Genus Otocinclus Cope, 1871

Otocinclus Cope, 1871: 112 .

Type-species: Otocinclus vestitus Cope, 1872 . Type by subsequent monotypy. Gender masculine.

Macrotocinclus Isbrücker, Seidel, in Isbrücker et al., 2001: 20. Type-species: Otocinclus affinis Steindachner, 1877 . Type by original designation. Gender masculine.

Included species. Otocinclus affinis Steindachner, 1877; Otocinclus arnoldi Regan, 1909; Otocinclus batmani Lehmann, 2006; Otocinclus bororo Schaefer, 1997; Otocinclus caxarari Schaefer, 1997; Otocinclus cocama Reis, 2004; Otocinclus flexilis Cope, 1894; Otocinclus hasemani Steindachner, 1915; Otocinclus hoppei Miranda-Ribeiro, 1939; Otocinclus huaorani Schaefer, 1997; Otocinclus juruenae Ribeiro, Lehmann, 2016; Otocinclus macrospilus Eigenmann, Allen, 1942; Otocinclus mangaba Lehmann, Mayer, Reis, 2010; Otocinclus mariae Fowler, 1940; Otocinclus mimulus Axenrot, Kullander, 2003; Otocinclus mura Schaefer, 1997; Otocinclus tapirape Britto, Moreira, 2002; Otocinclus vestitus Cope, 1872; Otocinclus vittatus Regan, 1904; and Otocinclus xakriaba Schaefer, 1997 .

Diagnosis. Otocinclus is diagnosed as monophyletic based on five exclusive synapomorphies: ventral ridge of lateral ethmoid absent (char. 6.2); teeth on pharyngeal jaws reduced and arranged in single series (char. 30.1); dorsal-fin spinelet V-shaped (char. 47.1); esophageal diverticulum expanded into accessory gas bladder (char. 102.2); and contact organ present at base of caudal fin in males (104.1). In addition, there are four non-exclusive synapomorphies: anteroventral surface of mesethmoid with one pair of uncinate processes (char. 1.1); swimbladder capsule enlarged (char. 16.1); anterior portion of basipterygium open, bearing pair of fenestrae (char. 60.0); and possession of gap in pores of canal-bearing median plate series (char. 81.1).

Comparisons. The species of Otocinclus are further distinguished from other hypoptopomatins by having a relatively robust and deep body and head, even though adult body size is moderately small, and its maximum standard length reaches 45 mm. In the species of Otocinclus, the fenestrae in the compound pterotic are progressively larger and irregular towards the posterolateral margin (vs. small and all with same size); and the mesethmoid completely covered by plates (vs. mesethmoid exposed). Otocinclus can be further distinguished by the possession of a V-shaped dorsal-fin spinelet and a functional locking mechanism of the dorsal-fin spine (vs. dorsal-fin spinelet absent); the lateral line not extending to the last plate in the median series (vs. lateral line extending to last plate in the median series); the posterior truncation of the mid-dorsal series, which goes from the compound pterotic to the caudal peduncle (vs. anteriorly truncated, with eight plates or less, not passing the posterior tip of the dorsal fin); and by the posteriorly truncated midventral series which extends to the caudal peduncle (vs. extending to a point between the dorsal and anal fins or even absent). Additionally, Otocinclus is distinguished by the presence of secondary sexual dimorphism in mature males in the form of a contact organ at the base of the caudal fin, composed by a patch of odontodes modified into a swirl (vs. swirl absent).