caudal fins reddish-orange and, dorsal fin in both sexes with broad black submargin. Pethia dikhuensis can be distinguished from P. expletiforis in having a minute humeral spot (vs. absence), dorsal fin with a broad black submargin (vs. reddish-orange dorsal fin devoid of markings or bands), pelvic and anal fin with tinges of black (vs. absence), slender body (33.6–36.6 vs. 37.1–42.9% SL) and longer caudal peduncle (20.4–22.6 vs. 15.8–19.2); from P. macrogramma by the presence of broad black band on the interradial membrane submarginally (vs. a series of black oval blotches on middle of interradial membrane), greater number of circumpeduncular scales (14 vs. 12), fewer scale rows between lateral-line and anal fin (3½ vs. 4½), and a slender body (33.6–36.6 vs. 41.8–46.7% SL); from P. stoliczkana in having longer anal fin (15.9–18.0 vs. 12.2–13.4% SL), a minute black humeral spot on the 3 rd and 4 th lateral-line scales, prominent on the lower half of the scale and extending to one scale below the lateral line (vs. a prominent bar on lateral-line scales 3 and 4 extending to the scale above and below) and fewer scales between lateral-line and anal fin (3½ vs. 5½); from P. tiantian in having a minute humeral spot (vs. a dark vertical bar extending to the dorsal midline), and the last unbranched dorsal-fin ray strongly serrated with 14−19 serrae (vs. last unbranched dorsal fin ray slender and flexible with 6–11 short serrae); and from P. rutila in having a dorsal fin with reddish rays and black submargin in males (vs. dorsal fin tinged golden-yellow, with 2 longitudinal rows of black spots on inter-radial membrane posterior to last unbranched ray), reddish-orange caudal fin (vs. yellow); and greater number of circumpeduncular scales (14 vs. 12).

Additional characters distinguishing P. dikhuensis from its congeners occurring in the Western Ghats/southern peninsular India, Northeast India and Myanmar are provided in the discussion.

Description. Morphometric data for the holotype and 12 paratypes are provided in Table 1. A View TABLE 1 moderately deepbodied Pethia of adult size 38–45 mm SL; body slightly elongated, compressed laterally, its length 2.7–3.0 times its depth. Dorsal profile from tip of snout to occiput slightly arched, humped immediately posterior to occiput, rising evenly up to dorsal-fin origin, thereafter sloping ventrad towards caudal-fin base. Ventral profile curved up to pelvic-fin origin running almost straight towards anal-fin origin, sloping down sharply towards posterior end of anal-fin base, and almost straight or moderately curved up to hypural notch. Body depth greatest at dorsal-fin base. Head small, laterally compressed, length contained 3.5–3.8 times in SL and 1.2–1.5 times its depth. Eyes large, positioned slightly in anterior half of head, Mouth small, subterminal, rictus not reaching to vertical from anterior margin of orbit. Both lips thin, moderately fleshy, lower lip not interrupted. Barbels absent.

Dorsal fin with 4 unbranched rays (consisting of two supernumerary and one unbranched ray) and 8 branched rays, last unbranched ray stiff, proximal half smooth, strongly serrated with 14 (1), 15 (1), 16 (10), 18 (1) serrae distally ( Fig. 3A View FIGURE 3 ). Dorsal-fin origin slightly behind pelvic-fin origin, its distal margin slightly concave. Adpressed dorsal-fin rays reaching vertical through second branched anal-fin ray. Anal fin with 3 unbranched rays (consisting of two supernumerary and one unbranched ray), and 5 (13) branched rays, its distal margin straight with rounded corners.

Pectoral fin rounded, with i, 12 (3), 14* (7), 15 (1), 16 (2) rays, reaching almost one scales anterior to pelvicfin origin. Pelvic fin rounded, with 1 simple and 7 (12*) or 8 (1) branched rays. Adpressed pelvic fin reaching the anal opening in males, but not in females. Caudal fin forked, lower lobe slightly longer than upper lobe, 7 (1), 8 (1), or 10* (11) + 6 (1), 8 (1), or 9* (11) principal rays. Caudal fin procurrent rays: 3* (1), 4(4), 5(6), 6 (1), 8 (1) on upper lobe, and 4* (8), 5 (4), 6 (1) on lower lobe. Lateral line complete, with 21 (4), 22 (5), 23* (3), 24 (1) pored scales and 1 (7) non-pored scale on caudal-fin base, running straight, horizontally until anterior 3–4 scales, then descending due to intercalation of scale row above, sloping down in a smooth curve, thereafter ascending to median of caudal peduncle. Predorsal scales 7 (1), 8 (9) or 9* (3); prepelvic scales 10* (10) or 11 (3); circumpeduncular scales 14 (13). Scales between dorsal-fin origin and lateral line 4½ (13), between lateral line and anal-fin origin 3½ (13). A prepelvic axillary scale present, its exposed length one-third of pelvic-fin length. Caudal fin with six hypurals and one parhypural. The last three caudal vertebrae support the caudal fin. Epural free in region between pleurostyle and neural spine of preural centrum ( Fig. 3B View FIGURE 3 ). Total number of vertebrae 4+26, with 4+13 abdominal and 13 caudal vertebrae, including compound centrum. Predorsal vertebrae including the weberian apparatus 8. First dorsal pterygiophore inserted between 8 th and 9 th vertebrae. First anal pterygiophore inserted between 17 th and 18 th vertebrae.

Proximal tip of dentary flattened and exhibits a coronoid process with a blunt end, articulated with the maxilla; coronomeckelian bone well developed, shuttle shaped, a small notch on its posterodorsal side, its length almost equal to the length of the retroarticular bone; Meckel’s cartilage rod-shaped; anguloarticular pointed on dorsal side, posterodorsal side deep and concave at the base to articulate with quadrate bone ( Fig. 4A View FIGURE 4 ). Premaxilla length three times its depth, with a ventrally projected blunt tip and posterodorsal process stunted ( Fig. 4B View FIGURE 4 ). Palatine process of maxilla short, with plain posterolateral side ( Fig. 4C View FIGURE 4 ). Infraorbital (IO) bone five in series, IO1 well developed, pentagonal, perforated with minute foramen, orbital margin straight; IO2 broad with a thin tube like infraorbital sensory canal, length equals to IO4, perforated with a median foramen; IO3 broad, deep, ventral surface curved; IO4 well developed, slightly broader than second infraorbital; fifth infraorbital small; supraorbital large, well ossified, covering orbital cavity anterolaterally ( Fig. 4E View FIGURE 4 ). Gill rakers simple, 14 on medial margin of first ceratobranchial, one on angle and two on epibranchial. Lateral arm of fifth ceratobranchial curved, elongated, with three large foramina; the proximal half of fifth ceratobranchial with unicuspid pharyngeal teeth with slightly hooked tips, arranged in three rows, with formula 4,3,2 ( Fig. 4D View FIGURE 4 ).

Colouration in life. In males ( Fig 2A View FIGURE 2 ), dorsum and sides above lateral line, olive-green to reddish orange, silvery-red below lateral line, scales bordered with melanophores. Caudal peduncle reddish-orange. Dorsal, anal, pectoral and pelvic fins reddish-orange. Dorsal fin interradial membrane with broad black submargin and reddish distally, its rays translucently reddish; apical portion of supernumerary ray black; serrae reddish. Anal fin with tinges of black on unbranched rays. Cheek and operculum reddish-silvery with melanophores. A minute black humeral spot on the 3 rd and 4 th lateral line scale, prominent on lower half of the scale and extending to one scale below the lateral line, a large round to elliptical black blotch covering the 16 th to 18 th (2) or 17 th to 19 th (11*) lateralline scales. Sclera reddish-orange on anterior and posterior region, iridescent-green at dorsal half and silvery on ventral half. In females ( Fig. 2B View FIGURE 2 ), dorsum and sides above lateral-line olive green to silvery, lateral-line silvery below pigmented with melanophores. All fins hyaline, dorsal fin membrane with an inconspicuous black submargin, distal margin pale-yellow. Two supernumerary rays black apically, serrae orange distally. Cheek and gill cover silvery with melanophores. Sclera uniformly silvery with melanophores.

Color in preservative (70% ethanol): In males ( Fig. 1 A & B View FIGURE 1 ), dorsal and anal-fin rays hyaline, its interradial membrane black. Basal margin of the pectoral and pelvic fins hyaline, with a black tinge. Caudal fin hyaline with melanophore pigmentation on the rays towards basal margin. Nape and snout heavily pigmented with melanophores. In females ( Fig. 2C View FIGURE 2 ), sides of the body brown to beige. Abdomen greyish-beige to pale-yellow. Side of head and nape brown, interorbital region with melanophore pigmentation. Operculum and infraorbital 3 silvery. Dorsal, anal, pelvic and caudal fin hyaline. Prominent, black humeral and caudal spots.

Distribution. Pethia dikhuensis is currently known only from the type locality, Dikhu River, a tributary of the Brahmaputra, in Mokochung district, Nagaland, India.

Habitat. The type locality during the month of January 2022 had a water depth of 0.5 m, with numerous boulders and rocks ( Fig. 5 View FIGURE 5 ). Other species occurring with Pethia dikhuensis include: Balitora sp. , Amblyceps cerinum , Garra sp. , Mustura sp. , and Lepidocephalichthys sp.

Etymology. The species is named after its type locality, the Dikhu River.

Genetic analysis. Pethia dikhuensis is nested within the clade of Pethia in the phylogram confirming its generic status ( Fig. 6 View FIGURE 6 ). The two coxI sequences of Pethia dikhuensis ( ON667421 View Materials & ON667422 View Materials ) had 100% BLAST similarity with the sequences labelled as Pethia conchonius ( MG736404 View Materials ) in Barman et al. (2018), the barcode of which were derived from a specimen collected from Nagaland, India. The closest congener of P. dikhuensis is P. conchonius and P. sanjaymoluri Katwate et al. 2016 , from which it differs by a pairwise sequence distance of 4·4–4·6%. The new species differs from Pethia rutila ( MK572484 View Materials ) and Pethia stoliczkana ( MT483478 View Materials ) by a pairwise sequence distance of 8.3–8.5 & 9.0–9.1%, respectively.

Discussion. Pethia dikhuensis differs from the other congeners in the Ganges-Brahmaputra River system viz., P. canius , P. gelius , P. guganio ( Hamilton, 1822) , P. phutunio , P. ticto ( Hamilton, 1822) , P. shalynius (Yazdani & Talukdar, 1975) , P. aurea Knight, 2016 and P. arunachalensis Shangningam et al. (2020) . Pethia dikhuensis is distinguished from P. shalynius and P. arunachalensis in having a humeral spot (vs. absence); from P. gelius , P. phutunio , P. guganio , P. aurea and P. canius in having only two spots on the body (at humeral and caudal region) and a complete lateral-line (vs. three spots/blotches on anal fin origin, dorsal fin base, behind operculum or at caudal peduncle, and an incomplete lateral-line); and from P. ticto , P. atra ( Linthoingambi & Vishwanath, 2007) , P. khugae ( Linthoingambi & Vishwanath, 2007) , P. manipurensis ( Menon et al. 2000) , P. meingangbii (Arunkumar & Tombi Singh, 2003) , P. poiensis Shangningam & Waikhom, 2018 , P. ornata ( Vishwanath & Laisram, 2004) and P. yuensis (Arunkumar & Tombi Singh, 2003) , in having complete lateral-line (vs. incomplete).

Pethia dikhuensis differs from the other congeners of the Chindwin–Ayerwaddy drainage viz., P. didi ( Kullander & Fang, 2005) , P. erythromycter ( Kullander, 2008) , P. nankyweensis ( Kullander, 2008) , P. padamya (Kullander & Britz, 2008) and P. thelys ( Kullander, 2008) , in having a complete lateral-line (vs. incomplete); and from P. castor and P. pollux , recently described species from the Ayeyarwady drainage in Myanmar ( Conway et al. 2021), by the presence of a complete lateral line and prominent caudal blotch.

Four species of Pethia from the Western Ghats of peninsular India, namely P. punctata (Day, 1865) , P. setnai (Chhapgar & Sane, 1992) , P. striata Atkore et al. 2015 and P. lutea Katwate et al. 2014 , also have a complete lateral line. Pethia dikhuensis differs from P. punctata in having the humeral blotch on 3rd and 4th lateral-line scales, prominent on the lower half of the scale and extending to one scale below the lateral line (vs. humeral spot covering anterior half of fourth scale of the row below lateral-line), caudal-peduncle blotch overlapping 16 th to 18 th (n =2) or 17 th to 19 th (n =11) lateral-line scales (vs. overlapping 19 th to 21 st); from P. setnai in having a minute humeral spot (vs. prominent humeral mark 2–3 scales wide and 3–4 scale high), and the absence of a grey transverse bar below the dorsal-fin (vs. a prominent gray transverse bar, seen in both live and preserved specimens); from Pethia lutea in having more numbers of serrae on the unbranched dorsal-fin ray (14–18 vs. 8–12 serrae), a black band on the interradial membrane of the dorsal fin submarginally (vs. absence of any band); from P. striata in having more number of pored lateral-line scales (21–24 vs. 20–21), and in possessing a moderately rounded to elliptical blotch covering 1 scale high on 16 th to 18 th or 17 th to 19 th lateral-line scales compared to a large blotch, 3–4 scales high, covering 17 th to 19 th lateral-line scales.

Genetic analysis reveals that the closest congeners of P. dikhuensis is the widely distributed P. conchonius and the peninsular Indian endemic P. sanjaymoluri from which it differs in having a humeral spot and complete lateral line, respectively.

Pethia dikhuensis is currently known only from the type locality, Dikhu River , Mokochung district , Nagaland, India. The rivers of Nagaland are relatively less explored compared to other Northern Eastern states of India, and the description of P. dikhuensis suggests that the streams and rivers of this region may harbour many more undescribed species of fish, and therefore deserves increased survey efforts .