How do fish breathe underwater?

-- Mordechai Linfield, 8, Silver Spring

With all the talk this summer about those exotic snakeheads climbing out of ponds and trotting about in the open air, we almost forgot that most fish stay underwater.

Even though snakeheads seem to get tired of all that underwater breathing, it's a fascinating process. You could try it sometime, if only you had gills.

Here's how the process works:

Fish need oxygen, the same thing we breathe. But to get it, fish have to do some work.

Luckily, water contains oxygen (that's the O in H20). Fish take in water through their mouths (the nose is for smelling and is not part of the respiratory process). Once through the mouth, the water continues past what are called gill rakers, which filter out food particles and other material. The water then goes over the gills. Each gill has two rows of filaments, very thin membranes that exchange oxygen (good) for carbon dioxide (bad). The oxygen goes into the blood stream and the deoxygenated water passes out of the fish's body.

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It's a very efficient process. It has to be, since the oxygen content in water is only about 1/50th of what's in the air. Luckily for them, fish are able to extract up to 85 percent of the available oxygen in water.

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Really efficient fish species don't even need to swim around much to draw the oxygenated water across their gills. Other species, such as tuna, have to keep moving -- and keep their mouths open -- to keep the process going.

Further, different species require different amounts of oxygen. Trout prefer to stay in cool streams that tend to retain higher levels of dissolved oxygen. Sluggish fish, such as carp, can survive in warm, stagnant ponds that don't have as much oxygen.

And, closer to home, there's the case of the common goldfish. You might have noticed that goldfish often are found at the top of the bowl -- we're talking the live, upright ones here. The oxygen content is highest at the surface, where the water comes in contact with the outside air. So, the little fishy actually is trying to breathe easier. And, of course, he wants to be in position for those smelly fish food flakes that come falling from the sky each day.

-- Scott Moore

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