GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY
Harlequin sweetlips fish (Plectorhinchus chaetodonoides) with a cleaner wrasse (upper centre) inside a large sponge. Cleaner wrasse often make a specific object, such as a large sponge or coral outcrop, into their home. Larger fish then visit this "cleaning station" to have parasites picked from their skin. This is a symbiotic relationship: the visitors have harmful parasites removed and the wrasse get to eat them. Photographed in Borneo, Indonesia.
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