Corydoras is a genus of freshwater catfish in the family Callichthyidae and subfamily Corydoradinae. The species usually have more restricted areas of endemism than other callichthyids, but the area of distribution of the entire genus almost equals the area of distribution of the family, except for Panama where Corydoras is not present.[1] Corydoras species are distributed in South America where they can be found from the east of the Andes to the Atlantic coast, from Trinidad to the Río de la Plata drainage in northern Argentina.[2] Species assigned to Corydoras display a broad diversity of body shapes and coloration.[3] Corydoras are small fish, ranging from 2.5 to 12 cm (1.0 to 4.7 in) in SL.,[2] and are protected from predators by their body armor and by their sharp, typically venomous spines.[4]
The name Corydoras is derived from the Greek kory (helmet) and doras (skin).[5] Corydoras is by far the largest genus of Neotropical fishes with more than 160 species.[5] It is the sole genus in the tribe Corydoradini.[6] C. difluviatilis is recognized as the basalmost species of Corydoradini, exhibiting several plesiomorphic features compared to the other species of Corydoras.[6][3] The type species for this genus is Corydoras geoffroy.[5] Several hundred species are not yet classified, but kept by aquarists. These species are given 'C-numbers', originally devised by Hans-Georg Evers for the German fishkeeping magazine DATZ in 1993. In 2006, 153 C-numbers had been assigned, of which 32 had been assigned appropriate scientific names.[7]
The species C. barbatus, C. macropterus and C. prionotos have been reclassified into the genus Scleromystax.[6] Brochis had been differentiated from Corydoras due to the higher number of dorsal fin rays; however, Brochis has recently been suggested to be a synonym of Corydoras.[6] This is contested and has not been universally accepted. The sixray corydoras belongs in Aspidoras.[8]
Corydoras are generally found in smaller-sized streams, along the margins of larger rivers, in marshes, and in ponds.[2] They are native to slow-moving and almost still (but seldom stagnant) streams and small rivers of South America, where the water is shallow and very murky. Most species are bottom-dwellers, foraging in sand, gravel or detritus.[2] The banks and sides of the streams are covered with a dense growth of plants and this is where the corys are found. They inhabit a wide variety of water types but tend toward soft, neutral to slightly acidic or slightly alkaline pH and 5-10 degrees of hardness. They can tolerate only a small amount of salt (some species tolerate none at all) and do not inhabit environments with tidal influences. They are often seen in shoals.[2] Most species prefer being in groups and many species are found in schools or aggregations of hundreds or even thousands of individuals, usually of a single species, but occasionally with other species mixed in. Unlike most catfishes, which are nocturnal, these species are active during the daytime.[2] Corydoras are capable of breathing both water and air, often swimming to the surface to quickly ingest air before re-submerging. The frequency of this air breathing behavior increases when corys are exposed to water with low oxygen availability, allowing them to tolerate periods of aquatic hypoxia.[9]
Their main food is bottom-dwelling insects and insect larvae and various worms, as well as some vegetable matter. Although no corys are piscivorous, they will eat flesh from dead fishes. Their feeding method is to search the bottom with their sensory barbels and suck up food items with their mouth, often burying their snout up to their eyes.[10]