Nandus mercatus, Ng, 2008

Ng, Heok Hee, 2008, Nandus mercatus (Teleostei: Perciformes: Nandidae), a new leaf fish from Sumatra, Zootaxa 1963 (1), pp. 43-53 : 44-46

Nandus mercatus sp. nov.

( Figs. 1a , 2a )

Type material. Holotype; MZB 10987, 70.0 mm SL; Sumatra: Sumatera Selatan, market at Sekayu, 2°51'S 103°51'E; M. Kottelat & H. H. Ng, 16 May 2008.

Paratype; ZRC 51419, 39.8 mm SL; Sumatra : Sumatera Selatan, S tributary of ox-bow on Musi River, in the vicinity of Danau Calak, 2°57.190'S 103°58.690'E; M. Kottelat & H. H. Ng, 16 May 2008 .

Diagnosis. No unique diagnostic character was found for this species, with a combination of the following characters diagnosing it from all congeners: body depth 43.0–43.2% SL, 15 pectoral-fin rays, 29–32 lateral line scales, 11–12 scale rows below the lateral line, and slightly concave predorsal profile. Among Sundaic Southeast Asian congeners, Nandus mercatus is distinguished from N. nebulosus in having a slightly concave (vs. even or slightly convex) predorsal profile, rostral fossa reaching to almost half (vs. more than half) predorsal distance and fewer pectoral fin rays (15 vs. 17–19), and from N. prolixus in having fewer lateral-line scales (29–32 vs. 33–37) and deeper body (43.0–43.2% SL vs. 37.6–40.5). It differs from South Asian congeners N. andrewi and N. nandus in having fewer lateral line scales (29–32 vs. 42–55), fewer scale rows below the lateral line (11–12 vs. 14–18), and deeper body (43.0–43.2% SL vs. 24.3–37.1). Nandus mercatus is distinguished from N. oxyrhynchus in having fewer lateral-line scales (29–32 vs. 37–42) and a more gently-sloping predorsal profile.

Description. Morphometric data as in Table 1, meristic data as in Table 2. Body compressed, moderately elongate; dorsal profile with slight, but noticeable concavity in interorbital region. Snout profile acute. Dorsal surface of head with prominent rostral fossa, reaching to 47.8–48.2% of predorsal distance. Mouth moderately large, protrusible. Posterior end of maxilla extending almost to vertical through posterior orbital margin. Eye moderately large, diameter about one fifth of head length. Posterior edge of preopercle with fine serrations. Gill rakers short, club-shaped and denticulate. Teeth short, unicuspid, closely set and in many irregular rows on both upper and lower jaw.

Lateral line divided into two segments, with anterior segment more dorsally located than posterior segment. Upper lateral line beginning at dorsal origin of operculum, rising for distance equivalent to two or three scales rows and reaching greatest height above pectoral fin, thereafter sloping ventrally and ending at vertical through middle of anal-fin base (at 24th–25th scale). Lower lateral line beginning at vertical through middle of anal-fin base, vertically centered along length of caudal peduncle and continuing slightly past end of hypural plate.

Scales ctenoid, imbricate, and nearly uniform in size throughout body. Scales present throughout cheek region, preopercle, opercle, and area around eye, absent along midline of interorbital region. Area around nares and upper lip scaleless. Gular region and ventral region of head immediately adjacent scaleless; sensory pores present in this region. Sheath of scales surrounding proximal regions of dorsal and anal fins, forming ridges along sides of spines and attached to fin rays. Caudal fin scaled for about one quarter of length.

Dorsal fin with long base, anterior insertion anterior to vertical through posteriormost extent of opercle and posterior insertion at vertical through base of last anal-fin ray. Pectoral-fin origin anterior to pelvic-fin origin and shorter than pelvic fin. Pelvic fins not reaching urogenital opening or anterior insertion of anal fin. Longest ray of anal fin not reaching vertical through base of caudal fin.

Coloration. In 70% ethanol: light brown color on body, with mottled darker areas forming three reticulate vertical bars: first vertical bar over anterior half of dorsal-fin base, second at posterior third of dorsal-fin base and third at base of caudal fin. Lighter-colored areas of body with small dark brown spots: one running under base of pectoral fin and another anterior to anal-fin base. Two dark stripes running from eye: one dorsoposteriorly towards dorsal origin of operculum, terminating at posttemporal region, and second running posteroventrally and passing under posterior edge of maxilla, terminating at ventral edge of interopercle. All median fins with series of small brown spots forming transverse bars across fin membranes: dorsal and anal fins with four such bars, and caudal fin with six such bars. Pectoral fin hyaline, with small dark brown spot in middle of fin base. Pelvic fin with brown spots forming bands as in median fins, but much denser and brown coloration covering almost entire fin, except for irregular distal hyaline margin. Live coloration similar ( Fig. 2 ).

Distribution. Currently only known from the Musi River in the vicinity of Sekayu (2°51'S 103°51'E) in southern Sumatra ( Fig. 3 ).

Habitat. The single paratype of N. mercatus was obtained from streams draining into an ox-bow on the Musi River. The water was brown and of low pH (4.3).

Etymology. The specific epithet comes from the Latin noun mercatus, meaning a trading place or market. The name is used in reference to the method with which the holotype was obtained (purchased from a market), and honors the fish market, a valuable tool to ichthyologists for sampling fish diversity.

Figures

FIGURE 1. a. Nandus mercatus, holotype, MZB 10987, 70.0 mm SL; Sumatra: Sekayu; b. N. nebulosus, UF 165994, 63.1 mm SL; Sumatra: Penida River.

FIGURE 2. Live coloration of Nandus mercatus, paratype, ZRC 51419, 39.8 mm SL.

FIGURE 3. Map showing type locality of Nandus andrewi and collecting localities of N. nebulosus from Sumatra examined in this study.