Anemones belong in the Phylum Cnidaria, related to hard and soft-corals and hydroids. Early classifiers termed anemones "zoophytes" or animal plants, in reference to their flower-like appearance, but modern classifications group the anemones, jellyfish, hydras and corals as the phylum Cnidaria, Within the Cnidaria, Anemones are placed in the Class Anthozoa; Anemones are separated from other anthozoans in the sub-class Zoantharia with two main orders: The Actinaria or the so called true anemones and the Ceriantharia, or so called tube anemones which have greatly elongate bodies without basal discs, secreted mucous tubes buried in soft substrates.
All Anemones are efficient carnivores, capable of catching fish and small crustaceans by paralyzing them with stinging tentacles and devouring their prey. Although anemones may seem firmly attached to the rocks on which they sit, they are capable of very slow creeping from place to place.
Sea anemones live throughout the world's oceans, from poles to equator, and from the deepest trenches to the shores, as do fishes. But no one kind of either lives in all places. Of nearly 1000 species of sea anemones, only 10 are known to host anemonefishes, some species of these sea anemones harbour shrimps and crabs. These anemones, and their anemonefishes, shrimps and/or crabs exist only in shallow water, because within the cells of an anemone's tentacles and oral disc live microscopic, single-celled, called zooxanthellae. Like all plants, they require sunlight for photosynthesis, a process in which solar energy is used to make sugars from carbon and water. Some of these sugars fuel the algae's metabolism, but most of them "leak" to the anemone, providing energy to it. Therefore, the anemones that are host to clownfishes must live in sunny places. The amount of light in the sea diminishes rapidly with depth because water filters out sunlight. Turbidity also diminishes light penetration. So these anemones live at depths of no more than about 50 m, generally in clear water.