Researchers have discovered that Harlequin filefish use a sophisticated form of “chemical camouflage” to hide from predators by smelling like the coral they live in.

How the Chemical Camouflage Works

• “You Are What You Eat”: Harlequin filefish feed almost exclusively on branching Acropora corals. By digesting this coral, the fish transfers the coral’s chemical signature to its own body.
• The scent of the fish comes from its mucus, which is continuously released into the water. Chemicals from the eaten coral are absorbed into this mucus, making the fish smell just like the surrounding coral branches.
• While many insects use this type of camouflage, this study provided the first evidence of diet-induced chemical camouflage in a vertebrate. (The study was led by Rohan Brooker at James Cook University)

To prove this, researchers conducted studies with predators like the blue-spotted rock cod and small, coral-dwelling crabs:

• Fooling the locals. In tests, small crabs that live in coral branches were unable to tell apart the smell of their home Acropora coral and the smell of a filefish that had eaten that coral.
• Avoiding predators. When predatory cod were placed in tanks with filefish, they were much less likely to hunt or spend time near filefish that had recently eaten Acropora coral compared to those fed a different diet.
• The cover is blown. If the filefish hides in a different species of coral than the one it has been eating, it no longer smells right, and predators can easily track it down.

The filefish doesn’t just use scent; it combines this with several other, highly evolved, visual tricks to stay hidden:

• Visual Mimicry: The filefish has a spotted pattern that perfectly matches the appearance of Acropora coral polyps.
• Disruptive Coloration: The coloring on its body breaks up its outline, making it blend into the coral structure.
• Physical Masking: The tip of its tail is colored to look like the growing tip of a coral branch.
• “Locking” In: When resting, it can wedge itself into coral branches using its dorsal spine, staying completely still.

Do other fish or animals do this?

While the filefish study was the first to document this in fish, chemical camouflage is thought to be more widespread.

• Freshwater Catfish: Researchers have documented similar of chemical camouflage among freshwater catfish, which can make themselves “chemically invisible” to predators.
• Caterpillars: Certain caterpillars eat plants and then incorporate the plant’s compounds into their own cuticle to avoid being eaten by ants.
• Chemical “self-perfuming” likely exists in other coral-feeding organisms that live on the same food source, such as specific butterflyfish juveniles.

Predators often rely more on smell than sight to find prey on the coral reef. Scientists believe that chemical camouflage may be much more common in nature than previously thought.