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On different occasions I have noticed bubbles floating up from the gravel and I didn't understand where they're coming from. When i looked at the gravel, there was always a cory around. I thought it was a bubble of air trapped in the gravel and the cory looking for food, must have shifted the gravel and let the bubble out.

Until today. I finally know where they're coming from. My cories! One of them was just resting on the gravel in front of my eyes when I saw that he was, to put it simply, farting. It wasn't one bubble, no, there were a few of them.

Has anyone ever witnessed anything like this with any fish? I'm quite amused, and at least now we know where that smell's coming from...

Corys can utilize atmospheric oxygen, but unlike fish like gouramis and lungfish, they don't have any specialized organs for it. Instead, they quickly dash up to the surface and swallow some air, and then they dash back down. They absorb the oxygen from the air in their intestines, and then "farts" that you see is really just air that they swallowed earlier.

It's perfectly normal behavior.

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I'm not your neighbor, you Bakersfield trash.

I knew that they could do that, but I never actually see them doing that (well, not anymore). It was just so weirdly amusing. He looked so pleased at the time, that i couldn't have been anything but normal to him.

The air is usually expelled as it gulps some new air and it's usually so fast you don't notice it. Some cories can develop problems with getting rid of the air. It sounds like yours is really delayed. I'd wonder about whether he is stopped up a bit, making it difficult.

Do they get a varied diet?

Quite a varied diet. I'm not really worried, you konw. Or at least I didn't think there was a reason to be worried. They look really fine, I thought it was just a funny anecdote...

Ah, yes. I've seen this with my Pandas. Also, I've had one occasion when a Panda forgot to 'break wind' as it were, and his tail started slowly howering off the deck. Then, he'd turn round, as if trying to look at his tail, and see what was going on, to the point where I was mentally inserting the words "Hey! Who put the helium in my food?" as he tried to chase his tail, presumably to nail it to the deck. Then, suddenly, out popped a bubble, and normal equilibrium was restored, wherupon said Panda cruised off and rejoined his pals.

The process works as follows. Corydoras have evolved the ability to absorb atmospheric air via the intestinal lining, to enable them to survive in oxygen-depleted waters such as the waters of small pools and creeks toward the end of the dry season, just before the rains bring much needed relief. However, the process is now obligate: even in a superbly oxygenated aquarium, a Corydoras will still rise to the surface and take air. Gut complications arise if it doesn't.

Once the new air has been swallowed, the excess air from the last excursion to the surface, now depleted of oxygen, is expelled. Now, in order to have new air coming in at the head end, there's only one way out for the old air - out the back end. So, your Corydoras breaks wind.

Incidentally, the carbon dioxide produced by the fish isn't expelled into the spent air bubble. Because carbon dioxide is so soluble in water, it takes far less energy for the fish to pump it out via the gills. This has a secondary effect in its native waters: other fishes that can't breathe atmospheric air get their water further deoxygenated and loaded with carbon dioxide by the Corydoras, and start dying off if conditions are suitably extreme. The Corydopras, however, survive. And, as a corollary, not only reduce the competition for space and resources by effectively suffocating the opposition, they also provide themselves with some nice fresh juicy carcasses to munch on and stave off starvation. Sounds mercileslly cut throat, but that's Nature for you.

Of course, this latter scenario doesn't arise in some waters, such as the hillstreams and fresher upper reaches of rivers that are the home of long-snouted species such as Corydoras amapensis, and of course, Pandas. They're used to cleaner conditions and decent oxygenation year round, but still have the same basic adaptation: they've radiated out from the slower-water species over time to occupy the new niches. Which isn't surprising, given that Corydoras have been around in much their present form for, wait for it, 50 to 60 million years. There's a fossil, discovered in 1925, called Corydoras revelatus, dating back to the Eocene era, which looks a lot like a Bronze Cory!


Panda Catfish fan and keeper/breeder since Christmas 2002

"Incidentally, the carbon dioxide produced by the fish isn't expelled into the spent air bubble. Because carbon dioxide is so soluble in water, it takes far less energy for the fish to pump it out via the gills."

Quite the opposite, actually. CO2, being a nonpolar molecule, tends not to be very soluble in water unless forced; such is the purpose of CO2 diffusers-fish rely on opposite pressure to dispell the stuff. I have yet to hear of this corydoras suffocating theory, but I assure you, it's not ecologically efficient to reduce the populations of other fish, (specifically predators and those which don't compete with call. catfishes, such as tetras--when oxygen levels decrease, fish tend to migrate out of the area), lest the cory population booms, and in essence, starves itself---especially when you consider that many other scavengers sharing the same niche also use similar secondary respiratory organs (most noticeably, the loricariids).

Last edited by Cup_of_Lifenoodles at 18-Feb-2005 22:13

Quick comparison of solubilities of gases in water:

Oxygen: 0.047 cubic metres per cubic metre of water

Nitrogen: 0.0230 cubic metres per cubic metre of water

Carbon dioxide: 1.676 cubic metres per cubic metre of water.

All figures for 0°C. Although they decrease with increased temperature, the ratios remain roughly constant.

Source: Kaye & Laby's Tables of Physical & Chemical Constants.

Therefore carbon dioxide is a whole lot more soluble than the two main gaseous constituents of air. Therefore it takes less effort to pump it into the water than out of the bloodstream into the air.

Plus, when carbon dioxide dissolves in water, an equilibrium is set up between CO2 and H2CO3. Again, courtesy of Kaye & Laby. Pages 160 onwards.

The part about competition for resources was, admittedly, an extreme scenario. Usually, typical Cory habitats don't reach that critical point. But if they do, the Corys are better placed to survive than those fishes that don't breathe atmospheric air. Makes sense to me!



Last edited by Calilasseia at 18-Feb-2005 22:27

Panda Catfish fan and keeper/breeder since Christmas 2002

"Quick comparison of solubilities of gases in water:

Oxygen: 0.047 cubic metres per cubic metre of water

Nitrogen: 0.0230 cubic metres per cubic metre of water

Carbon dioxide: 1.676 cubic metres per cubic metre of water.

All figures for 0°C. Although they decrease with increased temperature, the ratios remain roughly constant."

Err, none of those gases are soluble in water , as they're all nonpolar. o2 and n2 expecially dont dissolve well, because STRONG covalent bonds between atoms prevents them from reacting with H2O.


"Therefore carbon dioxide is a whole lot more soluble than the two main gaseous constituents of air. Therefore it takes less effort to pump it into the water than out of the bloodstream into the air."

Uh, it's easier to pump it into the water because of negative pressure, as stated. Since there is more co2 inthe air as opposed to underwater, it's easier to push out the excess C02 into a smaller [CO2].

"Plus, when carbon dioxide dissolves in water, an equilibrium is set up between CO2 and H2CO3. Again, courtesy of Kaye & Laby. Pages 160 onwards."

...(and bicarb, but that's besides the point)therefore, it takes quite a bit of CO2 to raise levels to suffocative, as you had previously stated.

"The part about competition for resources was, admittedly, an extreme scenario. Usually, typical Cory habitats don't reach that critical point. But if they do, the Corys are better placed to survive than those fishes that don't breathe atmospheric air. Makes sense to me!"

...thusly, if they don't reach such extremes, then this behavior plays no role in "reducing the competition", so to speak.

Last edited by Cup_of_Lifenoodles at 18-Feb-2005 23:03

Er, if oxygen is *not soluble* in water, how do fish respire?

Panda Catfish fan and keeper/breeder since Christmas 2002

Saying o2 is insoluble is misleading, as chemically, it is indeed considered insoluble, although, in reality, it can still dissolve in water, but in very low []. However, due to Roaul's Law, the partial pressure of lower elevation areas such as those of most corydoras habitats, the lack of solubility due to warmer climates is cancelled out by the decrease in boiling point, and in turn, oxygen capacity.



Last edited by Cup_of_Lifenoodles at 19-Feb-2005 22:27

Ah. You seem to be using a narrow definition of 'soluble' as 'ionically dissociating'. Which suggests to me that you consider oxygen in water to be an enulsion rather than a solution. I'm also puzzled by the notion of an oxygen molecule as 'nonpolar' - there are still electrons available for hydrogen bonding because if there weren't, then there surely wouldn't be in a water molecule either? After all, most water molecules are covalently bonded. I seem to remember from my chemistry classes that the amount of ionic dissociation in pure water is very small - one molecule in 600,000,000. The equilibrium being shifted enormously the moment you dissolve an ionically dissociating substance (e.g. NaCl) in it. However, in pure water, almost all of the molecules are covalently bonded, and the bond dissociation energy of the O-H bond in water is pretty high - 492 KJ per mole. (Again, from Kaye & Laby).

I also consider that a work as definitive in the world of science as Kaye & Laby wouldn't talk about 'solubility of gases' as it does on page 186, unless the authors were pretty sure that those gases were indeed soluble in the sense that when you take pure water and pass a stream of gas bubbles through it, some of that gas ends up mixed with the water - most people would consider that a solution, and Kaye & Laby certainly seems to.




Panda Catfish fan and keeper/breeder since Christmas 2002

"Ah. You seem to be using a narrow definition of 'soluble' as 'ionically dissociating'."

Uh, not really--the ability of one substance to "dissolve" within another substance is based upon the fact that neither alters chemically. Ionic dissociation has nothing to do with the mentioned O2 solution, as the molecules are broken up into individual ions.

"Which suggests to me that you consider oxygen in water to be an enulsion rather than a solution."

What exactly are you talking about? I have yet to hear of oxygen was in a liquid state occuring in the natural world. Not to mention that, should the two liquids be mixed in a homogeneous fashion, one could consider an emulsion synonymous with a solution.

"I'm also puzzled by the notion of an oxygen molecule as 'nonpolar' - there are still electrons available for hydrogen bonding because if there weren't, then there surely wouldn't be in a water molecule either?"

Remember, oxygen is O2, not O. Therefore, valence electrons have bene distributed in such a way as to constitute a double bond. Slapping on new atoms to a molecule violates the octect rule, which rarely, if ever, occurs with O. Nonpolar means that the molecule is arranged in a symmetric fashion so as to not allow dipoles to form, and thus, the elections are distribured evenly. This is why O2 is so hard to dissolve in water, while passing ammonia gas over water renders the water deadly.

"After all, most water molecules are covalently bonded."
You mean "all" water molecules are cov. bonded.


"I seem to remember from my chemistry classes that the amount of ionic dissociation in pure water is very small - one molecule in 600,000,000. The equilibrium being shifted enormously the moment you dissolve an ionically dissociating substance (e.g. NaCl) in it."
The equilibrium of what?

"However, in pure water, almost all of the molecules are covalently bonded, and the bond dissociation energy of the O-H bond in water is pretty high - 492 KJ per mole. (Again, from Kaye & Laby)."
...Right, so therefore it's hard to pull apart an H2O molecule. What's your point?

"I also consider that a work as definitive in the world of science as Kaye & Laby wouldn't talk about 'solubility of gases' as it does on page 186, unless the authors were pretty sure that those gases were indeed soluble in the sense that when you take pure water and pass a stream of gas bubbles through it, some of that gas ends up mixed with the water - most people would consider that a solution, and Kaye & Laby certainly seems to."

As I had previously stated, O2 doesdiffuse into water, but only in very minimal amounts. It is, however, small enough as to be disregarded in most experiments. Thusly, it is considered by all practical means, "insoluble".


Last edited by Cup_of_Lifenoodles at 21-Feb-2005 11:45