Nature is full of impressive camouflage and mimicry, from color- and texture-changing octopuses to plant-impersonating stick bugs. To us and other animals that rely heavily on vision, these creatures are masters of disguise. But some other animals hunt by scent, and being out of sight isn’t the same as being out of mind.
To hide from scent-based predators and their strong sniffers, animals can employ chemical camouflage instead of the visual kind. Some of them limit or mask the chemical cues they produce. Others alter their scent to match that of the enemies they’re trying to evade. Some squirrels, for example, chew on shed snake skins and then lick themselves, which leaves the odor compounds from the skin on their fur and makes them smell more like a snake than themselves.
In Australia, scientists have found a fish that puts a different twist on this tactic. Instead of hiding by smelling like its predators, the harlequin filefish matches its scent to its own food so that it blends in with the olfactory background.
Harlequin filefish feed almost exclusively on Acropora corals and use them as overnight shelters. They mimic the coral visually with skin color, spot patterns, and sleeping positions that make them look like coral polyps. But biologist Rohan Brooker wanted to see if the chemical resemblance between the fish and their food was as strong as the visual one.
To find out, he turned to a panel of experts in coral smell: a crab species that lives and feeds on the same coral as the filefish, and another species that has a similar relationship with a different coral. Each crab was plopped in the middle of an aquarium that had a perforated chamber on either end, holding either a filefish or a piece of coral that the crab could smell but not see. The crabs could sniff out the difference between two types of coral and spent more time around the chamber that held the one they usually ate, but had a harder time distinguishing their favored coral from a filefish that had been eating that coral. They also showed more interest in filefish that had been eating their favored coral than a real piece of a different kind of coral, all of which suggest the fish smelled like their food.
The crabs had been fooled, but the researchers still wanted to see how well the filefish’s smell hid them from their actual predators, so they introduced cod to the aquarium and presented them with different combinations of filefish and coral whose smells either matched or were different. The cod paid little attention to the chambers when the fish were paired with the coral they usually fed on. When another coral was used and there was a mismatch in smells, though, the cod noticed and spent more time inspecting the chambers and looking for the filefish, suggesting they had picked up on the odor of potential prey.
To the noses of their predators and even bystanders, it seems that filefish are what they eat, and gain a chemical disguise from their meals—the first known case of a vertebrate doing so.