Mutualism is one of the most interesting forms of symbiosis, as it is a benefit to both species involved.
An unusual and ‘clever’ example of mutualism that we were fortunate enough to film in PNG was that of the Boxer crab, Lybia tesselata, which carries a pair of small anemones in it chelipeds (claws). When approached by a predator it waves these around presenting the stinging tentacles so as to deter the marauder. The anemones benefit from the small particles of food dropped by the crab during feeding.