Sea anemones

Sea anemones are closely related to corals, jellyfishes and hydrozoans.
They are armed with stinging cells. In many species, additional nourishment comes from a symbiosis with unicellular algae, like dinoflagellates, zooxanthellae or zoochlorellae, that live within the cells, as it happens in corals.
Several species of sea anemone live in association with invertebrates like hermit crabs, shrimps, crabs and various marine snails to their mutual benefit. In these mutualistic relationships, the symbiont receives protection from predators provided by the anemone's stinging cells, and the anemone utilizes the nutrients present in its faeces. However, the most famous association is surely that with the clownfish. Also, other fishes live associated with the anemone, such as the Banggai cardinalfish and the juvenile three-spot domino damselfish.
Sea anemones breed by liberating sperm and eggs through the mouth into the sea. The resulting fertilized eggs develop into larvae which, after being planktonic for a while, settle on the seabed and develop directly into juvenile polyps. Sea anemones also breed asexually, by breaking in half or into smaller pieces which regenerate into polyps.
A sea anemone is capable of changing its shape dramatically. The column and tentacles have longitudinal, transverse and diagonal sheets of muscle and can lengthen and contract, as well as bend and twist.