Co-operation can sometimes go beyond simple commensalisms and take the form of a real partnership
-- a mutualism. One dramatic example is the way that some species of fish and shrimps clean parasites from the skin of larger fish. The high densities of fish found on coral reefs gives rise to large numbers of lice and worms that live on the fish and draw their nutrition from them. At best, these infestations cause an irritation to the fish, at worst the parasites can seriously weaken their hosts. The cleaners therefore provide a vital service to other fish -- usually referred to as their "clients."
Which animals provide parasite cleaning?
Many different animals act as cleaners during a part of their lives. Various fish species, including the French angelfish (Pamacanthus paru) and several wrasses and gobies, perform this role as juveniles and some shrimps also fulfill this function. But the most famous are fish such as the wrasse Labroides dimidiatus, which spends its entire life as a cleaner. It is extremely active and can clean over 2,000 fish in a single day.
How does this work in practice?
Individual cleaner wrasses establish specific cleaning stations where they remain throughout the day, waiting for clients and aggressively defending these stations from other cleaner wrasses. And it is not just the cleaners that defend their stations; the clients themselves can sometimes end up waiting for the attentions of the wrasse. Where this happens, aggression can sometimes break out as fish try to jockey for priority.
Amazingly, the cleaners can distinguish between clients whose own territoriality means they have to remain in one place and cannot visit another cleaning station, and clients who are free to move to other stations. These latter "VIP" clients are given priority treatment by the cleaners.
Is this true mutualism?
While cleaners provide an undoubted benefit for their clients, they also occasionally supplement their diet by biting small chunks off them! Annoyed clients sometimes give chase, but remarkably the cleaners provide them with a better service next time, showing that they are capable of individually recognizing their clients. To try to prevent an aggressive client from pursuing the matter, the cleaners dash to the next client in line -- often a predatory fish -- and get to work on them. Cleaners also refrain from biting while a "VIP" client is waiting for service. This is because fish in the line watch the behavior of previous clients and if those dash off suddenly, so do the waiting fish. However, cleaners are not so careful when only their territorial clients are waiting!
How do cleaner wrasses advertise to their clients?
There is always a danger that a small fish, such as a cleaner wrasse, could end up as a quick snack for their larger clients. To try to prevent this, the wrasse advertise their role as cleaners via their distinctive and unique color patterns.