Cleaner Fish Make Other Reef Fish Smarter

By eating parasites that make the pufferfish's life a misery, the cleaner not only makes its client healthier, but smarter as well. Hans Gert Broeder/Shutterstock

If you're having trouble learning, maybe you need to shed some parasites. At least that's the case for coral reef fish, whose parasites interfere with their capacity to learn.

The beautiful waters of the Great Barrier Reef swarm with both internal and external parasites that feast on fish. Cleaner fish have made a niche for themselves, consuming more than 1,000 parasites a day off other grateful fish. In the process, they have established a complex and fascinating symbiosis with these “client” fish.

Fish with access to “cleaner stations”, where cleaners wait for clients, have been shown to grow faster and achieve greater sizes than cleanerless counterparts. In studying the benefits clients receive, Dr Lexa Grutter and Dr Derek Sun of the University of Queensland discovered that being cleaned improved the learning capacity of fish.

“When we're ill, our body shifts energy away from our brain to fight off the illness,” Sun said in a statement.

In Proceedings of the Royal Society B, Grutter, Sun, and their co-authors describe the responses of fish raised on reefs with and without cleaner wrasse (Labroides dimidiatus) to learning tests, as well as some fish artificially infected with parasites.

In one test of the fishes' learning ability, the authors repeatedly placed food with no smell on one of two plates and observed how easily the fish learned to associate food with that plate. They then investigated whether fish that had learned to associate one plate with food were able to adjust when the food location was swapped. A third test placed an abstract image in front of a food plate, as well as a different image before a non-food plate, and tested the fishes' capacity to learn to associate the correct image with food when the plates' locations were randomly shuffled.

Exposure to parasites didn't produce statistically significant effects on fish scores on the first two tests. However, fish raised on cleanerless reefs performed much worse on the last test, as did previously cleaned fish deliberately infected with parasites in the lab.

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