Why do big fish line up to be cleaned by tiny ones?

Big fish line up to be cleaned because tiny cleaners eat the parasites and dead skin that bother them. This is called cleaning symbiosis: two species help each other, one (the cleaner) removing and eating parasites from the surface of the other (the client). On coral reefs the best-known cleaners are small fish called wrasses, some of which feed almost entirely by cleaning larger fish and other marine animals. The client gets rid of pests, and the cleaner gets a meal.

The deal is run like a little shop. Cleaner wrasses work at cleaning stations, and when a visitor swims close they greet it with a dance-like motion, moving their rear end up and down. Bigger fish recognise a cleaner by the stripe along its body and by how it moves. The client then adopts a pose that lets the cleaner reach its body surface, its gills and sometimes its mouth. Cleaners are even safe from the large predators they work on, because both sides benefit from the service.

Where the arrangement is fully relied on, it pays off: in regions where bluestreak cleaner wrasses live only on clients' parasites, fish with access to cleaning have better body condition than fish without it.

It is not a perfect friendship. A cleaner sometimes cheats, eating mucus or healthy tissue instead of parasites, and some fish pretend to be cleaners. A blenny with the same look tears small pieces of flesh from bigger fish instead of cleaning them. Biologists have argued for decades about whether cleaning is selfless co-operation, mutual selfishness or even one-sided exploitation.

The idea is ancient. The Greek historian Herodotus described cleaning behaviour around 420 BCE, though his example, birds serving crocodiles, appears to occur only rarely.