Akrokolioplax bicornis (Wu, 1977)

E Zhang & Maurice Kottelat, 2006, Akrokolioplax, a new genus of Southeast Asian labeonine fishes (Teleostei: Cyprinidae)., Zootaxa 1225, pp. 21-30 : 24-29

Akrokolioplax bicornis (Wu, 1977)

(Figs. 1, 2B & 3B)

Epalzeorhynchos bicornis Wu, in Wu, Lin, Chen, Chen and He, 1977 : 357 (Liuku in Nu Jiang basin, Yunnan) . Chu and Cui, 1989: 232 (Baoshan, Yunnan) . Chen, 1998: 169 (Baoshan, Yunnan) .

Material examined

IHB 64.8.1, holotype, 108. 8 mm SL; China: Yunnan Prov.: Salween River at Baoshan, 25°01’N 99°01’E; no precise collector, Aug. 1964. IHB 80IV0033, 1 ex., 148.4 mm SL, China: Yunnan Prov.: Salween River at Baoshan; 25°01’N 99°01’E; no precise collector, Apr. 1980. IHB uncatalogued, 10 ex., 106.2-144.2 mm SL, China: Yunnan Prov.: Salween River at Baoshan; 25°01’N 99°01’E; S. Y. Huang, Aug. 1999. CMK 17278, 1ex., 72.8 mm SL; Thailand: Mae Hong Son Prov.; Salween River at Mae Sam Laep; 17°58’57"N 97°44’13"E; aquarium-fish collectors; donated by K. Kubota, Feb. 2002. CMK 16830, 1 ex., 93.0 mm SL; same data, Feb. 2000.

Diagnosis

See generic diagnosis.

Description

Morphometric data for the holotype and eleven additional specimens are given in Table 1. For general appearance, see Figure 1; for position and structure of the rostral flap on the snout, see Figure 2; for morphology of oromandibular structures, see Figure 3.

Body elongate and slightly compressed, with greatest depth immediately anterior to dorsal-fin origin and least depth of caudal peduncle closer to caudal-fin base than to posterior end of anal-fin base. Dorsal profile from tip of snout to dorsal-fin origin slightly convex; slightly concave from there to origin of dorsal procurrent caudal-fin rays. Ventral profile from tip of snout to anal fin straight; slightly concave from posterior end of anal-fin to origin of ventral procurrent caudal-fin rays. Head short, longer than wide. Eye medium, with a wide and slightly convex interorbital space, dorsolaterally located on posterior half of head. Snout obtuse, with a shallow furrow running along anteroventral border of lachrymal, from base of rostral barbel to lateral extremity of rostral fold (Fig. 2B); numerous minute tubercles on tip of snout (including the rostral flaps) and anterior portion of lachrymal. Two pairs of barbels; rostral pair well-developed, length equal to eye diameter, positioned at anterior end of shallow furrow on side of snout; and maxillary pair very minute, hidden in corner of mouth.

Body scales of moderate size; chest and belly scaled, scales slightly smaller than those on flank. Lateral line complete, scales 34* (11) or 35 (1), plus 3 on caudal-fin base; transverse scales 1/24/1/31/2 * (12). Circumpeduncular scales 16* (12). Predorsal scales 9 (5) or 10* (7), slightly smaller than those on flank and regularly arranged. Axillary scale present at base of pelvic fin. Four or five scales between vent and anal-fin origin.

Dorsal fin with 4 simple and 8* (11) branched rays, the last one split to base; last simple ray soft and smooth; distal margin of dorsal fin convex, its origin positioned immediately anterior to pelvic-fin origin. Pectoral fin with 1 simple and 13 (1) or 14* (11) branched rays, inserted immediately posterior to vertical through posteriormost margin of opercle, reaching more than midway between pelvic-fin origin and vertical through dorsalfin origin. Pelvic fin with 1 simple and 8* (11) branched rays, reaching five-sixths of distance to anal-fin origin, inserted closer to anal-fin origin than to pectoral-fin origin and midway between pectoral-fin insertion and posterior end of anal-fin base. Anal fin with 3 simple and 5* (11) branched rays, last one split to base; distal margin slightly concave; origin situated midway between pelvic-fin insertion and caudal-fin base, reaching more than halfway to caudal-fin base. Caudal fin forked, upper and lower lobes equal in length and shape, with pointed tips.

Vertebrae 22+12 = 34 (1), 23+12 = 35 (1), 22+13 = 35* (9), 23 +13 = 36 (1). Pharyngeal teeth in three rows, pattern 1, 3, 4 / 4, 3, 1 (2), with pointed and slightly curved tips. Gas bladder bipartite, anterior chamber oval or round and posterior chamber oblong and distally pointed. Gill rakers small and sparse.

Coloration

Body dark brown in formalin-preserved specimens, with some light spots dorsally on some scales, and grayish white ventrally. A dark midlateral stripe, indistinct anteriorly and faintly marked posteriorly, ending in a large dark blotch at caudal-fin base. Dorsal and anal fins with white distal margin, and blackish subdistal band emanating from pigment on rays. Caudal fin with a dark band along margin of both lobes, with indistinct inner edge, more conspicuous on lower than on upper lobe. Pelvic and pectoral fins grayish white.

Sexual dimorphism

Sex and state of maturation of specimens not determined, and no obvious external sexual dimorphism noted.

Distribution

Known from the upper Salween River (Nu Jiang in Chinese) basin in Yunnan (South China), Myanmar and Thailand.

Discussion

The most unusual feature of Akrokolioplax bicornis is the presence of a pair of moveable rostral flaps at the tip of the snout. Because of this feature, the species was assigned by Wu et al. (1977) to Epalzeorhynchos , a genus characterized by possessing a pair of moveable rostral lobes. Wu et al.’s placement of the species in Epalzeorhynchos was clearly made on the basis of the literature, as no comparative material was available to them. This certainly explains why they did not note the important difference in position and morphology of the moveable rostral lobes and flaps. In Akrokolioplax , the rostral flap is positioned far from the rostral barbel and sublachrymal groove, whereas the rostral lobe is located between the rostral barbel and the sublachrymal groove in Epalzeorhynchos . In addition, the two genera differ in details of morphology of the oromandibular structures. Akrokolioplax lacks an upper lip; the upper jaw is entirely covered by the rostral cap (vs. present, medially reduced and separated from upper jaw, completely covered by the rostral cap medially), the anterior margin of the lower lip is papillose (vs. entire), and the maxillary is conspicuously longer.

According to Yang and Winterbottom’s (1998) analysis of the relationships among five species then recognized in Epalzeorhynchos , A. bicornis probably has a closer affinity with Paracrossochilus or Crossocheilus than with the species of Epalzeorhynchos . This affinity is indicated by characters such as a rostal cap with 17-20 heavily papillose indentations, disconnected from the lower lip around the corner of the mouth; minute maxillary barbels; 8 branched dorsal-fin rays; and a lower lip with an exteriorly papillated anterior margin. However, both the rostral flap on the snout, located far from the rostral barbel, and the sublachrymal groove, which is diagnostic for Akrokolioplax , are missing in both Paracrossochilus and Crossocheilus . In Paracrossochilus , the upper lip is papillose, separate from the upper jaw, and is well developed on the side, with the median part absent or vestigial (vs. lower lip absent or vestigial in Akrokolioplax ); there is a groove separating the fleshy, papillose anterior part of the lower lip from the posterior part (vs. no groove), and the jaws are transversely straight (vs. arched). In Crossocheilus , the upper lip is welldeveloped and separated from the upper jaw, is margined with tiny projections, and is connected with the lower lip around the corner of the mouth (vs. lower lip absent or vestigial in Akrokolioplax ).

In Akrokolioplax , the upper lip is absent or vestigial, the lower lip has an exteriorly papillose anterior margin, and the upper jaw is connected, by a frenum, to the lower lip around the corner of the mouth. The above three characters are shared with both Rectoris (sensu Wu et al., 1977) and Pseudocrossocheilus (sensu Zhang & Chen, 1997). It is distinguished from these two genera by having a pair of moveable rostral flaps (vs. absent), the rostral cap disconnected (vs. connected) from the lower lip, the minute (vs. well developed) maxillary barbel, and fewer lateral line scales (36-37 vs. 40-46).

Akrokolioplax resembles Sinocrossocheilus (sensu Wu et al., 1977), with which it shares the rostral cap laterally disconnected from the lower lip around the corner of the mouth. This genus is distinguished from Sinocrossocheilus in the presence of 8 (vs. 7) branched dorsal-fin rays, 3 (vs. 2) rows of pharyngeal teeth; the upper jaw connected (vs. disconnected) by a frenum with the lower lip around the corner of the mouth; and absence (vs. presence) of a modified lower lip whose central portion protrudes to form a round fleshy pad that is anteriorly covered by papillae.