Why are reef fish so colourful?
Reef fish have very distinctive colour patterns. Their bright colours are used for species recognition and protection. Reef fish are very aware of the different colours and patterns that can be used as a signal for danger.
Fish use their bright colours for camouflage, blending in with the corals and plants of the reef habitat and they are able to ambush their prey and / or protect themselves against predators.
A number of reef fish have different colours according to whether they are male or female. In a word, where sex changes are the norm this could be an important way of attracting a mate. Fairy basslets are among the most spectacular examples, the females a a vibrant yellow and the males are a vibrant pink.
Many poisonous fish are brightly coloured and patterned. This colouring is a warning signal to predators that they are not good to eat. The Box fish species are a good example.
Some juvenile fish look completely different from the adult forms that for a long time were assumed to be of a different species. The emperor angle fish is patterned with blue and white lines and circles while the adult is entirely different. The patterns of the juvenile may make the smaller and more vulnerable fish less visible to predators. The differences may also prevent adult fish wasting energies on juveniles.
Red for Camouflage
Fish such as the red squirrel fish display red and white stripes. To our eyes, this makes the squirrel fish stand out in the ocean and possibly seem like an easy target for predators. In fact red may be the best colour for camouflage, as it is the first colour to disappear in the water column as it has the longest wavelength in the light spectrum. A red fish at depth will lose all colour and appear to be middy brown. The white stripes may serve to break up the outline of the fish thus completing its visual protection from predators. Nocturnal fish such as the solider fish and other squirrel fish, as well as many deep water predators such as trout and cod tend to be red.
Warning or Camouflage?
On land, red is considered a warning colour. Red frogs, snakes and insects advertise their toxicity in this way. Members of the poisonous scorpion fish family such as the lion fish use the same technique. However, since the colour red is more difficult to see underwater, its colour combined with a broken outline, may be more helpful for concealing this stealthy predator from potential prey.
Some fish who are not toxic have developed colours and patterns which mimic those of poisonous fish, thus giving them protection. Both the leatherjacket fish and the juvenile footballer trout display the same colours and patterns as the poisonous puffer fish the black saddled toby.
Many fish also use colour to look like their surroundings, so that potential predators simply swim right by. Others, such as the stone fish family, use colour to hide and ambush prey.
It is thought that the cleaner wrasse may induce a relaxed state in marine animals by rubbing its ventral fins over the larger fishes body. This action may also cause the fish to enhance the colours of its body, possibly making the parasites more obvious to the cleaner wrasse. Changing of colours with moods is common. Coral trout are known to develop black tail and fin tips prior to spawning, while male moon wrasse become blue when feeling bossy. When preparing a nest, the male sergeant major develops a white mask which advertises his intention to spawn.
The cleaner wrasse is boldly stripped in white, black and blue to advertise its cleaning services. Predators that would normally eat fish of this size, approach the wrasse cleaning station and allow it to remove parasites from the outer skin and inside the gills and mouth.
Most fish are constantly on guard from the prospect of being eaten. Some fish confuse their predators by concealing their eye with a black line and have a false eye spot at their dorsal fin like the butterfly fish species. Eye spots are particularly common in juvenile fish, thus fooling the predators who attach at the wrong end of the body. As with many fish, the strong patterns on their bodies break up the fish outline which makes it more difficult to identify. This is known as descriptive colouration.