By Doreen Walton
Male fish that nibble parasites off larger fish punish females of the species when they bite and drive off the bigger fish.
The cleaner fish sometimes bite bigger client fish to get a mouthful of mucus, which they prefer to the parasites.
This punishment by males on behalf of victims appears altruistic but a study in Science journal suggests otherwise.
Males seem to be acting in the interest of their stomachs by chasing females who cause the food source to leave.
"The male's dinner leaves if the female cheats," explained lead scientist Nichola Raihani from the The Zoological Society of London.
"By punishing cheating females, the males are not really sticking up for the clients but are making sure that they get a decent meal."
But when male cleaner fish, Labroides dimidiatus, usually larger than females, take a bite of mucus and drive off dinner, they get away with it.
"The males are less well behaved than the females a lot of the time but perhaps part of the reason the males are so likely to cheat is that females never punish males," Dr Raihani told the Science podcast.
True altruism is rare
For the experiment, cleaner fish were trained to eat off plates containing prawns, which they find tastier, and fish flakes. If they ate prawns, the plate was immediately removed.
The fish treated the plates like client fish, even performing a type of massage on them with their fins, a practise meant to encourage real clients on the reef to stay.
The researchers say that in humans "third party punishment" is often seen in terms of group-level benefits but this study on fish shows it working to the individual's advantage.
Dr Raihani believes true altruism is rare.
"When you see something that looks like it's altruistic, if you look hard enough, there's normally going to be a benefit somewhere down the line for the person that's doing that supposedly altruistic act," she said.