Filefish: Now you see it. Now you don’t.

Species: The slender filefish (Monacanthus tuckeri)
Habitat: Shallow waters in the Caribbean Sea

The slender filefish has a way to stay off the seafood menu – it has evolved the ability to become almost invisible. The fish can camouflage its body patterns and shape to match its marine surroundings in seconds (see video).

The small fish lives near soft corals on reefs and feeds on small crustaceans and zooplankton. But larger predators are on the lookout for a filefish snack.
To see an object for what it is, you need to be able to perceive its edges, which mark it out as being separate from the background. The filefish changes its coloration to create “false edges”. For example, it can make a dark, longitudinal stripe appear on its body that looks like a real edge. The eye sees this false edge, and so can miss the true outline of the fish. And if you don’t see the real outline, you won’t recognize what’s in front of you.

To alter its patterning, the fish gathers visual information from its surroundings, then its brain signals to pigment-containing cells in the skin. Depending on the signal, the pigment can either aggregate at the center of cells, covering a smaller area of the skin, or disperse to cover a larger area. The fish also has small, protruding skin projections, called dermal flaps that help it hide itself from others. They make the physical edges of the fish look less smooth or “fishlike” and more jagged and complicated. The dermal flaps often resemble underwater structures such as coral polyps, small clumps of sand or bits of algae.
This is enough to confuse predators and even practiced researchers!