Corydoras - Ecology

Ecology

Corydoras are generally found in smaller-sized streams, along the margins of larger rivers, in marshes and ponds. 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 clear. Most species are bottom-dwellers, foraging in sand, gravel, or detritus. 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. 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, Corydoras species are active during the daytime.

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.