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However, fish do develop gas in the gut, and this is expelled through the vent, much like that of most animals. The difference is in the packaging.

Fish package their excreta into a thin gelatinous tube before disposal. This includes any gas that has formed or been carried through digestion. The net result is a faecal tube that either sinks or floats, but as many fish practice coprophagia, these tubes tend not to hang around for too long.

DEREK SMITH
Long Sutton
Lincolnshire

a. I have on several occasions witnessed my cichlids passing wind--to the displeasure of my plub eel.

This seems to be a result of them taking in too much air while wolfing down flaked foods floating on the surface of the water. If the air was not expelled it would seriously affect their balance.

PETER HENSON
University of London

a. Most sharks rely on the high density lipid squalene to provide them with buoyancy, but the Sand Tiger shark, Eugomphodus taurus, has mastered the technique of farting as an extra buoyancy device. The shark swims to the surface and gulps air, swallowing it into its stomach. It can then fart out the required amount of air to maintain its position at a certain depth.

ALEXANDRA OSMAN
London