Fin massage relieves stress in surgeonfish.

Fortunately for some of its fishy inhabitants, they can call on a masseur to soothe their nerves – the first non-primate known to do so.

Surgeonfish (Ctenochaetus striatus) make regular use of cleaner wrasse (Labroides dimidiatus) to remove their parasites and dead skin. Marta Soaresof the ISPA University Institute in Lisbon, Portugal, noticed that the cleaners seem to offer another service too: they can placate an agitated surgeonfish by rubbing back and forth on its pelvic and pectoral fins.

Soares and her team set out to see if it was the social interaction or the feeling of the massage that kept the surgeonfish at ease. “We know that fish experience pain,” says Soares. “Maybe fish have pleasure, too.”

To test this, they studied two groups of eight surgeonfish. The team confined each fish in a small bucket for a short period to simulate the stresses they would encounter in the wild – predation, conflicts with cleaner fish or competition for food, for instance. They then placed the surgeonfish into tanks with a model cleaner fish. One group was given a stationary model, the other a model that moved back and forth, and so could provide physical stimulation.

All the surgeonfish readily approached the model, but those in the tank with the moving model were able to position themselves beneath it and use its fake fins to gain a back rub. These fish were more relaxed, as measured in terms of the steroid hormone cortisol, which is released in response to stress.