Members of the South American Corydoras genus are freshwater temperate and tropical catfish in the armored catfish family (Corydoradinae subfamily), and are commonly referred to as cory catfish, cory cats, or simply corys.
The C. barbatus, C. macropterus, and C. prionotos have been reclassified into the genus Scleromystax. 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. This is contested and has not been universally accepted. The sixray corydoras belongs in Aspidoras.
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, one of the reasons a soft sand substrate is preferable.
In several species of Corydoras, it has been observed that the fishes, after initial evasive reaction to threat, lay still; this is suggested to be a form of cryptic behavior. However, it is also argued that most species do not have cryptic coloration nor freezing behavior and continue to exist.
A few Otocinclus species (O. mimulus, O. flexilis, O. affinis, 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 avoid predation.
A unique form of insemination has been described in Corydoras aeneus. When these fish reproduce, the male will present his abdomen to the female. The female will attach her mouth to the male's genital opening, creating the well-known "T-position" many Corydoras exhibit during courtship. The female will then drink the sperm. The sperm rapidly moves through her intestines and is discharged together with her eggs into a pouch formed by her pelvic fins. The female can then swim away and deposit the pouch somewhere else alone. Because the T-position is exhibited in other species than just C. aeneus, it is likely that this behavior is common in the genus.
Most corys prefer soft, acidic water. They can, however, tolerate a wide range of water conditions, including temperatures that are cooler than tropical. They do not do well in fish tanks with high nitrate levels. This ion leads to the infection of the barbels, which will shorten and become useless. The barbels may also be affected by constant contact with a sharp substrate. They are more likely to thrive if there is an open area of substrate on the bottom of the tank where they can obtain submerged food.
These fish are easy to keep, being peaceful, small, hardy, active, and entertaining. Occasionally they will dart to the surface, sticking their snout above the water for an instant to take a "breath" of air. This behavior is perfectly normal and is not an indication that anything is wrong with the fish. If done in excess, this behavior may indicate poor water conditions.
Where investigated Corydoras sp. have been shown to be diurnal and crepuscular rather than nocturnal and activity can even peak at twilight. Corydoras are very good choices for a community aquarium, and are widely kept throughout the world. Their longevity in the aquarium is noteworthy; C. aeneus is said to have lived 27 years in captivity and 20 years is not too uncommon.