Balantiocheilos ambusticauda sp. nov.

(Fig. 1 a)

Balantiocheilos melanopterus (non Bleeker) Bleeker, 1865a: 35; 1865b: 176; Sauvage, 1881: 164; Tirant, 1885: [p. 161 of 1929 reprint]); Hora, 1923: 159; Chevey, 1932: 14; Fowler, 1937: 200, Figs. 185–186; Smith, 1945: 206; Taki, 1968: 10; Kottelat, 1985: 260; Rainboth, 1996: 86; Baird et al., 1999: 25; Kottelat, 2001: 38 (not figure [a cultivated specimen]).

Type material. Holotype. MHNG 2689.096, 105.1 mm SL; Thailand: Nakhon Sawan province, Bung Borapet; R. Geisler, February 1967.

Paratypes. MHNG 2158.45 (3), 102.4–120.9 mm SL; data as for holotype. ANSP 87831 (3), 52.6–54.6 mm SL; Thailand: Kemrat, 16°2'N 105°12'E; R. de Schauensee, 1936. ANSP 87209 (2), 110.0– 116.6 mm SL; Thailand: Bangkok; R. de Schauensee, 1936. MNHN A6333 (1), 126.0 mm SL; Thailand: Chao Phraya River; M. Harmand, 1883. MNHN 1847 (5), 78.6–107.5 mm SL; Thailand: Bangkok; F. Bocourt, 1862.

Diagnosis. Balantiocheilos ambusticauda is distinguished from its sole congener, B. melanopterus (Fig. 1 b), in having a shorter snout (27.5–33.9% HL vs. 33.2–39.1) that is rounded (vs. obliquely truncate) in specimens larger than ca. 80 mm SL (Fig. 2), posteriorly directed groove at rictus curved (vs. straight; Fig. 3), and narrower black margins on the pelvic and anal fins (on distal third of both fins or less vs. on distal half or more, with pelvic fins sometimes entirely black).

Description. Morphometric data are given in Table 1. Body moderately slender and strongly compressed; scales in transverse lines ½5/1/4½ (1) or ½6/1/4½ (8). Predorsal profile gently sloping dorsally, then sloping gently ventrally from origin of dorsal fin to end of caudal peduncle; predorsal scales 11 (6) or 12 (3). Ventral profile less convex than dorsal profile, sloping gently ventrally to anal-fin origin, then sloping more steeply dorsally from there to end of caudal peduncle. Lateral line complete, with 32+2 (6), 33+2 (2) or 34+3 (1) scales. Scales with strongly convergent radii. Vertebrae 17+14=31 (1), 18+14=32 (4), 17+16=33 (2) or 18+15=33 (2).

Head narrow, snout produced, with a rounded tip. Nostrils small and situated in front of eye. More than 50% of eye in anterior half of head, visible from ventral, but not dorsal, aspect. Suborbital bones fairly large, narrowest directly below eye. Mouth subterminal, with well-developed groove separating lips from both upper and lower jaws. Lips thick and fleshy, with numerous longitudinal striae. Upper lip produced, entirely visible from below when mouth is closed. Lower lip with prominent lobe produced posteriorly; lobe deeply incised medially to form chevron-shaped posterior margin, and with short, curved groove extending posteriorly at rictus (Fig. 3). Gill membranes separate at isthmus. First gill arch with 4+7 (3), 5+10 (4) or 6+10 (2) rakers. Gill rakers short, 25% length of opposing filaments on epibranchial and 20% length of opposing filaments on ceratobranchial. Pharyngeal bones stout, with three rows (2,3,5–5,3,2) of hook-shaped, conical teeth.

Dorsal fin moderately high, with ii,8½ (9) rays and concave distal margin, sharply pointed at apex; origin slightly anterior to pelvic-fin origin. Tip of adpressed dorsal fin reaching vertical through base of first anal-fin ray. Last unbranched ray longest, osseous proximally; spinous part with 12–15 serrations on posterior edge.

Pectoral fin long, with i,12,i (1), i,13,i (4) or i,14,i (4) rays and extending almost to pelvic-fin origin (reaching to 1–2 scale rows anterior to pelvic-fin origin); distal margin slightly convex. Pelvic fin with i,8 (1); i,8,i (1) or i,9 (7) rays, extending to slightly more than midway between pelvic-fin origin and anterior part of anal-fin base; distal margin slightly convex. Anal fin with ii,5½ (9) rays, strongly concave distal margin and last three posterior rays approximately equal in length; when adpressed against body, extending to middle of caudal peduncle. One scale row between urogenital opening and anal-fin origin. Caudal peduncle strongly compressed and moderately long; circumpeduncular scales ½3/1/3½ (9). Caudal fin deeply forked, with i,8,8,i (1) or i,9,8,i (8) rays. Upper and lower caudal lobes pointed; lower lobe slightly longer than upper.

Pronounced sexual dimorphism absent. Breeding tubercles absent and body depth between sexes not different.

Coloration. Preserved material with a brown body and head. Dorsal, anal and caudal fins hyaline with a thick black margin band. Proximal anterior part of dorsal fin dark brown. Dorsal and ventral margins of upper and lower caudal fin lobes with faint dark band in some individuals. Posterior third of pelvic fin black. Pectoral fin hyaline.

Distribution. Lower and middle Mekong and Chao Phraya river drainages in mainland Southeast Asia (Fig. 4). The original distribution of this species included the Chao Phraya River drainage from Bangkok upriver to the lower Nan River (Smith, 1945) and in the Mekong from Vietnam and the Great Lake (Tonle Sap) to the lower Nam Ngum River (Taki, 1968). The specimen identified as Balantiocheilus from Chiengmai by Fowler (1934: 127) is actually a juvenile Poropuntius.

Etymology. The name of this species comes from the Latin ambustus (burned around, scorched) and cauda (tail), in allusion to the black edge of the caudal fin. This name evokes the Thai name for the fish (pla hang mai, meaning “burnt tail fish”; Smith, 1945: 206). A noun in apposition.