Obvously an anemone is not needed to keep clowns, it is just more interesting. I agree w/slakker, what is your source?
Here is one : )
University if Michigan Museum of zoology
Animal Diversity Web
Amphiprion ocellaris
clown anemonefish
Behavior:
Amphiprion ocellaris exists in a symbiotic relationship with sea anemones, specifically, Heteractis magnifica, Stichodactyla gigantean, and Stichodactyla mertensii (Myers 1999). These fish are dependent upon the anemone for shelter. In open waters these fish are more susceptible to predators and are poor swimmers. In addition, anemones provide protection for the nests (Fautin and Allen 1992). The anemones are observed to generally do better with a host fish and may also benefit possibly from fish consumption of parasites and increased water circulation from fanning (Fautin and Allen 1992). Anemonefishes are protected from the sting of the anemone by their mucus (Fautin and Allen 1992). There is a period of acclimatization that must occur before the fish is immune to the anemone sting. This involves a process in which the fish swims around the anemone rubbing its belly and ventral fins on the ends of the tentacles (MarineBio 1998).
As the juveniles search for an anemone, their survival is contingent upon finding an anemone to reside in, but this process is complicated by the dynamics within the anemone. Because of the hierarchical dynamics within the anemone, the new juvenile fish enters the system at the bottom and is exposed to the worst aggression and may be driven away (Thresher 1984). A. ocellaris are able to find one of the three species of sea anemones by olfactory clues, due to imprinting that occurred while in the nest (Arvedlund and Nielsen 1996). (Arvedlund and Nielsen, 1996;