Otocinclus arnoldi Regan 1909

Distinguished from all congeners species except Otocinclus mimulus, by having five pectoral branched rays (vs. six pectoral branched rays), and except from Otocinclus mimulus and Otocinclus xakriaba by having the prootic involved in the contact with the hyomandibular articular condyle (vs. only compound pterotic contacting the hyomandibular articular condyle). Differs from Otocinclus mimulus by having a not elevated patch of enlarged odontodes on the postero-dorsal parietosupraoccipital tip and 15 caudal vertebrae (vs. having an elevated patch of enlarged odontodes on the parietosupraoccipital and typically having 16-17 caudal vertebrae). Can be further distinguished (except for Otocinclus affinis, Otocinclus hasemani, Otocinclus xakriaba, Otocinclus mimulus and Otocinclus flexilis) by possessing an iris operculum, and from Otocinclus affinis, Otocinclus mimulus and Otocinclus xakriaba by having a not elevated patch of enlarged odontodes on the postero-dorsal parieto-supraoccipital tip. Differs also from all remaining congeners (except for Otocinclus xakriaba, Otocinclus flexilis, and Otocinclus mimulus) by having a lateral trunk coloration composed of either a row of 3-6 distinct dark blotches or a distinct dark stripe extending from the compound pterotic to the base of the caudal fin, or a diffuse mixture of those two color patterns (Ref. 86395).

Otocinclus arnoldi is a species of armoured catfish native to South America, where it occurs in the lower río Paraná drainage, in the lower and middle rio Uruguai and in the río de La Plata. This species reaches a maximum length of 4.8 centimetres (1.9 in) (SL).

Named in honor of German aquarist Johann Paul Arnold (1869–1952), who "presented" type specimen to the British Museum.