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Although sea anemones look like flowers, they are predatory animals. These invertebrates have no skeleton at all. They live attached to firm objects in the seas, usually the sea floor, rock, or coral, but they can slide around very slowly. Sea anemones have very long lives. Hermit crabs sometimes attach sea anemones to their shells for camouflage.

There are over 1,000 species of anemones found in coastal waters worldwide, in shallow waters (including coral reefs), and in deep oceans. In tropical waters clown fish always live near anemones. They are immune from (and protected by) the stinging tentacles. The clown fish help the anemone by cleaning the tentacles (as the fish eat detritus) and perhaps by scaring away predators.

Sea anemones come in many shapes, sizes, and colors. Radially symmetric, they have a columnar body with a single body opening. Their mouth is surrounded by tentacles. The tentacles protect the anemone and catch its food. Sea anemones are carnivores that eat fish, mussels, zooplankton (like copepods, other small crustaceans, and tiny marine larvae), and worms. They are studded with microscopic stinging capsules. Sea anemones are usually about one to four inches (2.5 to 10 cm) across, but a few grow to be six feet (1.8 m) across. Sea anemones are eaten by very few animals. Their predators include the gray sea slug.

Sea anemones reproduce by lateral fission (in which an identical animal sprouts out of the anemone's side) and by sexual reproduction (in which anemones release eggs and sperm, producing free-swimming larvae).