BioEd Online

It turns out that even for a fish, eavesdropping might help in figuring out who you can trust not to stab you in the back. The sea bream Scolopsis bilineatus apparently spies on cleaner fish Labroides dimidiatus before deciding which of them to employ.

Cleaner fish help out their larger 'clients' by eating parasites, for which they gain protection — although their preferred diet is the more nutritious mucous coating of the bigger fish. 'Cheating' behaviour, in which cleaners tuck into the mucous not the parasites, is rare, prompting biologists to wonder why.

Redouan Bshary of the University of Neuchâtel, Switzerland, and Alexandra Grutter of the University of Queensland, Australia, set up a series of experiments to work out why the two species seem to get along so well instead of turning on one another. It seems that by feeding on parasites, instead of their preferred mucus, while other clients watch them at work, the cleaner fish build a reputation that stands them in good stead.

Nosy neighbours

Bshary and Grutter placed a client fish in the centre of an aquarium divided by two partitions. From here it could watch two cleaners, one at either end of the tank, through a two-way mirror. This way, the client could observe the cleaners without the cleaners being aware of it.

One of the cleaners appeared to cooperatively interact with a model client fish that had had prawn, the cleaner fish's preferred food, smeared on it. The other cleaner fish swam randomly around its prawn-free model.

Having observed the two cleaners, the client fish chose to spend significantly more time with the more reputable and seemingly helpful cleaner, the researchers report in Nature¹. "The client fish pay attention to what the cleaners are doing," says Bshary.

In a second experiment, Bshary and Grutter set up an artificial pulley system attached to a series of plates offering food to the cleaner fish. On one set of plates were prawns whereas the other plate held less appetizing food flakes. As soon as the cleaner fish fed on prawn, the system would automatically remove both plates. Eventually the cleaner fish learned to eat the flakes even though both plates were offered.

This shows that the cleaners are prepared to settle for second best in order to ensure a reliable food supply. "They are willing to feed against their preference," says Bshary.

Fishy friends

Both experiments show that a state of mutualism can exist between the two fish, the researchers say. The cleaners feed on their second-choice diet to reduce conflict with their current client fish and build a reputation that allows future employment by other picky clients.

Such 'image-building behaviour' is a well-known facet of human behaviour. Military generals, for instance, often base their tactics at least partly on what intelligence they perceive the other side to have, based on monitoring their enemy's behaviour. But seeing such sophisticated behaviour in animals such as fish is a surprise, says Lee Dugatkin, an animal-behaviour expert at the University of Louisville, Kentucky.

"That level of strategy behaviour across species is remarkable," says Dugatkin, adding that the find is "a major advance in the study of evolution of social behaviour".

Bshary thinks that other animals might also unconsciously manipulate their perceived image in this way, regardless of how 'smart' they are. He suspects that hormones, rather than conscious choice, help to dictate the fishy cooperation.

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