An Orange Spotted Filefish

An orange-spotted filefish, Oxymonacanthus longirostris, is seen in front of its favorite food, corals in the Acropora genus, in 2010. Orange-spotted filefish (Oxymonacanthus longirostris), which feed exclusively on Acropora corals in Australia, ingest chemicals in the corals

that cause them to take on the scent of their food. This hides the filefish from their own predators, such as cod. 

This is the first time scientists have discovered an animal chemically camouflaging itself via its diet, said study leader Rohan Brooker, currently a postdoctoral student at the Georgia Institute of Technology in Atlanta.

Visual camouflage is well known in the animal kingdom, from the cheetah’s spots to stick-like stick insects to owls that blend into trees. These masters of disguise are especially impressive to humans, because we rely so heavily on vision. However, many other animals interpret the world mostly by smell.

“Most of the literature on camouflage focuses on visual methods, but many animals use smell more. For these animals, chemical camouflage may be far more important to stay hidden,” said Brooker, whose results appear December 9 in the journal Proceedings of the Royal Society B.

Brooker got the idea for studying filefish camouflage when he read about the Biston robustum caterpillar, which takes on the smell of the plants it eats via molecules in those plants. By doing so, the caterpillar “hides” from predatory ants. 

Since filefish feed exclusively on Acropora and coral reefs are places where predators often hunt, Brooker believed the filefish might also be using a form of chemical camouflage.

Brooker, then at James Cook University in Australia, and colleagues captured filefish near Lizard Island Research Station on the Great Barrier Reef. The team placed the fish in large aquariums and divided them into two groups—one that ate an exclusive diet of the coral species Acropora spathulata and one that ate only Pocillopora damicornis, which is not part of the filefish’s regular diet. The fish ate this diet for four weeks.

To find out if the fish smelled like coral, the team also captured two species of small crab, Tetralia glaberrima and Trapezia cymodoce, which dwell inside Acropora and Pocillopora, respectively. These crabs were added to both fish groups.

As expected, T. glaberrima crabs clearly preferred the smell of the filefish that had eaten Acropora over those fed Pocillopora—indicating the fish were taking on Acropora’s scent.The filefish’s scent was so strong, in fact, that some crabs treated the filefish as if they were coral, the study says. 

Hiding in Plain Sight
But the team needed to take the work one step further and find out if the coral smell masked filefish from predators.

So Brooker and colleagues caught a predatory cod species and added it to the aquariums.

The cod was less active and spent less time hunting around the filefish that ate Acropora than around the fish that ate Pocillopora, indicating that the cod could not detect the Acropora-eating filefish.

“It’s a clever study design and a nice contribution to the literature on chemical camouflage. They showed that by smelling like coral, filefish can blend in and avoid predators,” said Jelle Atema, a marine biologist at Boston University who was not involved in the study.

Atema also points out that the results of the study may not be quite as unique as the researchers think, as he has documented chemical camouflage among freshwater catfish.

However, this was the first time that diet was documented as the source of this camouflage in fish.

“I suspect that this method of hiding is probably a lot more common than any of us guessed,” study leader Brooker concluded.