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A sea anemone (order Actiniaria) usually resembles a flower with a crown of tentacles surrounding a disk on top of a column-like body. They can retract these tentacles completely, appearing as if they are no more than a harmless blob.
The tentacles have stinging cells called nematocysts that are used to immobilize or kill their prey, which can be anything from tiny plankton to fish much larger than itself. The sea anemone pulls its victim into its central body cavity, which is almost entirely made of a giant digestive gland. When the anemone is antagonized, some species produce special stingers called acontia (singular: acontium) through the pores in its column.
It may look like sea anemones are immobile, rooted in place like a plant, but this is only an act, part of their disguise. They can actually glide slowly along the ocean floor using their muscular base. Crafty hunters, sea anemones are excellent at appearing as if they are just a part of the flora and fauna of the sea in order to avoid attackers and fool their prey.
Varying in size, a sea anemone can be anywhere from a couple of inches wide (5cm) up to three feet (1 meter) across in some tropical species (Stoichactis). They live very long lives; some have been known to survive almost a hundred years. A sea anemone can reproduce sexually or by budding a new family member from the base of their column.
Some sea anemones have been found at some of the ocean's deepest spots. The deepest is noted at about 30,000 feet (9,000 meters). Most sea anemones, however, live in tide pools or tropical coral reefs. The large tropical species, are often so colorful with tentacles that range from pink to yellow to red, that they blend in with the other inhabitants of active tide pool communities.
Sea anemones often develop symbiotic relationships with other sea creatures. The most famous is the clownfish (Amphiprion) who lives within the protective tentacles of the sea anemone, immune to its poison, and feeds on the crumbs of its host anemone's meals.
Some sea anemones live affixed to the shells occupied by hermit crabs (Eupagurus). When the crab moves out, the only thing that comes with them is their anemone friend.