From New Scientist

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

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

Now you see it. Now you don’t. 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 on website – link below).

The small fish hangs around near soft corals on reefs and looks for small crustaceans and zooplankton to munch on. But larger predators are on the lookout for a filefish snack.

Justine Allen of Brown University in Providence, Rhode Island, was amazed by how fast the fish camouflaged themselves when she saw them while diving in the Caribbean. It took them just 2 seconds to match the colours of the sea fans, or gorgonians, as they swam along.

“It was just incredible watching the animal go from one gorgonian to the next and really quickly change its pattern,” she says.

Living on the edge

Allen and her colleagues have been photographing and videoing the vanishing act at Little Cayman in the Cayman Islands. They have also studied the fish’s skin in the lab to put together a picture of how the camouflage works.

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 colouration 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 recognise 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 centre of cells, covering a smaller area of the skin, or disperse to cover a larger area.

Master of disguise

The fish also has small, protruding skin projections, called dermal flaps that help it hide itself from others. “They are like itty-bitty trees on the skin,” Allen says.

They make the physical edges of the fish look less smooth or “fishlike” and more jagged and complicated, she says. 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. “These animals have tricked my visual system more than once,” Allen says. “The slender filefish’s fast, adaptive camouflage behaviour is remarkable.”

Journal reference: Biological Journal of the Linnean Society, DOI: 10.1111/bij.12598

IMAGES:
On the menu? (Image: Justine Allen/Marine Biological Laboratory/Brown University)
Off menu (Image: Justine Allen/Marine Biological Laboratory/Brown University)

For more on this story and video go to: http://www.newscientist.com/article/dn27753-zoologger-the-fish-that-can-vanish-in-2seconds-flat.html#.VYVpF-t-RIc