Fishes survive easily in water but they also are exposed to dirt and vermin.  That is why they use the services of cleaner fish.  The latter are in general small fishes which satisfy their hunger by eating leftovers – small insects, dead or ill skin, algae – off big fishes that they can find.  The big fishes are in no way annoyed in having cleaner fishes as parasites.  On the contrary, they cooperate by opening their mouth so that the small fish can perform its cleaning job.

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