Key to species of Tatia:

1. Nasal ossified, tubular, not sutured to mesethmoid. Caudal peduncle depth less than 13.1% SL. Adult size between 28.0-46.6mmSL……………………………………………2

1’. Nasal ossified with wide medial flanges partially sutured to lateral margins of mesethmoid (Fig. 8). Caudal peduncle depth greater than 13.6% SL. Adult size greater than 48.0 mmSL...………...……………………………..…..…………4

2. Lower jaw slightly protruding beyond upper; prevomer small, with rostro-lateral process limited in size; ribs 5-6; post-Weberian vertebrae 29-30. Mature males with externally elongate deferent duct and swollen anal-fin base (Fig. 26). Ground color whitish with brown spots, pattern mottled or irregularly striped. Ventral surface of head covered with short brown irregular stripes. Body with characteristic dark band along lateral line …………………………………… ……………… Tatia gyrina (Fig. 25, Amazon and Suriname)

2’. Jaws equal in length; prevomer arrow-shaped with welldeveloped rostro-lateral process; ribs 7-9; post-Weberian vertebrae 32-34. Mature males with short emergent deferent duct and thick anal-fin base (Fig. 18 and 32). Ground color brownish. Ventral surface of head whitish or brownish around chin barbels; no dark band along lateral line…3

3. Cranial fontanel relatively narrow with two openings, anterior one between mesethmoid and frontal and posterior one limited to frontal (Fig. 31); infraorbital 1 with short ventral process, restricted to anterior border of eye; pectoral-fin I,4; snout depth 44.8% HL or less ………………… …………………………… Tatia meesi (Fig. 30, Essequibo)

3’. Cranial fontanel wide, with single opening (Fig. 17); infraorbital 1 with pronounced ventral process, almost reaching ventral border of eye; pectoral-fin I,5; snout depth 46.8% HL or more ………………………………………… ……… Tatia caxiuanensis (Fig. 16, Curuá, lower Amazon) 4. First nuchal plate small, rounded (Fig. 11); pectoral fin usually I,4, sometimes I,5; eye relatively small, 17.4-24.2% HL; last two precaudal centra posterior to the Weberian apparatus alternatingly unribbed and ribbed ………………… 5

4’. First nuchal plate large, somewhat pentagonal (Fig. 37); pectoral fin usually I, 5, sometimes I, 4; eye relatively large, 21.4-33.0% HL; all post-Weberian vertebrae consecutively ribbed…………………………………………………………6

5. Pectoral-fin spine, 22.0-27.1% SL; mouth width 47.9-52.9% HL; post-Weberian vertebrae 31-33; color pattern brownish dorsally, irregularly speckled over sides of body; caudal fin with irregular dark bars …………………………….. ……………… Tatia neivai (Fig.33, upper Paraná-Paraguay)

5’. Pectoral-fin spine, 19.8-21.8% SL; mouth width 39.0-43.2% HL; post-Weberian vertebrae 34; color pattern greyishbrown dorsally, with small dark chromatophores over lateral and dorsal parts of body; caudal fin pale, with small spots over distal portions of rays ………………………… ………………..…… Tatia boemia (Fig. 10, upper Uruguay)

6. Postcleithral process long, 15.6-21.8% SL, almost reaching vertical through origin of dorsal fin; pelvic-fin tip whitish ………………………………………………………7

6’. Postcleithral process short, 12.0-14.4% SL, not reaching vertical through origin of dorsal fin; pelvic-fin tip dark …............. Tatia nigra (Fig. 36, central and lower Amazon).

7. Snout length 23.1-33.3% HL; eye 27.1-42.9% HL; ribs 7; post-Weberian vertebrae less than 34 ……..……………. 8

7’. Snout length 36.1-44.3% HL; eye 21.4-26.5% HL; ribs 9-11; post-Weberian vertebrae more than 33 ……….………… 9

8. Narrow elliptical cranial fontanel (Fig. 23); eye 37.0-42.9% HL; pectoral fin with 5 branched rays; post-Weberian vertebrae typically 32-33, rarely 30; sides of body usually dark with light rounded spots, or sometimes uniformly pale brown …………………… Tatia galaxias (Fig. 22, Orinoco)

8’. Large cranial fontanel (Fig. 41); eye 27.1-30.3% HL; pectoral fin with 4 branched rays; post-Weberian vertebrae 29-30; sides of body dark with light, narrow, irregular, longitudinal stripes… Tatia strigata (Fig. 40, central Amazon and Negro)

9. Cranial fontanel long, extending between mesethmoid and frontal (Fig. 28); mature male genital papilla without skin flap around deferent duct (see Fig. 9); notch absent from distal margin of modified anal fin in mature males (i.e., central fin rays not reduced); third anal-fin ray in males comparatively long, between 7.5-10.0% SL; post-Weberian vertebrae34-36…………………………………………………10

9’. Cranial fontanel short, opening restricted to frontal (Fig. 8); mature male genital papilla with thick skin flap around deferent duct; notch present in distal margin of modified anal fin in mature males (i.e., central fin rays reduced); third anal-fin ray in males comparatively short, between 5.2-7.0% SL; post-Weberian vertebrae 38-39 ……………… …………………… Tatia aulopygia (Fig. 7, upper Madeira)

10. Body coloration variable: usually with ellipsoid spots present on body and caudal fin (Fig. 27 b,c), or sometimes coloration uniformly brown; caudal fin pale (Fig. 27a); large adult specimens (> 72 mm SL) usually with prevomerine teeth; male modified anal fin with 3-6 elongate antrorse denticulations along distal anterior margin of third unbranched ray (Fig. 29) …………………… Tatia intermedia (Fig. 27, Guyana, Suriname, French Guiana, Amapá and eastern Pará, Brazil, lower Amazon and Tocantins)

10’. Body coloration mottled with irregular blotches or stripes; caudal fin with whitish spots or blotches; no prevomerine teeth, even in large adult specimens; male modified anal fin with sharp distal tip and 1-3 short antrorse denticulations along distal anterior margin of third unbranched ray (Figs.14 and 20)…………………………………………… 11

11. Head width 76.0-80.1% HL; mouth width 48.1-52.3% HL; pectoral-fin spine uniformly dark brown, without transverse bands; anal-fin rays usually iii, 6-7; modified anal fin in mature males with first unbranched ray unsegmented (Fig. 21) …................ Tatia dunni (Fig. 19, upper Amazon)

11’. Head width 86.6-93.4% HL; mouth width 54.0-59.7% HL; pectoral-fin spine usually with tranvese dark bands; anal-fin rays usually iii, 7-8; modified anal fin in mature males with first unbranched ray divided into 3-4 segments (Fig. 15) ..…… Tatia brunnea (Fig. 13, Negro and Suriname, French Guyana)

4’. First nuchal plate large, somewhat pentagonal (Fig. 37); pectoral fin usually I, 5, sometimes I, 4; eye relatively large, 21.4-33.0% HL; all post-Weberian vertebrae consecutively ribbed…………………………………………………………6

5. Pectoral-fin spine, 22.0-27.1% SL; mouth width 47.9-52.9% HL; post-Weberian vertebrae 31-33; color pattern brownish dorsally, irregularly speckled over sides of body; caudal fin with irregular dark bars …………………………….. ……………… Tatia neivai (Fig.33, upper Paraná-Paraguay)

5’. Pectoral-fin spine, 19.8-21.8% SL; mouth width 39.0-43.2% HL; post-Weberian vertebrae 34; color pattern greyishbrown dorsally, with small dark chromatophores over lateral and dorsal parts of body; caudal fin pale, with small spots over distal portions of rays ………………………… ………………..…… Tatia boemia (Fig. 10, upper Uruguay)

6. Postcleithral process long, 15.6-21.8% SL, almost reaching vertical through origin of dorsal fin; pelvic-fin tip whitish ………………………………………………………7

6’. Postcleithral process short, 12.0-14.4% SL, not reaching vertical through origin of dorsal fin; pelvic-fin tip dark …............. Tatia nigra (Fig. 36, central and lower Amazon).

7. Snout length 23.1-33.3% HL; eye 27.1-42.9% HL; ribs 7; post-Weberian vertebrae less than 34 ……..……………. 8

7’. Snout length 36.1-44.3% HL; eye 21.4-26.5% HL; ribs 9-11; post-Weberian vertebrae more than 33 ……….………… 9

8. Narrow elliptical cranial fontanel (Fig. 23); eye 37.0-42.9% HL; pectoral fin with 5 branched rays; post-Weberian vertebrae typically 32-33, rarely 30; sides of body usually dark with light rounded spots, or sometimes uniformly pale brown …………………… Tatia galaxias (Fig. 22, Orinoco)

8’. Large cranial fontanel (Fig. 41); eye 27.1-30.3% HL; pectoral fin with 4 branched rays; post-Weberian vertebrae 29-30; sides of body dark with light, narrow, irregular, longitudinal stripes… Tatia strigata (Fig. 40, central Amazon and Negro)

9. Cranial fontanel long, extending between mesethmoid and frontal (Fig. 28); mature male genital papilla without skin flap around deferent duct (see Fig. 9); notch absent from distal margin of modified anal fin in mature males (i.e., central fin rays not reduced); third anal-fin ray in males comparatively long, between 7.5-10.0% SL; post-Weberian vertebrae34-36…………………………………………………10

9’. Cranial fontanel short, opening restricted to frontal (Fig. 8); mature male genital papilla with thick skin flap around deferent duct; notch present in distal margin of modified anal fin in mature males (i.e., central fin rays reduced); third anal-fin ray in males comparatively short, between 5.2-7.0% SL; post-Weberian vertebrae 38-39 ……………… …………………… Tatia aulopygia (Fig. 7, upper Madeira)

10. Body coloration variable: usually with ellipsoid spots present on body and caudal fin (Fig. 27 b,c), or sometimes coloration uniformly brown; caudal fin pale (Fig. 27a); large adult specimens (> 72 mm SL) usually with prevomerine teeth; male modified anal fin with 3-6 elongate antrorse denticulations along distal anterior margin of third unbranched ray (Fig. 29) …………………… Tatia intermedia (Fig. 27, Guyana, Suriname, French Guiana, Amapá and eastern Pará, Brazil, lower Amazon and Tocantins)

10’. Body coloration mottled with irregular blotches or stripes; caudal fin with whitish spots or blotches; no prevomerine teeth, even in large adult specimens; male modified anal fin with sharp distal tip and 1-3 short antrorse denticulations along distal anterior margin of third unbranched ray (Figs.14 and 20)…………………………………………… 11

11. Head width 76.0-80.1% HL; mouth width 48.1-52.3% HL; pectoral-fin spine uniformly dark brown, without transverse bands; anal-fin rays usually iii, 6-7; modified anal fin in mature males with first unbranched ray unsegmented (Fig. 21) …................ Tatia dunni (Fig. 19, upper Amazon)

11’. Head width 86.6-93.4% HL; mouth width 54.0-59.7% HL; pectoral-fin spine usually with tranvese dark bands; anal-fin rays usually iii, 7-8; modified anal fin in mature males with first unbranched ray divided into 3-4 segments (Fig. 15) ..…… Tatia brunnea (Fig. 13, Negro and Suriname, French Guyana)