Why do most fish have a dark back and a pale belly?

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Look at the colouration of your typical fish and you'll see that it has a paler belly and a darker back. But why is that?

This is called counter-shading and is an anti-predator adaptation. Most fish show this, having a dark-coloured dorsal surface, or back, and a paler ventral surface or underside.

Predators looking down from the surface will probably see a dark substrate and the fish should blend in because its back is a similarly colour. Bottom-dwelling predators looking up will see the sky and might not spot the fish because its belly is also pale.

Fish that spend their lives upside down, such as the Upside-down catfish (Synodontis nigriventris) have reverse counter-shading. Their bellies are dark and backs pale — the opposite of fish that swim the normal way up.

This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.