Cleaner fish are fish sought out by client fish who queue up to have their parasites removed.

Dr Grutter said the interactions between the cleaner fish Labroides dimidiatus and its client fishes are a well-known example of mutually beneficial behaviour involving different species.

Cleaners may cooperate and remove parasites from client fish, however they may also cheat by feeding on client mucus, which they prefer. As such there is a temptation for cleaners to cheat.

"Our experiments suggest image scoring is one of several alternative mechanisms used by clients to avoid exploitation by cheating cleaners," Dr Grutter said.