Schistura rebuw, Choudhury & Dey & Bharali & Sarma & Vishwanath, 2019

Choudhury, Hrishikesh, Dey, Abhinit, Bharali, Ratul Ch., Sarma, Dandadhar & Vishwanath, Waikhom, 2019, A new species of stone loach (Teleostei: Nemacheilidae: Schistura) from Arunachal Pradesh, India, Zootaxa 4551 (1) : -

Schistura rebuw , new species

( Figs. 1 & 2 )

Type material. Holotype. GUMF 0 301, 69.8 mm SL; India: Arunachal Pradesh: East Kameng District: Pachai stream near Seppa : Kameng River (Brahmaputra basin); 27°23ʹ20ʺ N, 93°03ʹ20ʺ E, 400 m above sea level; A. Dey & party, 16 February 2017.

Paratypes. GUMF 0302 /5, 5 specimens, 41.8–71.1 mm SL, data same as holotype . ZSI FF7956, 53.0 mm SL, data same as holotype .

Diagnosis. Schistura rebuw is distinguished from its congeners by the combination of the following characters: 10–11 blackish bars on a greyish-beige body; pre-dorsal bars mostly broken and/or incomplete, coalescing dorsally in a more or less alternate fashion to form irregular saddles; bars below and behind the dorsal fin mostly regular, rarely broken or incomplete, coalescing dorsally to form regular saddles; a complete caudal bar; all bars over-layered on a faint mid-lateral stripe, mostly discernible as a connection between the caudal and its preceding bar, sometimes continuous and reaching vertically through pectoral fin base; vermiculated head and cheeks; a small suborbital slit in females and a suborbital flap in males; a complete lateral line; a prominent axillary pelvic lobe; and a forked caudal fin.

Description. See Figure 1 for general appearance and Table 1 for morphometric data. Body elongate, nearly oval in cross-section up to dorsal fin, compressed caudad. Head slightly depressed at nape, its anterior profile arched up to nares. Dorsal body profile gently arched up to a point slightly ahead of dorsal fin, followed by a small depression, then descending gently up to caudal-fin base. Ventral body profile nearly straight, with a gentle ascent after anal-fin origin. Short adipose crests along dorsal and ventral midlines of caudal peduncle, often appearing notched in some specimens when viewed laterally.

Mouth arched, nearly semi-circular, about 1.5–2.1 times wider than long, with thick, fleshy, pleated lips; upper lip with median incision, lower lip with wide median interruption. Prominent processus dentiformis present on upper jaw, with corresponding median depression on lower jaw ( Fig. 3a ). Anterior nostrils pierced in front of short, broadly-pointed tube-like flaps. Barbels in three pairs: inner rostral reaching slightly beyond the angle of mouth; outer rostral reaching the anterior margin of the orbit; maxillary barbel extending up to anterior margin of opercular bone. Lips, barbels covered by numerous unculi.

Dorsal fin with 4 (7) simple and 7½ (2), 8½ (5) branched rays; its origin slightly ahead of pelvic fin. Anal fin with 3 (7) simple and 5½ (7) branched rays; fin not reaching hypural fold when adpressed. Pectoral fin with 9 (1) or 10 (6) branched rays; adpressed fin reaching nearly half (or slightly more than) the distance between pectoral and pelvic origins. Pelvic fin with 6 (1) or 7 (6) branched rays; adpressed fin reaching about two-third the distance between its origin and anus. Caudal fin forked, with 8+7 (2) or 9+8 (5) branched rays; its lower lobe equal to or slightly longer than the upper. Anus situated posteriorly, just about an eye diameter ahead of anal fin.

Body covered by minute, deeply-embedded cycloid scales, except on head and ventrally up to anal-fin origin. Lateral line complete, with 76–97 pores. Cephalic lateralis system consists of 6–7 supra-orbital, 3 + 8–10 infraorbital, 9 preoperculomandibular and 3 supra-temporal pores. Intestine nearly straight, without any discernible loop behind stomach ( Fig. 3b ). Air bladder within ossified capsule, with a small free posterior chamber ( Fig. 3c ). Axillary pelvic lobe present. Minute tubercles on entire body and in rows along interradial membranes of fins. Vertebrae: 37 (1) or 38 (1).

Coloration. In preservative ( Fig. 1 ): Ground colour grey or greyish-beige dorso-laterally, darker dorsally, fading ventrad up to three-fourths or level of pectoral-fin laterally; dull creamy-white ventrally. Dorso-lateral region of head up to level of rostral barbels along the infra-orbital pore canal darker grey than body, except lightgreyish nasal flaps; light beige to creamy-white ventrally. Unless otherwise stated, all markings blackish. Body with 10–11 bars, weakly contrasted (except in holotype: 69.8 mm SL, and smallest specimen: 41.8 mm SL); those in front of dorsal fin somewhat irregular, sometimes broken and/or incomplete, usually wider or subequal to interspaces, reaching up to level of pectoral-fin base, coalescing dorsally in an alternate pattern to form Y-like (rarely X-like) saddles; those below dorsal fin complete, sometimes broken, reaching up to level of pelvic fin origin; those behind dorsal fin mostly regular, complete, subequal to or narrower than interspaces, reaching up to level of anal fin base, except the single bar immediately behind anal fin, which sometimes reaches the ventral midline to form a complete ring. In some specimens ( Fig. 1e ), bars much faded, showing only remains of saddles (possibly due to preservation). Caudal bar complete, sometimes not reaching dorsal or ventral midlines. Bars overlaid on a faint, thick mid-lateral stripe, mostly discernible as a connection between the caudal and its preceding bar, sometimes (a few specimens below 53 mm SL) continuous, reaching vertical through pectoral fin base. Head and cheeks vermiculated.

In the smallest specimen ( Fig. 1d , 41.8 mm SL), pre-dorsal bars coalesce dorsally in a more irregular and vermiculated pattern. Bars sharply contrasted and overlaid on a thin mid-lateral stripe, which appear to connect them. Caudal bar incomplete, connected to its preceding bar by thick, prominent mid-dorsal and mid-lateral stripes.

Fins with hyaline interradial membranes; dorsal fin rays proximally blackish until a little beyond branching points, sometimes prominent as an oblique band; proximal two-thirds of first four or five fin rays of pectoral and pelvic fins blackish; anal fin rays sometimes with faint blackish oblique band; caudal fin rays blackish, sometimes with two closely-set V-shaped bands (often indiscernible). Black blotch sometimes present at origin of dorsal-fin base, surrounding the simple and first two branched rays. Barbels greyish black except for maxillary barbell, which is creamy white, its base sometimes blackish.

Coloration in life ( Fig. 2 ): Ground colour metallic beige to golden yellow dorso-laterally, creamy white ventrally. Bars dark-greyish black, distinct, with shiny olive tinge. Head slightly darker than body; vermiculated patterns on head and cheek iridescent olive. Fins hyaline, reddish in freshly caught males, gradually fading in aquarium; all markings on fins blackish. Nasal and rostral barbels reddish-brown, maxillary barbel creamy, with slight olive tinge.

Sexual dimorphism. Males with a short suborbital flap, approximately parabolic in shape ( Fig. 4a ). Females with a suborbital slit in the position corresponding to the posterior edge of the flap in males ( Fig. 4b ).

Notes on Biology. One dissected female (GUMF 0302/5, 71.1 mm SL) contained about 700 immature ova of approximate diameter of 0.6 mm.

Distribution and Habitat. Presently, the species is known only from the Pachai stream, joining the Kameng River, Brahmaputra drainage, near Seppa, East Kameng District, Arunachal Pradesh, India ( Fig. 5 ).

The species was collected from a fast-flowing stretch of the stream about 10–20 m width and 0.5–2.0 m depth. The river bed comprises of pebbles and large rocks, mostly covered in algae ( Fig. 6 ). Analyzed during the month of February 2016, the water was clear, pH 7.5, D.O. 6.3 ppm, temperature 16°C, salinity 0.04 ppt and conductivity 9.12 µS. Other associated fish species caught along were Amblyceps apangi , Garra annandalei , Glyptothorax striatus , Devario aequipinnatus , Pseudecheneis sulcata and Psilorhynchus arunachalensis .

Etymology. The specific epithet is derived from the Nishing (an ethnic community in Arunachal Pradesh) word ‘ rebuw ’, referring to loach-like fish. A noun in apposition.

Figures

FIGURE 1. Schistura rebuw, India: Arunachal Pradesh: East Kameng district; holotype, male, GUMF 0301, 69.8 mm SL, a. dorsal, b. lateral and c. ventral views; side views of paratypes: GUMF 0302/5, d. 41.8 mm SL, e. 51.5 mm SL, g. 71.1 mm SL; and f. ZSI FF7956, 53.0 mm SL.

FIGURE 2. Live coloration of Schistura rebuw, GUMF 302/5, 59.5 mm SL (laterally inverted).

FIGURE 3. Illustrations showing: a. lip structure (GUMF 0301, 69.8 mm SL); b. digestive tract (GUMF 0302/5, 54.2 mm SL); and c. air bladder (GUMF 0302/5, 71.1 mm SL) of Schistura rebuw. Scale bars 1 mm.

FIGURE 4. Photograph showing: a. suborbital flap (GUMF 0301, 69.8 mm SL); and b. suborbital slit (GUMF 0302/5, 71.1 mm SL) in Schistura rebuw.

FIGURE 5. Map showing the type locality of Schistura rebuw (filled triangle).

FIGURE 6. The Pachai stream near Seppa, the habitat of Schistura rebuw.