Nothing beats a good spa treatment to erase the tension of a tough week at workâ€”the manicure, pedicure, massageâ€¦the works. Reef fishes (and not just the females) know how to benefit from a little pampering at stations manned by cleaner wrassesâ€”the wrasses set up shop in a particular spot on the reef (or your tank), and soon the other fish know where to go to get a little TLC. We have always looked at the cleaning function and mutualismâ€”the â€œpatronâ€ gets cleaned of parasites and the cleaner gets a good mealâ€”as the sole purpose of the transaction. But is that all that goes on?
It turns out the answer is decidedly no. In addition to nibbling off parasites, it was noted that cleaner wrasses â€œmassageâ€ their clientsâ€”gently stroking them with their fins. They seemed to give extra attention to piscivorous fish (that might make a meal of the wrasse), and even more attention when the client was hungry. So the massage seems to pacify the client fish and make it less likely the wrasse gets eaten.
When researchers tested this in aquaria, they found something even more surprising:Â fish which have gotten the full massage treatment are less likely to be aggressive to ALL the other fish in the tank. Aquaria were set up with an assortment of fish, plus or minus cleaner wrasses, and observed. To illustrate the experiment, itâ€™s convenient to think of a closed aquarium like high school; you have your football jock/bullies (here they used slingjaw wrasses), your, er, fish geek targets (fang blennies and two-tone wrasses), and everyone else. In normal conditions the slingjaws will bully the blennies. This happens often in our aquaria when bigger or more aggressive species are mixed with peaceful community fishâ€”the jocks beat up the geeks. But, in tanks with cleaner wrasse stations, the bullies attacked only half as often. One might think that this was simply because the bullies were too busy getting cleaned, but the calming effect lasted even when the cleaning stopped.Â In fact, it seemed the longer and more thorough the massage aspect, the more docile the client fish was rendered.
Can reefkeepers exploit this natural mutualism? It would appear so in tanks large enough to provide sufficient food for the cleaner wrasse. One of the largest contributors to disease susceptibility and increased mortality in fish is stress. Adding a cleaner wrasse would create a â€œsafe havenâ€ for fishes to visit, and the fishes that get serviced leave not only feeling clean and sexy, but less aggressive. This benefits even the fish that do not get cleaned as the general tank atmosphere would be more peaceful. This benefits the wrasse as wellâ€”relaxed fish which are not as concerned with avoiding predators spend more time at cleaning stations, this giving the cleaner wrasse a better meal.
Cheney, K.L., Bshary, R., and Grutter, A.S. 2008. Cleaner fish cause predators to reduce aggression toward bystanders at cleaning stations. Behavioral Ecology 19(5) pp 1063-1067.
Photo credit: Nick Hobgood