Sea anemones are animals belonging to the phylum Cnidaria, which includes the jellyfish, corals and sea pens. They live in all oceans from the shore to a depth of 10,000 metres, and range in size from one centimetre to almost two metres in diameter. They attach themselves to rocks, wharves and other hard surfaces, or construct burrows in mud and sand. Sea anemones have a flat upper surface, with a central mouth surrounded by tentacles, a tubular body, and a flat base that attaches to the substrate. A sea anemone uses its tentacles to capture prey and defend itself against predators.
Every tentacle is covered with thousands of tiny stinging capsules called nematocysts.
Each capsule contains a coiled hollow thread with a barb on the end. The threads carry a minute amount of poison capable of paralysing or killing small animals. When a small fish, shrimp or crab comes into contact with the tentacles, hundreds of the capsules burst open and fire their barbed threads like harpoons, which pierce the skin of the animal and inject their poison. The thread remains attached to the tentacle so, like a harpooned whale, the victim is held by its captor. The anemone moves all the nearby tentacles into position to sting and hold its prey until it is subdued by the poison.