Ever since the blockbuster movie "Finding Nemo" hit the big screen Clownfish, particularly the Percula Clownfish species like Nemo, have become the most popular fish in the world, particularly with children (of all ages). In the beginning of the movie Nemo's parents were seen swimming through the tentacles of an anemone which was their home. Without a doubt, this was cute beyond belief, is remembered by almost everyone and connects Clownfish and anemones into eternity.
All, or almost all, saltwater aquarists who have or are planning to have one or more Clownfish in their aquariums would really like to see the Clownfish frolicking in the tentacles of an anemone in their tank. Many new saltwater aquarists who are contemplating adding a Clownfish ask the question: "Do Clownfish need to have an anemone to survive in a tank?" The answer, of course, is "no", but ironically, if there were no anemones in the wild, there would be no Clownfish in the oceans as the Clownfish has no defense against predators in the wild except the stinging tentacles of the anemone, which Clownfish are immune to, thanks, in part, to the thin film of mucus on their skin. In the wild you will never find Clownfish far from their host anemone.
Clownfish and anemones have a symbiotic relationship in the wild. The anemone provides refuge from predators while the Clownfish actually provides a portion of the food required by the anemone in the form of detritus produced by the Clownfish and the small fish which the Clownfish captures and inserts into the many tentacles of the anemone where are then stung and transported to the host anemone's mouth where it is quickly consumed. This ritual is quite fascinating to watch in an aquarium. We used to feed the a Rose Anemone (Entacmaea quadricolor) in a tank at an LFS we worked at by dropping a half dozen Rosy Reds (freshwater minnows) into a tank that had several False Percula Clownfish (Amphiprion ocellaris) hosting the anemone. The Clowns would chase the Rosy Reds all over the tank and catch them one by one, then take them over tot he anemone, stuff them into the tentacles then go after another one. It was quite amusing to watch.
Clownfish do not just dive into any anemone they see. Normally they are quite particular as to which anemones they will host, even rejecting the particular anemone that their Clownfish species typically hosts in the wild. Clownfish/Host Anemone Matches will give you an idea of which Clownfish species will host which anemone species, but this does not always work out right. Quite often a particular Clownfish will completely ignore its supposed "preferred" anemone in favor of another anemone that it isn't supposed to like to host.
Some Clownfish will host something like a coral instead of an anemone. We once had a mated pair of wild caught Red Saddleback or Fire Clownfish (Amphiprion ephippium) that we had purchased for breeding purposes. After we had the pair for about a month the female became quite belligerent and not only killed the male, she would attack anything that would be put in the tank, including a hand that entered the water while cleaning the tank. She would even leap out of the tank when going after your hand if it was near the water's surface. We moved this Fire Clownfish to a 92g show tank and introduced one anemone after another to the tank, but she would not host any of them.
We had a Toadstool Coral in the tank which had grown to quite a large (about 8" across) size and after a month or two she decided to host the Toadstool. She would spend all of her time in the long tentacles of the Toadstool when she wasn't either chasing food or the occasional object that entered the tank. From time to time the Toadstool would withdraw its tentacles and skin over for a week or so before shedding. When the Toadstool was skinned over the Fire Clownfish became more irritable, even attacking a hand that dallied too long while putting food in the tank. When the Toadstool would finally complete its shed, the Fire Clownfish would go back to hosting it again.
While there is no solid research to back it up, many aquarists believe that tank raised Clownfish lose their attraction to anemones after a few generations out of the wild. Anecdotal evidence does seem to show however, that there may be some truth to this theory. If a Clownfish has been in the wild it has to have been in the company of an anemone or it wouldn't be alive, while a tank raised Clownfish may have never seen an anemone its entire life.
Overall, however, different Clownfish species seem to prefer certain species of anemones over other anemones. This would be expected with wild caught Clownfish as they were occupying a certain anemone when they were captured, so it would be like us moving back into the house we grew up in after going on a trip for a few weeks.
Clownfish/Host Anemone Matches shows the generally accepted preferences of certain Clownfishes for anemones. These preferences are not carved in stone, as there will always be exceptions, but for the most part they seem to work a majority of the time.