Do fish pass gas?

We answered! Experts reveal the shocking truth behind the mysteries of science.

Laryssa Tartt, II Bamberg, Germany

Yes, in a sense some fish do pass gas, though not in the same way humans do. The bubble trail from the back end of a fish results more from swimming at different water depths than, say, eating a can of beans, explains Ted Pietsch, an ichthyologist (fish expert) at the University of Washington in Seattle.

Most fish depend on their swim bladder (a balloon-like, elongated body sac) to stay neutrally buoyant, or afloat, at any given depth. As they move vertically up and down through water, they need to constantly change the amount of air in their swim bladders to adjust to pressure at different depths. Fish need more air in the bladder to float comfortably at lower sea levels; they require less air when swimming closer to the surface. While some fish gulp in surface air and "burp" it out of the mouth, many species of fish (including sardines and herrings) possess a duct or tube leading from the swim bladder to the anus. "They force some of that excess gas out backwards," says Pietsch.

Human flatulence, or passing gas out of the anus, is caused mainly by millions of gas-producing bacteria that line the large intestine. Bacteria munch on high fiber foods like beans, onions, and cabbage, which contain lots of oligosaccharides (a group of sugars the body has no enzyme to break down). The bacteria chow down on leftovers passed on from the small intestine. While metabolizing, or turning the nutrients into energy, the bacteria give off gaseous waste that make you go "toot."

Scientists aren't sure digestive systems in fish can pass gas (the swim bladder is separate from a fish's gastrointestinal tract). But Pietsch believes fish intestines could produce some aquatic kaboom.

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