Why are tropical reef fish so colorful?

It seems like coral reef fish are much more brightly colored than most other fish, especially freshwater fish from temperate regions. Why is this?

Camouflage?

Although some tropical fish have an amazing ability to merge into their surroundings. Many of them stick out like dogs balls which would suggest, to me, that camouflage is not the reason.

Could it be that they are camouflaged in a different part of the light spectrum, and what we’re seeing is the visible (to us) byproduct.

I don’t know. Good point.

Water adsorbs low wave lengths so many fish are bright red as a camouflage color.

But all of these explanations seem like they should be true anywhere, and yet fish in the tropics, especially reef fish, are usually much more brightly colored than temperate fish. What could explain the difference?

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.

To expand on our binary friend’s post: Tropical reefs can be colourful places. Corals, anemones, rocks, sand, etc. can be many different hues. The shallow waters allow riplling light to play on the submarine surfaces. So strips and spots and whatnot may make the fish harder for predators to see. In the cold deep ocean there’s not so much interplay between light and the environment. Camouflage can simply be ‘dark on top, light on the bottom’. Cold water supports more life than warm water. IME diving in cold water is like swimming in soup, what with all of the plankton and such. With lower visibility fish don’t need such garish designs.

Evolution works by elimination: it’s basically a negative process. So these kinds of things are more easily understood by asking why such and such is not otherwise. For example, the creationist canard: if evolution were “true,” why aren’t rabbits green? Well, because they don’t need to be: brown obviously works just fine. Rabbits evolved a strategy of fertility over perfect camouflage: they just produce enough bunnies to keep their predators well enough fed to allow enough bunnies to live to breed more bunnies.

With reef fish, the bright colors probably provide advantages for sexual selection, for example. (They’re also not quite as bright when seen without artificial or surface light.) And reef fish tend to stay pretty darn close to the reef: within darting distance of a hidey hole. So hiding in plain site is probably not the only strategy available to them; they can hide for real. (Anyone who’s ever tried to catch a fish in a reef tank knows this is true; it’s pretty well impossible, and once you’ve introduced a fish into a large reef tank it’s there until it dies or until you disassemble the reef.)

(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.)

Sexual Selection appears to be part of it.

You could ask the same question for brightly colored animals and plants on land as well. And there are lots of reasons for color variation. Some poisonous animals are brightly colored to advertise the fact - from an evolutionary perspective, it does no good to be poisonous if a predator only finds out about it after you’re dead.

A reef is a very complex ecosystem. I wouldn’t be surprised to learn that the vast amount of color is just an evolutionary adaptation to allow all the fish to sort out their roles in the community. Note that pelagic species in the tropics don’t seem to have those bright colors. Dolphins, whales, sharks, barracuda, rays… It seems to be mostly restricted to the fish that live in a large community of species.

Eesh. “Palette.” And me an artist.

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.

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

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.

In fact given the mineral deficient status typical of tropical waters if pigmentation were dependent on minerals we would expect reef fish to be among the most bland.

Compared to open water.

I know; I almost said “except blue,” but didn’t. Same goes for birds and flowers, btw.

Absolutely not, when comparing it to *fresh *water, as I explicitly was.

http://www.amonline.net.au/fishes/fishfacts/fish/bconspic.htm This beuty is according to a book I used to have about them, is very hard for a preditor to see. I doubt it.

The thing is that very few fish are open water species. There is little food in the open ocean. Most fish are either inshore or bottom feeders that may travel the open ocean from one feeding point to another. Even large species typically considered open ocean fish such as sharks and tuna feed and live primarily inshore and around sea mounts, which is where the food is.Since these fish are feeding in and living primarily inshore or on the botoom that is where they will most likely be targetted by predators, as such there is no validity in saying they are “open water” species.

That is my point. Almost all fresh water has a much higher level of minerals that can be epxloited in pigments than reef waters. Reef waters have been long since leached of material of bilogical utility: metals, phosphates, nitrates and so forth. In contrast fresh waters are usually quite high in biologically active minerals because they have recently been eroded from the land.