Clownfish and sea anemones have a symbiotic, mutualistic relationship, each providing a number of benefits to the other. The individual species are generally highly host specific, and especially the genera Heteractis and Stichodactyla, and the species Entacmaea quadricolor are frequent clownfish partners. The sea anemone protects the clownfish from predators, as well as providing food through the scraps left from the anemone's meals and occasional dead anemone tentacles. Biting or even eating anemone tentacles is common. In return, the clownfish defends the anemone from its predators, and parasites. The anemone also picks up nutrients from the clownfish's excrement, and functions as a safe nest site for the clownfish. The nitrogen excreted from clownfish increases the amount of algae incorporated into the tissue of their hosts, which aids the anemone in tissue growth and regeneration. It has been theorized that activity of the clownfish results in greater water circulation around the sea anemone. Studies on anemonefish have found that clownfish alter the flow of water around sea anemone tentacles by certain behaviors and movements such as "wedging" and "switching." Aeration of the host anemone tentacles allows for benefits to the metabolism of both partners, mainly by increasing anemone body size and both clownfish and anemone respiration.
Clownfish and certain damselfish are among the few species of fish that can avoid the potent poison of a sea anemone. There are several theories about how they can survive the sea anemone poison:
The mucus coating of the fish may be based on sugars rather than proteins. This would mean that anemones fail to recognize the fish as a potential food source and do not fire their nematocysts, or sting organelles. Some theorize that eating anemone tentacles is used to process sugar and maintains the mucous coating.
The coevolution of certain species of clownfish with specific anemone host species and may have acquired an immunity to the nematocysts and toxins of their host anemone. Experimentation has shown that Amphiprion percula may develop resistance to the toxin from Heteractis magnifica, but it is not totally protected, since it was shown experimentally to die when its skin, devoid of mucus, was exposed to the nematocysts of its host.