not a species from Argentina:
Otocinclus affinis Steindachner,1877

published in:
Lehmann, P., F. Mayer & R.E. Reis (2010):
Re-validation of Otocinclus arnoldi Regan and reappraisal of Otocinclus phylogeny (Siluriformes: Loricariidae).
Neotropical Ichthyology 8 (1): 57-68

distribution (from publication):
In his revision of Otocinclus Schaefer (1997) reported three lots of O. affinis from the La Plata basin in Argentina (USNM 176023 and USNM 177900) and rio Uruguai in southern Brazil (MCP 9388), significantly extending the geographical range of that species from the coastal rivers of Rio de Janeiro and São Paulo to the La Plata basin. We re-examined these three lots and all 60 specimens have five branched rays in the pectoral fins (except one specimen in USNM 176023 which has six rays in one side). The color pattern is rather faded, but the broad longitudinal stripe or series of blotches can be still seen. For this reason, these specimens are re-identified as O. arnoldi and O. affinis is consequently restricted to the coastal rivers of the Brazilian States of São Paulo and Rio de Janeiro.

López et al. (2003) refer to the above mentioned paper of Schaefer (1997) for the occurrence of this species in Argentina.