Hello, Frank Indiviglio here. Among the 150+ species of catfish classified in the genus Corydoras, all native to Central and South America, we find some of the most beloved of all aquarium fishes. Yet despite their willingness to breed in captivity, not many aquarists make the effort. As we will see, however, the droll cory cats employ one of the fish world’s most unusual breeding strategies…they are definitely worth a closer look!
Startling Reproductive Behavior
Female cory cats are larger than males, and their body has a thicker, more rounded appearance. In some species, the male’s dorsal fin is thinner and higher than the female’s and is held in a noticeably more erect position.
Corys may breed in either pair or group situations. Males chase gravid females, with the pair eventually lying motionless and perpendicular to one another (this is known to aquarists as the ‘T-Position”). The male lies on his side, and, amazingly, the female then uses her mouth to withdraw sperm from his vent.
Fertilization – Internal or External?
How fertilization actually occurs is still open to some discussion. Most ichthyologists believe that the sperm exits the female’s gills, and is shunted to a unique cup that she forms with her pelvic fins.
Upon obtaining the sperm, the female lays 1-5 eggs into this cup. She then moves off to a pre-cleaned site, usually a plant or the aquarium glass, where she glues each egg individually. She repeats this process with the same or another male until her clutch of 10-25 eggs is laid, an ordeal that may last 3-6 hours.
Some researchers have suggested that the sperm passes through the female’s digestive tract, to be released along with the eggs, or that the female expels the retained sperm upon the eggs. In any event, a most extraordinary means of fertilization…why such a strategy would evolve has not, to my knowledge, been explained.
While Corydoras cats may spawn spontaneously, the most consistent results will be obtained if cool, highly oxygenated water is added to the aquarium, simulating the drop in temperature and rise in water levels that accompanies the start of the rainy season in most Corydoras habitat. Dropping the aquarium’s water level beforehand may also help.
While various species differ in their requirements, the formula that I have used for bronze corys (C. aeneus) seems to work well in general. After dropping the water level for 2-3 weeks and maintaining it at 76 F, I add water of 60 F in an amount equal to 1/3 the volume of the tank.
Caring for Eggs and Fry
The eggs can be left in place and the adults removed (some are egg predators), or they can be carefully transferred to a rearing aquarium (they are fairly large and sticky for a day or so after deposition).
The fry should be housed in shallow water (3-4 inches) at first, as constantly rising to the surface for air will weaken them significantly.
Cory cats are, despite their “scavenger” reputation, highly specialized predators on tiny invertebrates. Their fry are best reared on live blackworms, brine shrimp and Daphnia, with animal-based flakes and pellets being added as they grow.
Plan to be in it for the long haul…a bronze cory in my collection is nearing 21 years of age!
Please write in with your questions and comments. Thanks, until next time, Frank Indiviglio.