Key to species of Ancistrus from the Río de La Plata and coastal basins from southern Brazil

1 Maxillary barbel longer and free from the lower lip .................................................................................................... 2

- Maxillary barbel short and not free from the lower lip................................................................................................. 6

2 Male with small pale dots on the head becoming vermiform lines on the trunk; female with very small pale dots all over the body ................................................................. A. claro (upper Rio Paraguai basin, Chapada dos Guimarães)

- No distinct color pattern among male and female ........................................................................................................ 3

3 Mandibulary teeth row 12.2 to 15.0% of HL ..................................................... A. cuiabae (upper Rio Paraguai basin)

- Mandibulary teeth row more than 15.2% of HL........................................................................................................... 4

4 Occipital depth 17.9 to 18.7% of SL .................................................................. A. cirrhosus (lower Río Paraná basin)

- Occipital depth less than 16.7% of SL.......................................................................................................................... 5

5 Pear-liked body shape; supracleithral width 30.3 to 37.0% of SL .................... A. piriformis (lower Río Paraná basin)

- Not pear-liked body shape; supracleithral width 26.3 to 31.3% of SL ............. A. pirareta (lower Río Paraguay basin)

6 Orbital diameter 8.3 to 8.7% of HL ............................................................ A. formoso (Rio Formoso basin, cave fish)

- Orbital diameter more than 11.7 of HL ........................................................................................................................ 7

7 Mandibulary teeth row 13.4 to 15.5% of HL .......................... A. abilhoai n. sp. (upper and middle Rio Iguaçu basin)

- Mandibulary teeth row more than 15.5% of HL........................................................................................................... 8

8 Occipital depth 18.8 to 22.0% of SL ................................................................ A. agostinhoi n. sp. (Rio Jordão basin)

- Occipital depth less than 18.8% of SL.......................................................................................................................... 9

9 Interopercular spines 12 to 22 ................................................................... A. mullerae n. sp. (lower Rio Iguaçu basin)

- Interopercular spines 22 to 35..................................................................................................................................... 10

10 Postopercular plates 8 to 12; plates between anal and caudal 11 to 12 .............. A. taunay (middle Rio Uruguai basin)

- Postopercular plates 5 to 8; plates between anal and caudal 12 to 13 ............................................................................ .......................................................................................................... A. multispinis (coastal basins of southern Brazil)