Sea anemonesSea anemones living in tropical waters are brilliantly colored.

The sea anemone has a soft, muscular, cylinder-shaped stalk with a broad, circular foot at its base. Atop the stalk is a mouth surrounded by circular rows of hollow tentacles. The tentacles, which eject poisonous stinging threads to paralyze prey, grasp small animals and sweep them into the mouth. Usually, the sea anemone lives attached by its foot to shells, rocks, or wharf pilings. It can, however, slowly slide along the ocean floor on its foot or swim by moving its tentacles. When disturbed, the animal contracts its body, pulls its tentacles inside its mouth, and draws its mouth together like a pouch.

Sea anemones reproduce both asexually and sexually. Asexually they usually reproduce by dividing in two, each half forming a new animal. Sea anemones vary greatly in size, depending on the species. Some tropical species are up to three feet (90 cm) in diameter. The brown sea anemone, found in the Northern Hemisphere, is about three inches (7.5 cm) in diameter.

Which Polyp Looks Like a Flower?

A sea anemone (uh NEHM uh nee) looks a lot like a flower. But of course, it isn't. A sea anemone is a stinger that lives almost its whole life as a polyp. Some sea anemones also go through a larva stage.

The body of a sea anemone is a hollow column. The mouth and the tentacles are at the top. Like most stingers, a sea anemone uses its tentacles to capture prey and push it into its mouth.

A few sea anemones move around much in the same way that hydras do. Some creep along on the suction foot. Others somersault. And still others swim by flexing their bodies.

If a sea anemone is threatened, it pulls its tentacles into its body. This makes the animal look like a colorful ball.

Why Is That Sea Anemone Riding a Crab?

Some sea anemones have special relationships with other animals. The relationships are special because each animal helps the other in some way. This sea anemone, for example, may spend much of its life clinging to the shell of the hermit crab. Why? The hermit crab helps the sea anemone catch more food by moving it from place to place. In turn, the sea anemone's tentacles help protect the hermit crab from its predators.

Some fish also have special relationships with sea anemones. One kind of fish, the clown fish, lives among the sea anemone's stinging tentacles. That keeps it safe from its predators. In turn, the clown fish chases away predators that might want to eat the sea anemone. The clown fish may also help keep the sea anemone clean.

The brown sea anemone is Metridium dianthus. Sea anemones belong to the class Anthozoa.