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For anyone who has ever found clowns a bit sinister comes news that even clownfish are dressed to scare 鈥� but the warning message they send to other animals is unique. Whereas many small animals are brightly coloured to let predators know they are poisonous, clownfish use their colourful bodies to advertise the fact that nearby sea anemones are dangerous.

Clownfish are a familiar feature of dentist office aquaria, and also found fame in the film Finding Nemo. But although the fish are instantly recognisable to many, researchers have given little thought to why they are so flamboyantly coloured. Did their bold orange and white stripes evolve as camouflage? Or do they help the fish distinguish members of their species from individuals belonging to related clownfish species?

To find out, Sami Merilaita at the University of Turku in Finland and Jennifer Kelley at the University of Western Australia in Perth explored the scientific literature on the biology and evolution of the 30 or so species of clownfish. Each species of clownfish lives in close association with a species of venomous sea anemone, so the researchers looked at the biology of the anemones too.

Defensive colours

The two researchers found that the clownfish species that hung out in anemones with short, highly-venomous tentacles had fewer white stripes than those that lived with anemones sporting long, less venomous tentacles. The correlation between fish coloration and the defensive ability of the partner anemones was so strong, says Merilaita, that clownfish colours probably evolved as a form of defence system to warn predators to keep away.

This conclusion may not seem surprising: many small animals 鈥� from poison-dart frogs to venomous blue-ringed octopus 鈥� use bright colours to warn off would-be predators. It鈥檚 such a common evolutionary strategy that biologists have given it a name: aposematism.

But the clownfish鈥檚 鈥渁posematic鈥� coloration is unique because it is a warning on behalf of a different species 鈥� the sea anemone.

鈥淭o my knowledge, there are no other animals using warning coloration that would [then] rely on another organism [for defence],鈥� says Merilaita. He thinks the rarity of this situation may be because clownfish and sea anemones share such a close 鈥渕utualistic鈥� relationship: the anemones offer the clownfish protection, while the clownfish defends the anemones from parasites. 鈥淲ithout such a close relationship, the warning signal would be meaningless and unlikely to evolve,鈥� says Merilaita.

For Tim Caro, an evolutionary ecologist at the University of California, Davis, the study highlights the need to investigate whether other animals living in symbiotic relationships have outsourced their warning colours to another species. 鈥淭he study opens us to thinking more broadly about aposematic advertisements,鈥� he says.