not a species from Argentina:
Otocinclus flexilis Cope,1894

published in:
Lehmann, P. (2006):
Anatomia e relações filogenéticas da família Loricariidae (Ostariophysi: Siluriformes) com ênfase na subfamília Hypoptopomatinae.
Tese de doutorado. PUCRS, Porto Alegre, Brazil. ix + 420 p.
(usage of unpublished data authorized by Roberto Reis)

Lehmann (2006) limits the distribution of O. flexilis to tributaries of the Patos lagoon system and resurrects O. arnoldi from the synonymy with O. flexilis. Records of Otocinclus flexilis from Argentina are now to be considered Otocinclus arnoldi.

extract from publication:
Otocinclus arnoldi from the La Plata basin is resurrected from the synonymy of O. flexilis described from rio Jacuí basin, based on two diagnostic characters: the presence of five branched pectoral-fin rays vs six in O. flexilis, and the presence of the raised tuft of enlarged odontodes at the tip of the supraoccipital posterior process, in juvenile as well as in adults of O. arnoldi