Sea anemones that are host to clownfishes, like many tropical actinians and some temperate ones, harbour unicellular algae within the cells of their tentacles and oral disc (see Introduction). A portion of the sugars produced by these plants through photosynthesis are "leaked" to their host. This may be the anemone's major source of energy. The widely flared oral disc of many host actinians serves not only to accommodate fish, but its large surface area is well adapted for intercepting sunlight.
However, actinians, like all coelenterates, capture and digest animal prey with their nematocysts. We have found small fish, sea urchins, and a variety of crustaceans (shrimps and crabs) in the coelenteron of host anemones. They also appear to feed on planktonic items conveyed by the currents. Although the energy they derive from photosynthesis may be sufficient to live, the anemones need sulfur, nitrogen, and other elements in order to grow and reproduce. These animals are not voracious predators: their prey probably consists of animals that bump into them (e.g. a fish fleeing a more active predator) or stumble over them (e.g. a sea urchin, which has no eyes). Therefore, the supply is probably small and irregular. A more predictable source of these nutrients may be from wastes of their symbiotic fish. This issue deserves to be studied scientifically. Anemones of some species are capable of absorbing nutrients directly from seawater through their thin tissues, and that may be another source of nutrition for these animals as well.