Ocean Watch

In a recent e-mail, an artist asked me about bubbles. She knows fish breathe oxygen from the water but isn't sure how it works or what happens to the carbon dioxide given off.

"I want to be sure I don't put bubbles in my watercolors where they don't belong," she writes.

Given the tiny amount of oxygen in the world's waters, I think it's a wonder fish breathe at all. And here in Hawaii's warm waters, it doesn't get much worse.

The first hitch is that there's not much oxygen around to work with. Bodies of water get nearly all their oxygen from the air above them, and that's only about one-fifth oxygen.

Also, oxygen has dissolving issues: It does it best in water that's both fresh and cold. The Great Lakes, therefore, have far more of this vital gas than tropical oceans.

Fresh water, for instance, at 41 degrees Fahrenheit, contains one-twenty-third as much oxygen as the air above it. Warm that water to 95 degrees, though, and the amount of oxygen in the water decreases to one-forty-third of the air. Make that warm water salty, and it contains only one-fifty-second as much oxygen as air.

Still, even with such minuscule amounts of oxygen available, fish breathe just fine because they have crackerjack respiratory organs: gills.

Unlike our lungs, gills are a one-way street. Water flows in a fish's mouth and out through the gills. The way the fish achieves this unidirectional stream depends upon the species. Some fish constantly swim forward to move water through; others pump water over their gills with muscles in the mouth area.

The most misunderstood of these pumpers are moray eels. Their jaw-working motion, and the sharp teeth it reveals, looks threatening, but the eels are simply breathing. That's why, if you hold your hand or foot still during a bite, the eel will soon let go to take a breath, leaving a minor puncture wound instead of a more serious slicing wound.

Or so the theory goes. I've never known anyone who could remain motionless with a moray eel clamped onto his or her body.

Anyway, as water passes over a fish's gills, oxygen goes into the bloodstream and carbon dioxide goes out. And like oxygen, this carbon dioxide dissolves in the water.

Nature, however, is full of wonderful exceptions. At least 374 fish species in the world breathe both water and air. This enables the creatures to survive a wide range of conditions, both aquatic and terrestrial.

Some of these fish are famous. There are South America's lungfish, Southeast Asia's walking catfish and the more common mudskippers, found throughout the world.

I will add to this list of superior survivors our underappreciated tilapia of the Ala Wai Boat Harbor and Ala Moana Beach Park. When the water there gets low in oxygen, these adaptable fish swim to the surface and gulp air through their mouths.

So to answer my reader's question: Painting bubbles rising from a fish's mouth is almost never realistic. But remember Nemo, SpongeBob and the Little Mermaid. Realism isn't everything.