Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species are recognized: one in the genus Premnas, while the remaining are in the genus Amphiprion. In the wild, they all form symbiotic mutualisms with sea anemones. Depending on species, clownfish are overall yellow, orange, or a reddish or blackish color, and many show white bars or patches. The largest can reach a length of 18 centimetres (7.1 in), while the smallest barely achieve 10 centimetres (3.9 in).

Clownfish are native to warmer waters of the Indian and Pacific oceans, including the Great Barrier Reef and the Red Sea. While most species have restricted distributions, others are widespread. Clownfish live at the bottom of shallow seas in sheltered reefs or in shallow lagoons.

Clownfish are omnivorous and can feed on undigested food from their host anemones, and the fecal matter from the clownfish provides nutrients to the sea anemone. Clownfish primarily feed on small zooplankton from the water column, such as copepods and tunicate larvae, with a small portion of their diet coming from algae, with the exception of Amphiprion perideraion, which primarily feeds on algae. They may also consume the tentacles of their host anemone.

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. 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. 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 the clownfish use their bright coloring to lure small fish to the anemone, and that the 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 behaviours 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.

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.
  • 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.

Clownfish are the best known example of fish that are able to live among the venomous sea anemone tentacles, but there are several others, including juvenile threespot dascyllus, certain cardinalfish (such as Banggai cardinalfish), Bucchich's (or anemone) goby and juvenile painted greenling.

Clownfish make up 43% of the global marine ornamental trade, and 25% of the global trade comes from fish bred in captivity, while the majority are captured from the wild, accounting for decreased densities in exploited areas. Public aquaria and captive breeding programs are essential to sustain their trade as marine ornamentals, and has recently become economically feasible. It is one of a handful of marine ornamentals whose complete life cycle has been closed in captivity. Members of some clownfish species, such as the maroon clownfish, become aggressive in captivity; others, like the false percula clownfish, can be kept successfully with other individuals of the same species.

When a sea anemone is not available in an aquarium, the clownfish may settle in some varieties of soft corals, or large polyp stony corals.[citation needed] Once an anemone or coral has been adopted, the clownfish will defend it. As there is less pressure to forage for food in an aquarium, it is common for clownfish to remain within 2-4 inches of their host for their entire lifetime. Clownfish, however, are not obligately tied to hosts, and can survive alone in captivity.