Sea anemones are unique group predatory animals that are closely related to corals and jellyfish. Like jellyfish, sea anemones can sting prey with nematocyst cells which either sting or inject a toxin. They mainly prey on mussels, sea urchins, small fish, and crabs. Anemones are highly sensitive to industrial pollution, oil spills, sewage, or sludge. Many anemone species, such as the aggregating anemone (Anthropleura elegantissima) are quite adept at cloning themselves in order to reduce predatory threats. In some parts of the intertidal zone, whole rocks can be covered with sea anemones that all came of one individual. Researchers are looking at the locally abundant giant green anemone (Anthopleura xanthogrammica) to see if a chemical in its tissues that stimulates heart muscles can be used for medical purposes.