It appears to be defenseless since it doesn't have a shell, and, indeed, they can't be out of the water for very long. Most kinds of anemones prefer to live low enough on rocks and pilings so that they are exposed at low tide only on some days and then for only an hour or two. The sea anemone has a ring of tentacles around its mouth. It eats plankton or small fish and shrimp by harpooning its prey with its stinging tentacles and then swallowing the stunned animal whole. Those same stingers come in handy when nearby anemones begin to crowd too close.
Many sea anemones reproduce by cloning, literally splitting in two.
They can recognize intruders not from their own clone and attack them with stinging cells. Usually the anemones retreat, leaving an uninhabited border between them.