The genus Otocinclus Cope (1872) of the siluriform family Loricariidae is diagnosed as monophyletic on the basis of shared derived characters of the cranial and hyobranchial skeleton, dorsal gill arch musculature, and gut. Otocinclus are relatively small herbivorous catfishes restricted to small streams and quiet slow-flowing margins of larger rivers, most frequently living in close association with aquatic macrophytes and terrestrial marginal grasses extending into the water column. Otocinclus species share a novel modification of the distal esophageal wall which is developed into an accessory blind diverticulum that may function in aerial respiration and for providing additional modulatory positive buoyancy for remaining in the upper water column at stream margins. Otocinclus has no junior synonyms, however several nominal species originally described in Otocinclus are here formally re-assigned to other genera in the subfamily Hypoptopomatinae. Otocinclus cephalacanthus Ribeiro 1911, O. depressicauda Ribeiro 1918, O. francirochai Ihering 1928, O. laevior Cope 1894, O. leptochilus Cope 1894, O. maculipinnis Regan 1904, O. nigricauda Boulenger 1891, and O. paulinus Regan 1908 are all placed in the genus Microlepidogaster Eigenmann & Eigenmann 1889; O. obtusos Ribeiro 1911 was placed in Pseudotothyris Britski & Garavello 1984; the genus Nannoptopoma Schaefer 1996 was erected for O. spectabilis Eigenmann 1914 in the tribe Hypoptopomatini; O. gibbosus Ribeiro 1908 is removed from Otocinclus, yet remains of undetermined generic status. Thirteen species are recognized in Otocinclus: O. affinis Steindachner 1877 of the lower Parana/Paraguay and Uruguay basins and coastal streams of southeastern Brazil; O. bororo n. sp. of the upper Rio Paraguay; O. caxarari n. sp. of the middle Rio Guapore/Mamore system; O. flexilis Cope 1894 of the lower Parana-Paraguay and Uruguay basins and coastal streams of southeastern Brazil; O. basemani Steindachner 1915 of northern Brazil; O. hoppei Ribeiro 1939 of the upper Amazon, Tocantins and Paraguay basins and coastal streams of northeastern Brazil; O. huaorani n. sp. of the upper Amazon and Orinoco basins; O. macrospilus Eigenmann & Alien 1942 of the upper Amazon basin of Colombia, Ecuador, and Peru; O. mariae Fowler 1940 of the lower Amazon, upper Madeira and Paraguay basins; O. mura n. sp. of the middle Amazon River; O. vestitus Cope 1872 of the upper Amazon and lower Parana basins; O. vittatus Regan 1904 of the Amazon, Orinoco, Parana/Paraguay, and Tocantins basins; and O. xakriaba n. sp. of the rio Sao Fransisco basin. Two species are placed in synonymy: Otocinclus arnoldi Regan 1909 and O. fimbriatus Cope 1894 are junior synonyms of O. flexilis. Keys to the species of Otocinclus and genera of the Hypoptopomatinae are provided. A descriptive treatment of the osteology and cranial myology is provided for O. vittatus. Detailed analysis of meristic and morphometric variation based on geometric morphometric procedures is provided for the phenetically similar species pairs O. mariae and O. vittatus, O. bororo and O. huaorani in an a posteriori evaluation of separate species status. The phylogenetic relationships among Otocinclus species, and the phylogenetic position of Otocinclus among genera of the Hypoptopomatinae, are determined based on analysis of 27 morphological features using cladistic parsimony. Monophyly of Otocinclus was confirmed; within Otocinclus, a clade comprised of O. affinis and O. flexilis is the sister-group to the remainder of the genus. Within that latter clade, O. hasemani and O. xakriaba are the first and second-level sister-groups to the remainder of the genus, within which relationships among species are not fully resolved with available data. The phylogenetic biogeography of Otocinclus is informative regarding the historical relationships among major river drainage basins, particularly of those river systems of the Brazilian Shield. A biogeographic hypothesis is proposed based on the area cladogram derived from the species-level phylogenetic relationships, which suggests successive vicariance and speciation in the non-Amazonian regions of endemism of southeastern and eastern South America, followed by speciation and dispersal within the Amazon, Orinoco and upper Paraguay basins. The pattern of vicariance revealed by the Otocinclus species-level phylogeny is congruent with the geologic history of the major river drainage basins of the Brazilian Shield. This result suggests that, for Otocinclus and perhaps other loricariid catfishes, much of their generic and species-level diversification occurred prior to the formation of the Amazon basin.
