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).