WORKING FOR A LIVING

Cleaner wrasses are small marine fish that feed on the parasites living on the bodies of larger fish. Each cleaner owns a 'station' on a reef where clientele come to get their mouths and teeth cleaned. Client fish come in two varieties: residents and roamers. Residents belong to species with small territories; they have no choice but to go to their local cleaner. Roamers, on the other hand, either hold large territories or travel widely, which means that they have several cleaning stations to choose from. The cleaner wrasses sometimes 'cheat'. This occurs when the fish takes a bite out of its client, feeding on healthy mucus. This makes the client jolt and swim away.

Roamers are more likely to change stations if a cleaner has ignored them for too long or cheated them. Cleaners seem to know this: if a roamer and a resident arrive at the same time, the cleaner almost always services the roamer first. Residents can be kept waiting. The only category of fish that cleaners never cheat are predators, who possess a radical counterstrategy, which is to swallow the cleaner. With predators, cleaner fish wisely adopt an unconditionally cooperative strategy.