i took this photo several years ago from a saltwater aquarium. here is a small information about clownfishes.

Clownfish live in a mutual relationship with sea anemones, or in some case settle in some varieties of soft corals, or large polyp stony corals. Once an anemone or coral has been adopted, the clownfish will defend it.

The anemone is required in nature because reef life is dangerous for small, brightly coloured fish with very poor swimming abilities; in an aquarium lacking predators it is not needed. For this reason, clownfish never stray far from their host. In an aquarium, where they don't have to forage for food, it is very common for clownfish to remain within 6 to 12 inches of their host for an entire lifetime.

Clownfish and damselfish are the only species of fish which can avoid the potent stings of an anemone. There are several theories for how this avoidance is accomplished. Firstly, the slime coating of the fish may be based on sugar rather than proteins so anemones fail to recognize the fish as food and do not fire their nematocysts, or sting organelles.

Secondly, the mucous coating may mimic the anemone's own coating, a theory that is bolstered by the fact that it takes several days for a clownfish to adapt to a new species of anemone. There is no adaptation period when a clownfish is moved to another anemone of the same species.

Thirdly, their unique movements, which are unlike any other fish, may let the anemone know that they are not food. This theory is bolstered by the fact that juvenile clownfish, which have no coating, will immediately seek refuge in any compatible anemone and will not be stung. Juvenile clownfish will not survive for long without the protection of an anemone, and few find one before being eaten.

Clownfish live in their anemone in groups. Usually a female lives with other males. When the female dies the head male changes sex and becomes the female.

ref:wikipedia