Astyanax dnophos, Lima & Zuanon, 2004

Lima, Flávio C. T. & Zuanon, Jansen, 2004, A new species of Astyanax (Characiformes: Characidae) from the rapids of the lower rio Xingu, Brazil, Neotropical Ichthyology 2 (3), pp. 117-122 : 118-120

Astyanax dnophos , new species

Figs. 1 - 2

Hemigrammus sp. “Xingu”: Stawikowski, 2001: 30-31, photo (“Altamira...Balneario Pedral...unteren Rio Xingu”).

Holotype. INPA 22072 (31.4 mm SL), Brazil, Pará, Altamira, rio Xingu, Costa Júnior , 03°29’37”S, 52°20’10”W; J. Zuanon, 25 Sep 1997.

Paratypes. All localities in Brazil, Pará State, rio Xingu , municípios deAltamira and Senador José Porfírio . INPA 20507 (1, 40.2 mm SL) ; MZUSP 84417 (1, 27.6 mm SL); same data as holotype . INPA 20516 (2, 26.3-46.7 mm SL) ; CAS 219762 (1, 28.5 mm SL); Costa Júnior , rapids downstream the campsite, 3°16’21”S, 52°12’07”W; J. Zuanon, 24 Sep 1997 . INPA 20517 (3, 1 cs, 23.8-27.7 mm SL), Costa Júnior , 3°16’21”S, 52°12’07”W; J. Zuanon, 23 Sep 1997 . INPA 20513 (3, 18.5- 28.4 mm SL) ; MZUSP 84416 (1, 26.0 mm SL); near Ilha da Bela Vista , 3°24’22”S, 51°43’03”W; J. Zuanon, 27 Sep 1996 . INPA 20510 (2, 25.8-27.6 mm SL), corredeira Arini , 3°24’15”S, 51°41’53”W; J. Zuanon, 8 Sep 1997 . INPA 20505 (1, 30.0 mm SL) ; UMMZ 243665 (1, 25.5 mm SL); Furo do Ramiro , 3°15’21”S, 52°05’06”W; J. Zuanon, 13 Sep 1997 . INPA 20512 (9, 18.6-25.9 mm SL), Sr. Izaltino Barbosa’s island , 3°16’21”S, 52°12’07”W; J. Zuanon, 5 Oct 1996 . INPA 20506 (4, 14.6-24.8 mm SL), pedral (rocky rapid) below Sr. Izaltino Barbosa’s island , 3°16’42”S, 52°04’11”W; J. Zuanon, 14 Sep 1997 . ZUEC 4482 (3, not measured), pedral (rocky rapid) below Sr. Izaltino Barbosa’s island , 3°16’21”S, 52°12’07”W; J. Zuanon, 14 Sep 1997 . MZUSP 82460 (3, 1cs, 29.6-33.2 mm SL), Belo Monte , rocky pool, near the rapids, 3°05’S, 51°46’W; M. Goulding, 26 Sep 1983 .

Diagnosis. Astyanax dnophos can be distinguished from its purported congeners by the presence of a dark, broad, midlateral stripe extending from the snout to the caudal-fin base, formed by superficial pigmentation (vs. dark midlateral stripe, when present, relatively narrow, not extending onto head, and formed by subjacent pigmentation, being only apparent in specimens stored for a relatively long period in formalin (see Lima & Toledo-Piza, 2001). Useful characters that might help to tell apart the new species from other small characids are a long anal fin, with 24-26 branched rays, and a large eye (43.0-49.4 % in head length).

Description. Morphometric data of holotype and paratypes presented in Table 1. Body compressed, moderately slender, greatest body depth at dorsal-fin origin. Dorsal profile of head convex from upper lip to vertical through middle of orbit; slightly concave from latter point to tip of supraoccipital spine. Predorsal profile of body slightly convex, dorsal-fin base posteroventrally inclined, straight. Body profile straight from end of base of dorsal fin to adipose fin; slightly concave between latter point and origin of dorsalmost procurrent caudal-fin ray. Ventral profile of head and body convex from lower lip to anal-fin origin. Body profile along anal-fin base straight, posterodorsally slanted. Ventral profile of caudal peduncle slightly concave.

Jaws equal, mouth terminal. Maxilla reaching vertical through anterior margin of orbit. Premaxillary teeth in two rows (Fig. 4). Outer row with 2(1), 3*(7), 4(13), or 5(4) tricuspid teeth. Inner row with 5 tetra- to pentacuspid teeth. Maxilla with 5 or 6(2) tri- to tetracuspid teeth. Dentary with 4 large, penta- to heptacuspid teeth followed by 11-12(2) smaller unito tricuspid teeth ( Fig. 3 ).

Scales cycloid, circuli present on exposed area of scales, with few slightly divergent radii extending to posterior margin of scales. Lateral line complete, with 32(1), 33(2), 34(1), 35*(9), 36(5), or 37(6) perforated scales. Horizontal scale rows between dorsal-fin origin and lateral line 5, not including scale of predorsal series situated just anterior to first dorsal-fin ray. Horizontal scale rows between lateral line and pelvic-fin insertion 4. Predorsal scales 7(2), 8(14), 9*(3), or 10(1). Circumpeduncular scales 12(5), 13(5), or 14*(3). Single row of 10-12 scales covering base of anteriormost anal-fin rays.

Dorsal-fin rays typically ii,9 (ii, 10 in one specimen). Dorsal- fin origin slightly anterior to middle of standard length. Base of last dorsal-fin ray at vertical through anal-fin origin. First dorsal-fin pterygiophore inserting behind neural spine of 8 th (2) vertebra. Adipose fin present. Anal-fin rays iii(8) or iv*(17), 24(8), 25(11), or 26*(6). First anal-fin pterygiophore inserting behind hemal spine of 15 th (2) vertebra. Pectoral-fin rays i,12(6), 13(14), 14(2), or 15*(3). Tip of pectoral fin reaching vertical through pelvic-fin insertion. Pelvic-fin rays i,7. Caudal fin forked, lobes slightly pointed, similar in size. Principal caudalfin rays 10+9(2). Twelve (1), or 13(1) dorsal procurrent caudalfin rays, and 10(1), or 11(1) ventral procurrent caudal-fin rays. First gill arch with 7(1), or 9(1) epibranchial, 8(1), or 9(1) ceratobranchial, 1(2) on cartilage between ceratobranchial and epibranchial, and 2(2) hypobranchial gill-rakers. Four (2) branchiostegal rays, 3(2) on anterior ceratohyal, and 1(2) on posterior ceratohyal. Vertebrae 34(2). Supraneurals 4(2).

Color in alcohol. Overall color light beige. Some specimens still retaining guanine on opercle, infraorbitals, and sides of body. Dark chromatophores densely concentrated on dorsal surface of head, anterior surface of lower jaw, and maxilla. Numerous relatively large dark chromatophores on infraorbitals and opercle. Longitudinal dark stripe, wide as the iris, across the eye. Two longitudinal dark stripes on trunk: one dorsal, light brown, formed by relatively small dark chromatophores, continuous with dark pigmentation at top of head and extending to upper surface of caudal peduncle. Second stripe dark, mid-lateral, broad (about as wide as eye), continuous with dark pigmentation on snout, eye, infraorbitals, and opercle, formed by relatively large dark chromatophores. Dorsal margin of stripe relatively straight, lower margin slightly concave anteriorly, tapering towards caudal peduncle. Humeral spot present, merged into midlateral stripe, just perceptible as darker area with superior extension into light area intermediate to both stripes. Light area between dorsal and midlateral stripes, and below midlateral stripes, with few, scattered dark chromatophores, except for small concentration on base of anteriormost anal fin rays. Dark pigmentation on basal portions of medial caudal-fin rays, continuous with dark midlateral stripe. Scattered dark chromatophores on anal, dorsal, adipose, and caudal fins. Pectoral and pelvic fins with few dark chromatophores. Anal-fin base with narrow, dark area (lacking in clear-colored specimens). Color intensity variable among paratypes, some small specimens (e.g., INPA 20510, 20512) much clearer, which apparently is related to the degree of chromatophore contraction immediately before preservation.

Color in life. Description based on specimens photographed in field aquaria just after capture (see Fig. 2 ; cf. also Stawikowski, 2001:31). Dorsal region from above eye to origin of adipose fin gray to brownish. Cream to whitish colored narrow stripe running from postorbital region to base of caudal fin, with second narrow iridescent yellow stripe just below and parallel to it. Humeral spot barely visible as darker, greenish area over anterior portion of iridescent lateral stripe. Broad black mid lateral band running from snout and across eye to caudal fin base. Eye with upper third of iris iridescent greenish yellow to bronze colored.Ventral region from gular area through region above anal fin base and lower portion of caudal peduncle light beige. Concentration of chromatophores forming dark area above anterior portion of anal-fin base. Pectoral and pelvic fins yellow; two or three anteriormost dorsal-fin rays and basal portions of caudal and anal fin with yellowish tint.

Geographic distribution. Astyanax dnophos is only known from the lower rio Xingu, in the region of Altamira, Pará, Brazil.

Ecological notes. Groups of up to a hundred individuals of A. dnophos were observed in rapid areas with rocky bottom and moderate to fast current (0.40 – 1.5 m. s-1). Although occasionally seen foraging on open water near the bottom in shallow riffles, most of the fish groups were found in dark, shadowed shelters among large rocks or under boulders in river stretches up to 2 m deep. The fish were seen swimming in loose groups protected from direct current in the shelter and foraging on drifting and stirred up particles in the water column. In these low light conditions, the fish’s iridescent lateral stripe and the bright upper portion of the iris outstands against the dark background. The large eyes characteristic of this species may be related to the low light conditions in these shelters and to the supposedly visually oriented drift feeding strategy displayed by A. dnophos .

Etymology. From the Greek dnophos , darkness, gloom, in allusion to the habitat occupied by the new species.

Figures

Fig.3. Astyanaxdnophos,paratype,MZUSP82460, 29.6mmSL. Upper and lowerjaws,lateral view,left side.Scale bar:1 mm.

Fig. 2. Astyanax dnophos, paratype, INPA 20505, 30.0 mm SL: Brazil, Pará, rio Xingu; in life.