• Why Cooperation Is Vital
    Awake!—2005 | September 8

    • Because of its aquatic habits, the hippopotamus gets cleaned by both feathered and finned “friends.” When a hippo is in the water, fish called black labeos, a species of carp, “vacuum” away algas, dead skin, and parasites​—practically anything clinging to the animal. They even clean its teeth and gums! Other species of fish also help out​—some by cleaning wounds and others by using their long snouts to probe and nibble between the hippo’s toes and in other awkward spots.

      Of course, fish also attract, and thus need to be rid of, unwanted hangers-on, such as crustaceans and external bacteria, funguses, and lice, as well as damaged or diseased tissue. To that end, marine fish usually head for their local cleaning station. There, brightly colored gobies, wrasses, and cleaner shrimps give their clients a good working over, getting a meal for their efforts. Large fish may even have an entire team of cleaners servicing them!

      Client fish have various ways of signaling their desire to be cleaned. For example, some adopt unusual poses​—head down, tail up. Or they might hold their mouth and gills wide open, as if to say: “Come in. I won’t bite.” The cleaners readily oblige, even if the client is a fearsome predator, such as a moray eel or a shark. While being cleaned, some clients change color, perhaps to make parasites more visible. In aquariums without cleaner fish, sea fish “soon become infested by parasites and grow sickly,” says the book Animal Partnerships. “But as soon as a cleaner fish is put into the aquarium it sets to work to clean them up, and as if they know what is happening the others queue up to be cleaned.”