THE NEOTROPICAL CASCUDINHOS - SYSTEMATICS AND BIOGEOGRAPHY OF THE OTOCINCLUS CATFISHES (SILURIFORMES, LORICARIIDAE)

The genus Otocinclus Cope (1872) of the siluriform family Loricariidae is diagnosed as monophyletic on the basis of shared derived character s of the cranial and hyobranchial skeleton, dorsal gill arch musculatu re, and gut. Otocinclus are relatively small herbivorous catfishes res tricted to small streams and quiet slow-flowing margins of larger rive rs, most frequently living in close association with aquatic macrophyt es and terrestrial marginal grasses extending into the water column. O tocinclus 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 m argins. Otocinclus has no junior synonyms, however several nominal spe cies originally described in Otocinclus are here formally re-assigned to other genera in the subfamily Hypoptopomatinae. Otocinclus cephalac anthus Ribeiro 1911, O. depressicauda Ribeiro 1918, O. francirochai Ih ering 1928, O. laevior Cope 1894, O. leptochilus Cope 1894, O. maculip innis Regan 1904, O. nigricauda Boulenger 1891, and O. paulinus Regan 1908 are all placed in the genus Microlepidogaster Eigenmann & Eigenma nn 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. gibbos us Ribeiro 1908 is removed from Otocinclus, yet remains of undetermine d generic status. Thirteen species are recognized in Otocinclus: O. af finis Steindachner 1877 of the lower Parana/Paraguay and Uruguay basin s and coastal streams of southeastern Brazil; O. bororo n. sp. of the upper Rio Paraguay; O. caxarari n. sp. of the middle Rio Guapore/Mamor e system; O. flexilis Cope 1894 of the lower Parana-Paraguay and Urugu ay basins and coastal streams of southeastern Brazil; O. basemani Stei ndachner 1915 of northern Brazil; O. hoppei Ribeiro 1939 of the upper Amazon, Tocantins and Paraguay basins and coastal streams of northeast ern Brazil; O. huaorani n. sp. of the upper Amazon and Orinoco basins; O. macrospilus Eigenmann & Alien 1942 of the upper Amazon basin of Co lombia, 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 bas ins; O. vittatus Regan 1904 of the Amazon, Orinoco, Parana/Paraguay, a nd Tocantins basins; and O. xakriaba n. sp. of the rio Sao Fransisco b asin. Two species are placed in synonymy: Otocinclus arnoldi Regan 190 9 and O. fimbriatus Cope 1894 are junior synonyms of O. flexilis. Keys to the species of Otocinclus and genera of the Hypoptopomatinae are p rovided. A descriptive treatment of the osteology and cranial myology is provided for O. vittatus. Detailed analysis of meristic and morphom etric 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 s pecies status. The phylogenetic relationships among Otocinclus species , and the phylogenetic position of Otocinclus among genera of the Hypo ptopomatinae, are determined based on analysis of 27 morphological fea tures using cladistic parsimony. Monophyly of Otocinclus was confirmed ; within Otocinclus, a clade comprised of O. affinis and O. flexilis i s the sister-group to the remainder of the genus. Within that latter c lade, O. hasemani and O. xakriaba are the first and second-level siste r-groups to the remainder of the genus, within which relationships amo ng species are not fully resolved with available data. The phylogeneti c biogeography of Otocinclus is informative regarding the historical r elationships among major river drainage basins, particularly of those river systems of the Brazilian Shield. A biogeographic hypothesis is p roposed based on the area cladogram derived from the species-level phy logenetic relationships, which suggests successive vicariance and spec iation in the non-Amazonian regions of endemism of southeastern and ea stern South America, followed by speciation and dispersal within the A mazon, Orinoco and upper Paraguay basins. The pattern of vicariance re vealed 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 lo ricariid catfishes, much of their generic and species-level diversific ation occurred prior to the formation of the Amazon basin.