There are few cases of different species going out of their way to help one another. Indeed, mutual kindness often ends in acrimonious divorce as one species tries to take more than the other is willing to give. One classic mutualism between species that has withstood the test of time involves a group of fishes called cleaner wrasse and their fish clients. As the name suggests, wrasse clean their clients by eating their ectoparasites. Occasionally, however, they cheat their clients; instead of eating the parasites, they nibble their clients. New research by an international group of scientists led by Marta Soares in Portugal sheds new light on how this mutualism persists despite the selfishness of a handful of wretched wrasse. The answer: a good massage.
What does a cheating wrasse do when caught consuming its client? In fish, as in humans, sometimes an apology is offered. Previous research found that cleaners seek reconciliation while massaging their riled clients with their fins. This ‘tactile stimulation’ calms clients and keeps them from leaving the cleaning station. However, being caught once does not stop the wrasse from nipping at her client again later. Tactile stimulation has thus been viewed as a form of manipulation, benefiting the wrasse but harming the client. Soares and her colleagues wondered whether there was not more to the story. Might clients submit to cheaters because they get something out of it?
The researchers hypothesized that tactile stimulation was not unlike a human massage: something to enjoy, while at the same time lowering levels of the stress hormone cortisol (with attendant health benefits). To test the effects of tactile stimulation on client fish, the scientists first stressed them by isolation and next determined whether a massage calmed them down. Because wrasse do not offer massages when prompted, Soares and her colleagues built model wrasse masseuses that either massaged or remained stationary.
The team found that the stressed fish spent significantly more time with the moving model than with the stationary model when given the choice. In addition, their cortisol levels dropped in proportion to the time spent contacting the model. The authors have yet to determine whether this physiological change leads to direct health benefits. If it does, however, the current experiments imply that the client profits from the manipulative tactile stimulation of the cheaters. They cool down with a relaxing rubdown while wrasse nibble away at their client’s mucus.
Health benefits of massage in fish and man reveal a remarkable continuity in the effects of physical contact in vertebrates. We are unexpectedly tied to our aquatic forebears in our shared enjoyment of a calming touch, even if provided by a wooden model fish.