SYDNEY: A species of tropical reef fish that regularly visits smaller ‘cleaner fish’ seeks stress release in a similar way to humans, researchers have found – they love a good massage.
The striated surgeonfish (Ctenochaetus striatus), a species that inhabits coral reefs in the Great Barrier Reef off the coast of Queensland in northeast Australia, is the first non-human species found to have the ability to lower stress levels through physical contact alone, without any social aspect.
Scientists had previously thought that small fish called cleaner wrasses exploit the surgeonfish and other larger species by getting a free meal when cleaning debris of their skin. However, a new study has found that the wrasses actually lower the stress of surgeonfish by gently massaging their skin.
“The discovery of a positive effect of physical contact in a reef fish … resolves a long-standing paradox described in cleaning mutualism involving cleaner wrasses and their clients,” said Alexandra Grutter, marine biologist from the University of Queensland in Brisbane, and co-author of the report published in Nature Communications today.
Cleaner wrasses were believed to exploit their cleaning ‘clients’ by feeding on the debris and ectoparacites – tiny parasites that live on the skin’s surface – they remove from the scales. They also help themselves to healthy tissue and mucus from their client fish in the process. And being bigger than the wrasses, the clients provide the added benefit of being able to ward off predators.
Previous studies have suggested that the cleaner wrasses were cheating their client fish by taking advantage and not giving up much in return. Researchers wondered why the client fish would make regular visits to the wrasses – and ‘waiting in line’ for the opportunity to be stimulated (or cleaned) – if they were getting a bad deal.
But it seems as if the cleaner wrasse aren’t as selfish as previously thought, according to Grutter, who found that through direct physical contact with the sturgeonfish, the cleaner fish provide real health benefits.
Massage for stress relief
The positive health effects associated with massage in humans are well documented. The electrical signals transmitted by touch during a massage have fitness-enhancing benefits such as reducing injuries, relieving muscle tension and stimulating blood flow and the nervous system, as well a reduction in stress levels which can be measured in the level of stress-related hormones.
Until now, no non-human species has been proven as seeking out similar physical contact without the added benefit of receiving it from someone with which they have beneficial social bonds. Non-human primates partake in some form of massage, although studies are unclear as to whether or not the physical contact – which includes grooming – is also a way of socialising and creating relationships, as it is difficult to separate the two.
Lower levels of stress hormones
Grutter’s study involved exposing surgeonfish to mechanical cleaner wrasse models, around which the surgeonfish positioned themselves to receive a massage. When compared to their non-massaged counterparts, the stimulated surgeonfish had significantly lower levels of cortisol, a type of stress hormone. “[It] shows that physical contact alone, without any social context, is enough to produce short term benefits,” said Marta Soares, a marine biologist from Unidade de Investigação em Eco-Etologia in Lisbon, Portugal and co-author of the report.
Like the sturgeonfish, many other species of fish regularly pay a visit to the cleaner wrasses, although it is unclear if others fall into the same category as the surgeonfish. “There are no reasons to believe that these benefits are not also extended to remaining fish species that interact with cleaner fish [regularly],” said Soares, “[It] should be rather uniform among vertebrate species.”
Culum Brown, a marine biologist from Macquarie University in Sydney, said the results were exciting, but not necessarily for the reasons the report focused on. “[The study] provides an explanation for our observations of the social interactions between cleaners and clients,” he said. “[But] I still feel there are some other potential explanations (besides the fact that fish enjoy massage as much as humans) and some controls that need to be done to find out if this reduction in stress levels might also be associated with sociality.”