Clownfish and certain damselfish are the only species of fishes that can avoid the potent poison of a sea anemone. There are several theories about how this is accomplished:
Clownfish live in small groups inhabiting a single anemone. The group consist of a breeding pair, which cohabit with a few non-reproductive, "pre-pubescent", and smaller male clownfish. When the female dies, the dominant male changes sex and becomes the female. This life history strategy is known as sequential hermaphroditism. Because clownfish are all born as males, they are protandrous hermaphrodites (pro=first; androus=male). This is in contrast with another form of hermaphroditism, known as protogyny, in which all fish are born as females but can change to males later.
Clownfish lay eggs on any flat surface close to their host anemones. In the wild, clownfish spawn around the time of the full moon and the male parent guards them until they hatch about 6 to 10 days later, typically 2 hours after dusk. Clownfish are omnivorous: in the wild they eat live food such as algae, plankton, molluscs and crustacea; in captivity they can survive on live food, fish flakes and fish pellets. They feed mostly on copepods and mysids, and undigested food from their host anemones.
Depending on the species, clownfish can lay hundreds or thousands of eggs. Clownfish were the first type of marine ornamental fish to be successfully bred in captivity on a large scale. It is one of a handful of marine ornamentals whose life cycle has been closed in captivity.
Clownfish are a popular fishes for the reef aquarium. Clownfish are now tank-bred to lower the number taken from the wild. Compared to wild-caught clownfish, tank-bred clownfish are more disease resistant and also less affected by stress when introduced to the aquarium.
When a sea anemone is not available in an aquarium, they may settle in some varieties of soft corals, or large polyp stony corals. If the fish settles in a coral, it could agitate the fish's skin, and, in some cases, may kill the coral. 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 an entire lifetime.
Clownfish that are far removed from their parents through captive breeding may not have the same instinctual behavior to live in an anemone. They may actually have to be coaxed into finding the anemone by the home aquarist. Even then, there is no guarantee that the anemone will host the clownfish.
The characters Marlin and Nemo in the Disney 2003 animated film Finding Nemo are ocellaris clownfish. The movie was correct in showing that Marlin was unharmed by the anemones sting. Also, the movie was correct in showing that Marlin could be stung by the jellyfish because jellyfish and anemones have different nematocysts. As was shown, clownfish are not immune to the sting of a jellyfish.
Most young children accidently call clownfish Nemo or Nemo fish due to Disney's Finding Nemo film