How Fish Use Camouflage to Avoid Detection
When predators are at large, it obviously pays to be inconspicuous. One way is to blend into the background, hoping to avoid detection by a hunter. Camouflage, or crypsis, is used widely by fish species for this purpose. A fish whose color matches its environment will live a great deal longer than one that stands out. Many species that are brightly colored as adults are comparatively plain as juveniles, adopting camouflage colors during the early stages of life for greater comparative safety. There are two main elements to blending in to the background: color matching and contour elimination.
How do fish achieve camouflage in the natural world?
The extent to which fish match the colors in their environment varies between species. Some merely adopt a kind of drab overall color, whereas others have perfected the chameleon-like art of reacting and changing to their immediate surroundings, an ability known as adaptive camouflage.
This is most often seen among fishes that live on or near the substrate. Although marine flatfishes are the masters of this technique, many loaches and catfish are also highly adept. For fish that live in the main water column, such as penguinfish and silver dollars, silvery scales can reflect the light and colors of their underwater world. Some take this even further -- Indian glassfish and glass catfish have silvery bellies but otherwise virtually transparent bodies, which act as camouflage by allowing the habitat's dominant colors to show right through them.
How do fish maximize the benefits of camouflage?
For camouflage to be at its most effective, fish have to find ways of pulling off the trick of breaking up the outline and contours of their body. One common means of achieving this is by counter-shading. In this, the dorsal surface is dark in color and the belly is light, which allows the animal to blend in when seen from above against a deep background and from below when pictured against the sky.
Another means of breaking up contours is known as disruptive coloration. Color pattern features such as stripes allow different parts of the body to blend into the background and break up its outline. Angelfish, tiger barbs and kuhli loaches, among others, use this to good effect, especially against a background of vertical plant stems, such as reeds at the water's edge. A band across the face is also a common feature among fish, helping to break up the contrast and to hide the eye.
Can predators use camouflage as well?
Camouflage is not only used to escape the notice of predators; it can be used by predators themselves to sneak inconspicuously to within striking distance of prey. The cichlid Nimbochromis livingstonii is a master of this; its mottled coloration provides excellent camouflage against the substrate of Lake Malawi. This adaptation comes with an unusual accompanying behavior pattern: the fish "play dead" and lie on their sides against the bottom of the lake. This behavior, which leads to their local name of "sleeper" combines with their cryptic coloration to help them escape the notice of both potential predators and prey.