Colorful tropical saltwater fish

Why are tropical reef fish so colorful?

The shallowness and high clarity of the water required for reef growth. The relative abundance of food allowing this sort of adaptive experimentation by not cramming every living thing into a marginal utilitarian corner.

Well you got most of the answer right. Reef fish are brightly coloured because they live in shallow, clear water. There is absolutely no point having bright colours if you live in deep or turbid waters. All fish are the same colour in the dark. Many non-reef fish will develop bright colours in breding season when they habitualy come to the surface.

However coral reefs do not have an abundance of food. The reason why coral reefs have such clear water is because there is nothing in it: No alage, no sediment, no nutrients. In both relative and absolute terms they are extremely unproductive systems with very little available food. While a coral reef might provide marginally more food than an open ocean they provide far less food than any other inshore system. Contrast them with the continental shelf in temperate regions or magroves in tropical regions and they exteremly unproductive. Yet temperate shelf fish and mangrove fish are not noted for having bright colours.

Quote:

Originally Posted by lissener

And reef fish tend to stay pretty darn close to the reef: within darting distance of a hidey hole.

Some do. The vast majority do not. Most reef fish are forced to be wanderers precisely because there is so little food available.

(I have a theory, too, that saltwater fish have more colors in their "palate" than most freshwater fish because most pigments are mineral based, and saltwater is far richer in minerals than freshwater. But I have no cite for that; just something I've always wondered is true.)

Most biological pigments aren't mineral based in any meaningful sense. In fact most fish are restricted to the same pigment all their tetrapod desendants are forced to use: melanin. As such fish can produce red, yellow and brown colours using pigments, just like people. Fish, and indeed most animals, derive their blue and green colours from refraction patterns, which are dependent on the alignment of polymers and have no mineral component whatsoever.
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