Otocinclus arnoldi Regan, 1909

A, Pablo Lehmann, Mayer, Fernanda & Reis, Roberto E., 2010, Re-validation of Otocinclus arnoldi Regan and reappraisal of Otocinclus phylogeny (Siluriformes: Loricariidae), Neotropical Ichthyology 8 (1), pp. 57-68 : 60

Otocinclus arnoldi Regan, 1909

Fig. 3

Otocinclus arnoldi Regan, 1909: 234 . Type locality: La Plata, South America. Holotype: BMNH 1908.12.5:13, 43 mm SL.

Diagnosis. Otocinclus arnoldi is distinguished from all other Otocinclus species except O. mimulus , by having five branched rays in the pectoral-fin ( vs. six branched pectoralfin rays), and except from O. mimulus and O. xakriaba by having the prootic involved in the contact with the hyomandibular articular condyle ( vs. only compound pterotic contacting the hyomandibular articular condyle). It is distinguished from O. 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 - see Table 2). It is further distinguished (except for O. affinis , O. hasemani , O. xakriaba , O. mimulus and O. flexilis ) by possessing an iris operculum, and from O. affinis , O. mimulus , and O. xakriaba by having a not elevated patch of enlarged odontodes on the postero-dorsal parieto-supraoccipital tip. It is also distinguished from all remaining Otocinclus species (except for O. xakriaba , O. flexilis , and O. 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 ( Fig. 3).

Distribution. Otocinclus arnoldi occurs in the tributaries of the lower río Paraná drainage, lower and middle rio Uruguai, and the río de La Plata ( Fig. 5).