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Sea Anemone

Actiniaria

Photo: A brilliant purple sea anemone nestles in among soft corals and other reef inhabitants

Photo: A brilliant purple sea anemone nestles in among soft corals and other reef inhabitants (View larger version)

Photograph by David Doubilet

Published

The ornately colored sea anemone (uh-NEM-uh-nee) is named after the equally flashy terrestrial anemone flower. A close relative of coral and jellyfish, anemones are stinging polyps that spend most of their time attached to rocks on the sea bottom or on coral reefs waiting for fish to pass close enough to get ensnared in their venom-filled tentacles.

Their bodies are composed of an adhesive pedal disc, or foot, a cylindrical body, and an array of tentacles surrounding a central mouth. The tentacles are triggered by the slightest touch, firing a harpoon-like filament into their victim and injecting a paralyzing neurotoxin. The helpless prey is then guided into the mouth by the tentacles.

There are more than 1,000 sea anemone species found throughout the world’s oceans at various depths, although the largest and most varied occur in coastal tropical waters. They run the full spectrum of colors and can be as small as half an inch (1.25 centimeters) or as large as 6 feet (1.8 meters) across.

Some anemones, like their coral cousins, establish symbiotic relationships with green algae. In exchange for providing the algae safe harbor and exposure to sunlight, the anemone receives oxygen and sugar, the bi-products of the algae's photosynthesis.

They form another, more famous symbiotic alliance with clownfish, which are protected by a mucus layer that makes them immune to the anemone's sting. Clownfish live within the anemone’s tentacles, getting protection from predators, and the anemone snacks on the scraps from the clownfish’s meals.

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