Evolution has produced some masterful solutions for animals to avoid predation. Look no further than the camouflage abilities of the dead leaf butterfly, the flower mantis, or the freaky vanishing octopus for proof. But, a new study takes trickery to a whole new level. Researchers have discovered that the tropical harlequin filefish camouflages its scent by eating the coral it lives on, so as to blend into the olfactory background. It’s the first time a vertebrate animal has been found to practice such smell-deception. Olfactory Chameleons It may come as a surprise that many undersea critters possess a keen sense of smell – it’s the sense that helps salmon return to their home rivers to spawn, for instance. Researchers therefore wondered if some fish used smell-camouflage to hide from predators. They chose to study the harlequin filefish, a tropical fish that lives near coral reefs. The filefish also feeds on the very coral it calls home. And, this coral, Acropora spathulata, is not particularly nutritious – at least not the best option around. So, scientists wondered, could there be another driving factor? The Hunger Games To answer this question, researchers placed sixteen filefish into four tanks. Two tanks were exclusively fed the Acropora coral, while the other two received a different species, Pocillopora damicornis, to feast on. Researchers first tested the fish’s chemical camouflage by bringing in some expert tasters – species of crabs that feed on one of the two types of coral. If anyone should know the smell of coral, scientists reasoned, it was these crabs. Researchers placed sections of pipe in the tank: one contained a filefish and the other a piece of coral. The crabs couldn’t see what was inside the pipes, forcing them to rely on smell alone. In both cases, researchers found, crabs would prefer the filefish fed on tasty coral over a piece of unappetizing coral – indicating that the fish smelled like their food. Also, though most crabs preferred tasty coral over a filefish fed on tasty coral, a high percentage still opted for the fish itself. In a second experiment, researchers employed a real-life predator of the filefish: cod. When filefish smelled the same as the coral in their tanks, cod were uninterested in them, but when they stuck out scent-wise then cod were quite interested indeed. The results were published in the journal
Proceedings of the Royal Society B
this week. Masters of Disguise The filefish, then, are able to thwart predators through a chemical cloak of invisibility, just based on what they eat. The researchers say this is the first evidence that chemical crypsis caused by diet can be seen in vertebrates – proving that vision isn’t the only sense to be fooled. And that the animal world is even craftier than we’d originally thought.
