Genus Corydoras: general notes

The genus Corydoras has racked up spectacular successes in small shallow watercourses from the Orinoco basin to the Paraguay. A local population of C. æneus even got stranded in Trinidad when sea levels rose at the end of the last ice age. Over 138 species have been scientifically described, and almost 60 more have been assigned temporary "C-numbers." In their isolated streams, often two or three Corydoras species will be discovered living together ("sympatrically"). How they maintain their individuality as distinct species, by what subtle isolating mechanisms of pre-spawning behavior or timing, or perhaps characteristic odors, will provide material for many interesting studies in the new field of "ecological speciation." It seems a shame that we like to bundle them all together in mixed Cory herds and encourage them to hybridize.
 
There's a recognizably Corydoras-like fossil, 59 million years old, reported John Lundberg, who was exploring Amazonian fish diversity in Natural History, Sept 2001. That would place an identifiable Corydoras in the Palaeocene, along with the first primates and the radiation of flowering plants. The fossil, called Corydoras revelatus, was found in the 1920s by Prof. Cockerell in El Sunchal, Jujuy Prov., Argentina, and deposited in the British Museum. There are photos of the little fossil Cory at ScotCat.
 
Air gulping. Every so often your Cories will make a dash for the surface to gulp air. A highly-vascular section of the intestine serves as an accessory respiratory organ. The spent air, mostly exhausted of its oxygen, is expelled from the anus. In Corydoras, the swallowed air plays an even more important role in hydrostatic balance, keeping the fish neutrally buoyant.
 
Barbel erosion. Generally Cories are among the least demanding fishes. One problem with keeping Cories under imperfect conditions, however, is that their barbels may erode. The ineradicable folk tradition has long been that the barbels were being "worn away" by sharp gravel. I think that a fish that was prone to suffer this way could only evolve in habitats with very fine silty bottoms, and that consequently it would have a limited distribution, whereas every stream catchment area throughout wide stretches of the Amazon-Orinoco basin has its own Corydoras species— and often two of them, co-existing side by side. How could any organism evolve so delicately mis-tuned to the varying sands and gravels of its streambed environments and yet be successful over such a wide area? If you think some streambed gravels are rounder than others, keep a 10x loupe by you and check out all the streambed gravels you can find.  
 
Some Cory keepers feel that barbel erosion is more likely due to bacterial/fungal attack, and that it may be reversed when water conditions are improved. One passing mention of bacterial barbel erosion, in C. concolor, is in an article by Allen James (of ScotCat) archived among "Catfish of the Month" (April 2000) at Planetcatfish. Bacterial infections are generally secondary, however. The primary culprits in barbel erosion I feel may be skin flukes. A report of barbel "detachment" in ictalurid cats being aquacultured, which is ascribed to necrosis from Gyrodactylus (fluke) infestation, is mentioned in a Massachusetts Coastal Zone Management white paper, "Fish Health". Could these parasitic trematodes be an issue in Corydoras barbel erosion also?
 
My own experience suggests that barbel erosion might be related to long-term elevated nitrate levels. Some of my C. schwartzi have experienced eroded barbels when nitrates remained about 40 ppm. Is there a connection here? Almost like an answer to these concerns, RTR posted at AquariaCentral, 30 Aug 2001: "This particular myth has been around almost as long as I've been keeping fish, and it refuses to die. "One of my favorite test tanks used crushed glass substrate (not kiln-softened) and a school of C. arcuatus corys (personal favorites) with fractured glass slab "rockwork". A part of the same shipment of corys went into a nearby planted tank with which I had been having problems with a high-organic substrate. After just a few months, guess which tank had barbel erosion? And a few weeks after unifying the schools, guess who started recoving their injured barbels while living over crushed glass? I had in the past experienced occasional barbel problems in corys (and Brochis— they are more sensitive IME), and always had credited it to maintenance, and was able to clear it with good tank upkeep. That fact and loss of dwarf cichlids kept in organic-substrate tanks cured me of ever having a high organic substrate again. That problem tank was the last, and I'll never have such again. "I don't use the crushed glass any more either. I really just set it for a temp tank for the test. In the year+ it operated, I had no problems with it, except that it grew algae. I do have some crushed black glass substrate, but it has been kiln-softened to round the sharp edges. "Corys in the wild live over a wide variety of sustrates, from silt/mud to rocks, and they are adapted to substrate digging. You would expect some abrasion of the barbels over anything but fibrous peat (as used for killies), but if the substrate is clean, they will not suffer the secondary infections they will over polluted substrates. The secondary infections are what erode the barbels, just like fin rot does for the unpaired fins of free-swimming fish. "They do prefer more sandy substrates, and will dig more freely in soft sand than in gravel by a wide margin. But they can be kept over either without damage, so long as it is clean, and they can suffer erosion over either if they are not. I do not have much experience with Pictus cats, but I do know their barbels are nitrate-sensitive."
 
Corydoras links. Ian Fuller, chairman of the U.K.'s Catfish Study Group and the author of Breeding Corydoradine Catfishes and Identifying Corydoradine Catfish (2005), maintains a members-only wbsite, Corydoras World, devoted to articles by him and others on keeping, and especially on breeding Corydoras and their close relatives, Aspidoras and Brochis.  
 
You might also check the ID of your Corydoras among the huge repertory of Cory photos, accurately identified, in the "Corydoras Encyclopedia" at Yamamoto Yutaka's website "I Love Corydoras": Japanese fishkeeping is long-established, high-energy, and full of its own characteristic flavor!
 
Of course Corydoras figure at Planet Catfish, and Allen James has a Cory section at ScotCat , where you'll also find Ian Fuller's article "Starting with Corydoras."
 
Paul Schuman's "Crazy about Corydoras" was originally written for the Honolulu Aquarium Society. 
 
Eric Bodrock offers two articles on breeding rare Cories at his site All Oddball Aquatics (select "Articles" in the menu). He has bred over sixty species of Corydora and their Corydoradine kin through the years. His tanks share a central filtration system, and he finds that a spawning in one tank will set off a whole chain of spawnings through the system. If you're cories are spawning in one tank, it seems to me you could encourage other Cories by adding a gallon of water from the spawning tank.  
 
Dr David Sands' volume devoted to Cories, Keeping Aquarium Fishes: Corydoras, covers the Cories he knows so well in the wild and in the aquarium: keeping them happy, feeding and breeding them.
 
Rare Cory "contaminants." Sometimes quite rare Corydoras get shipped as "contaminants" with similar-looking but more commonplace species. Sort of like rubies "contaminating" a shipment of garnets, eh. Sharp-eyed and knowledgable Cory geeks lurk on fish-delivery day at a good LFS that isn't too far down the chain from the original importer, and they get to snap up the rare ones.
 
Mixed "breeds." What might look like rare "contaminants" in a shipment of farmed Cories are increasingly likely to be a sign that the whole bunch are casual hybrids. Hybridizing might be more excusable among amateurs. Confined in our aquaria, one or two individuals of mixed Cory species commonly find themselves thrown together by fishkeepers with a stamp-collecting bent, who think of them as merely "varieties." When Cories are put together like a bowl of mixed fruit, hybridizing can occur. Why is hybridizing such a poor idea? I've never been able to satisfy anyone who asked this question. Decreased fertility in the second generation never seems like a problem to the questioner somehow. I feel that it's disrespectful of the fishes. Hybridizers imagine that something "new" could be created, if you could select these spots and combine them with that dark head, etc.. I feel instead that some fine-tuned species could be lost if we blur all together into one general-issue "mixed Corydoras" aquarium strain. (In the U.S. this is already happening to Lake Malawi cichlids.) A fastidious aquarist won't force mixed company on the fishes in his or her care. When someone asks whether it's okay to mix Cories, I say, "Imagine that you've been abducted by aliens. Kind-hearted, responsible aliens, who have conscientiously done their homework: they studied your DNA, and now they set you up in the equivalent of a high-end Hospitality Suite. Your roommates, for the rest of your life-span (your DNA checked out as almost identical) are ...two chimpanzees...."
 
Spawning. More and more species of Corydoras are spawning in aquaria, even the species that had a reputation for being difficult, now that we understand the basic triggers that initiate their spawning cycle. In their native waters, apparently Corydoras spawn when temperatures drop in the first floods of soft water after a storm. Recreating this event can be as simple as a massive water change with cool water (under 70º even); a refinement is to time the water change to follow closely after a sharp drop in your local barometric pressure: check the Weather Channel. This sounds like a smart trick to remember. But then, I also like the charming true story of the devoted Cory lover who rattles sheets of galvanized tin to create a boom of thunder in the fishroom and sprinkles "rain" from a watering can!
 
There are variations on the Corydoras breeding theme. Follow the details in articles onbreeding various Cories at Scotcat.   
 
Cories are sensitive to medication. Years ago, when tests were done to test the salt-tolerance of freshwater fishes, Corydoras died first. Malachite green will poison Cories before any other fishes are affected; it took me years to learn never to use Malachite green on a Cory cat. I pass this on to you.
 
Corydoras genus at Wikipedia.