|Zebra oto—Otocinclus cocama|
Over a dozen, see text.
Otocinclus is the most basal genus of the tribe Hypoptopomatini of the subfamily Hypoptopomatinae. However, phylogenetic relationships are currently under study and this genus may eventually be relocated. Its monophyly is supported by seven derived features. O cocama, O. huaorani, O. bororo, O. mariae, and O. mura, and O. batmani form a monophyletic group within this genus. A monophyletic group is also formed by O. flexilis, O. xakriaba, and O. mimulus, which all share mimicry as a synapomorphy.
There are currently 17 recognized species in this genus: 
- Otocinclus batmani Lehmann A., 2006
- Otocinclus bororo S. A. Schaefer, 1997
- Otocinclus caxarari S. A. Schaefer, 1997
- Otocinclus cocama R. E. dos Reis, 2004 (Zebra oto)
- Otocinclus flexilis Cope, 1894 (Peppered oto)
- Otocinclus hasemani Steindachner, 1915
- Otocinclus hoppei A. Miranda-Ribeiro, 1939
- Otocinclus huaorani S. A. Schaefer, 1997
- Otocinclus macrospilus C. H. Eigenmann & W. R. Allen, 1942
- Otocinclus mangaba Lehmann A., F. Mayer & R. E. dos Reis, 2010
- Otocinclus mariae Fowler, 1940
- Otocinclus mimulus Axenrot & S. O. Kullander, 2003
- Otocinclus mura S. A. Schaefer, 1997
- Otocinclus tapirape Britto & C. L. R. Moreira, 2002
- Otocinclus vestitus Cope, 1872
- Otocinclus vittatus Regan, 1904
- Otocinclus xakriaba S. A. Schaefer, 1997
Otocinclus are widely distributed east of the Andes of South America, throughout the lowlands from northern Venezuela to northern Argentina, but are generally absent from the Amazon and the Orinoco lowlands.
Otocinclus, like other loricariids, are characterized by rows of armour plating covering the body, as well as the underslung suckermouth. They are generally small in size; Otocinclus tapirape is the smallest of the species (2.4 cm), while Otocinclus flexilis is the biggest (5.5 cm).
Otocinclus spp. have adaptations that allow them to breathe air. A duct forms at the junction between the esophagus and the stomach and expands into an enlarged, ring-like diverticulum, diagnostic of this genus, which allows air-breathing.
Otocinclus species are diurnal, and generally are found in small streams or along the margins of larger rivers, clinging to substrates using the mouth as a sucker, and feeding on algae or aufwuchs on roots, stones, macrophytes, and broad-leaved grasses. They inhabit well-oxygenated, moderate- to slow-flowing environments, often near river banks. They are found near the surface of the water, but are often associated with vegetation or other structures. They lay adhesive eggs and do not guard them; this is in contrast to many other loricariids where the male builds a nest and guards the eggs. These fish live in shoals or schools.
Otocinclus are able to breathe air. Prior to surfacing, they will release air through their gills and mouth. Upon returning to the bottom, the fish do not need to pump their buccal cavities, indicating they are absorbing oxygen from the swallowed air.
O. mimulus, O. flexilis and O. xakriaba are considered to be Batesian mimics of certain Corydoras species (C. diphyes, C. paleatus, C. nattereri, and C. garbei, respectively). These Corydoras species have bony plates of armor and strong spines as defenses, making them less palatable; by mimicking these species in size and coloration, Otocinclus spp. avoid predation.
In the aquarium
Otocinclus spp. are popular aquarium fish. They are often purchased as algae eaters, and are more suitable for the job in smaller aquaria than many of the larger loricariid relatives, such as Plecostomus. Water should be kept clean for these fish. As otos like company, they should be kept in groups of three or more individuals, unlike most other plecos. They are generally delicate fish, and only thrive in heavily planted aquariums. If not enough algae are present in the aquarium, their diets can be supplemented with algae wafers, blanched zucchini, and other plant foods.
- Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Otocinclus in FishBase. December 2011 version.
- Lehmann, Pablo A. (2006). "Otocinclus batmani, a new species of hypoptopomatine catfish (Siluriformes: Loricariidae) from Colombia and Peru" (PDF). Neotropical Ichthyology 4 (4): 379–383. doi:10.1590/S1679-62252006000400001.
- Axenrot, Thomas E.; Kullander, Sven O. (October 2003). "Corydoras diphyes (Siluriformes: Callichthyidae) and Otocinclus mimulus (Siluriformes: Loricariidae), two new species of catfishes from Paraguay, a case of mimetic association" (PDF). Ichthyol. Explor. Freshwaters 14 (3): 249–272. Retrieved 2009-06-23.
- Reis, Roberto E. (2004). "Otocinclus cocama, a new uniquely colored loricariid catfish from Peru (Teleostei: Siluriformes), with comments on the impact of taxonomic revisions to the discovery of new taxa" (PDF). Neotropical Ichthyology 2 (3): 109–116.
- Britto, Marcelo R.; Moreira, Cristiano R. (2002). "Otocinclus tapirape: A New Hypoptopomatine Catfish from Central Brazil (Siluriformes: Loricariidae)" (PDF). In Schaefer, S. A. Copeia 2002 (4): 1063–1069. doi:10.1643/0045-8511(2002)002[1063:OTANHC]2.0.CO;2.
- "Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes" (PDF). Copeia (3): 663–675. 1998. Retrieved 2009-06-23.
- Fenner, Robert. "The Ideal Algae Eater? The Littlest South American Suckermouth Catfishes, Genus Otocinclus". Retrieved 2007-05-04.