Identification key to species of Moenkhausia bearing a dark blotch on the upper caudal-fin lobe and hyaline lower lobe

1 Scales of the anterior portion of predorsal area arranged in a pair, followed by a single median row of scales extending onto dorsal-fin origin........................................................................................ M. lepidura (rio Orinoco basin and Amazonas lowlands, rio Tocantins, Madeira, and Negro basins, lower Tapajós and Trombetas drainages)

1’ Predorsal scales arranged in a single median row from the tip of supraoccipital spine to dorsal fin...................... 2 2 Humeral spot absent, caudal fin pigmentation consisting of roughly triangular dark spot at the middle caudal-fin rays, distinctly separate from the spot on the upper caudal-fin lobe...................................................................... M. celibela (rio Amazonas lowlands downstream of rio Maraú, rio Tapajós, lower rio Xingú and rio Jari). 2’ Humeral spot present, middle caudal-fin rays hyaline or when dark, continuous with the upper caudal-fin spot............ 3 3 Humeral spot displaced posteriorly, over the fifth to ninth lateral-line scale; upper caudal-fin lobe with scattered dark chromatophores from base to tip, frequently with a round dark spot at its middle portion................................................................................... M. hysterosticta (middle rio Tocantins basin and rio Trombetas). 3’ Humeral spot located over the third to fifth lateral-line scale; base of upper caudal caudal-fin lobe with a clear area, free of dark chromatophores....................................................................................... 4 4 Six longitudinal scale rows above the lateral line.......................... M. inrai (rio Maroni and Approuague basins) 4’ Five longitudinal scale rows above the lateral line........................................................... 5 5 Humeral spot inconspicuous, composed of few chromatophores...................................................................................... M. gracilima (rio Amazonas lowlands, Madeira, Tapajós, and Trombetas basins) 5’ Humeral spot conspicuous.............................................................................. 6 6 Humeral spot round; middle caudal-fin rays hyaline; first half of lateral line frequently easily visible through naked eye, due to conspicuous dark chromatophores concentrated around the lateral-line pores; mature males with two large, dorsally curved bony hooks on the anal fin, and tiny bony spines on distal portions of all fins............................................................ M. mikia (rio Orinoco and widely distributed in the Amazon basin except Tocantins-Araguaia basin) 6’ Humeral spot vertically elongate; middle caudal-fin rays dark; no evident dark chromatophores concentrated around the lateral-line pores; sexual dimorphism absent.................................................................. 7 7 Lateral line with 36–38 perforated scales....... M. loweae (rio Xingu and upper and middle rio Tocantins-Araguaia basin) 7’ Lateral line with 34–35 perforated scales................................................................... 8 8 Humeral spot wide, over three and a half scales or more horizontally, upper and lower edges turned anteriorly........... 9 8’ Humeral spot narrow, over one to two and a half scales horizontally, with upper and lower edges aligned in a straight line ... 10 9 Second humeral spot present, proximal half of anal-fin ray deeply pigmented, branched anal-fin rays 26–29....................................................................... M. megalops (rio Branco, rio Jari and rio Tapajós basin) 9’ Second humeral spot absent, anal-fin equally pigmented, branched anal-fin rays 21–26................................... M. lata (rio Orinoco basin, rio Negro, rio Amazonas between mouth of rio Negro and mouth of rio Tapajós, and lower rio Tapajós basin)

10 Unbranched dorsal-fin rays dark, branched anal-fin rays 25–29.... M. abyss (widely distributed in Amazon basin lowlands). 10’ Unbranched dorsal-fin rays hyaline, branched anal-fin rays 19–23.............................................. 12 12 Longitudinal thin line extending from the vertical through the middle portion of dorsal-fin origin, falling short of caudal-fin base; upper caudal-fin and middle caudal-fin rays weakly pigmented.......... M. icae (rio Solimões, rio Içá and rio Japurá) 12’ Longitudinal dark line extending from the humeral spot (or slightly posterior to it), becoming wider at vertical through the posterior third of dorsal fin to the caudal peduncle, frequently covered by guanine. Caudal-fin lobe mark variable, frequently presenting a semicircular darker spot on its middle portions, and faintly on middle caudal-fin rays...................................................... M. hasemani (widely distributed in the Amazon basin and drainages of the Guiana Shield).