Astyanax dnophos

biology_ecology

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.

A new species of Astyanax (Characiformes: Characidae) from the rapids of the lower rio Xingu, Brazil

description

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 ex

A new species of Astyanax (Characiformes: Characidae) from the rapids of the lower rio Xingu, Brazil

diagnosis

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).

A new species of Astyanax (Characiformes: Characidae) from the rapids of the lower rio Xingu, Brazil

distribution

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

A new species of Astyanax (Characiformes: Characidae) from the rapids of the lower rio Xingu, Brazil