Surgeonfish form schools numbering on the thousands at the Southern Line Islands, grazing the reef. (Courtesy of National Geographic Explorer-in-Residence Enric Sala)
Calm down, dear, I’ll rub your fins

Life on the reef can be stressful. Fortunately for some of its fishy inhabitants, they can get a massage to soothe their nerves.

Surgeonfish (Ctenochaetus striatus) make regular use of fish called cleaner wrasse (Labroides dimidiatus) to remove their parasites and dead skin. Marta Soares of the ISPA University Institute in Lisbon noticed that the cleaners seem to offer another service: 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 some 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 fake 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 by their hormone cortisol, which is released in response to stress.

Todd Anderson, a biologist at San Diego State University, who studies the ecology of reef fishes, says the study surprised him. “Normally I would think that physical contact would elevate stress in fish — as it should, for example, in prey experiencing attempted capture by a predator,” Anderson says. “However, the contact [in this study] is initiated by the client fish for an often beneficial relationship [that includes] removing parasites.”

Soares says this research may mean that pathways for sensory information processing in fish are more similar to those in humans than previously thought.

“Humans go to have massages when we feel sick or just to feel better, so maybe the reasons are basically the same,” she says.

Chelsea Whyte, New Scientist