It's not exactly a secret that many humans derive pleasure from massages, and previous research has shown other primates enjoy it as well. And now, we've found the first non-primate to use massages to relieve stress: the surgeonfish.
Surgeonfish are one of a number of fish species that enlist tiny cleaner wrasse fish to clean it, scraping off dead skin and potential parasites. Previous observations have also suggested that the wrasse rubbing on the pelvic and pectoral fins of the surgeonfish might serve to calm it down, in much the same way a masseur can relieve stress in a human.
To put this idea to the test, Marta Soares of Portugal's ISPA University Institute ran an experiment featuring two groups of eight surgeonfish. The fish were first exposed to all the stresses they would naturally experience in the wild, which included everything from predators to competition over food. They were then place in tanks that had models of cleaner wrasse. One set of models stayed completely stationary, while the other could move back and forth, which would allow them to simulate rubbing on the surgeonfish's fins.
While all the surgeonfish tried to get some use out of these fake cleaner fish, only those in the tanks with the moving models had any success. By positioning themselves below the model's "fins", the surgeonfish could get their equivalent of a backrub. It wasn't cleaning them, but it apparently still had an effect - these fish had significantly lower levels of the stress hormone cortisol than the fish in the tanks with stationary models. It seems this fishy back rub relaxed the surgeonfish just as a human massage might.
San Diego State researcher Todd Anderson comments on the research:
"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. However, the contact [in this study] is initiated by the client fish for an often beneficial relationship [that includes] removing parasites."
For her part, Soares says that this apparent pleasure and stress relief indicates a previously unconsidered benefit for the surgeonfish in their relationship with the cleaner wrasse. It also might well mean that fish experience tactile sensations in a way that's far more similar to humans than previously thought.