Is a Sea Anemone a Plant or an Animal? Sea Anemone Facts

Why sea anemones look like plants

It is easy to see why people mistake sea anemones for plants. Most spend their lives attached to a rock, often staying in the same spot for weeks or months. Their bodies are round and symmetrical, like a flower seen from above, and their tentacles spread out like petals in shades of red, green, pink and purple.

Even the name points to plants. Early naturalists named them after the anemone, a flowering plant in the buttercup family whose name comes from the Greek word for wind, which is why it is sometimes called the windflower. When a sea anemone is out of water at low tide, though, the “flower” disappears: it pulls in its tentacles and becomes a blob of jelly.

What makes a sea anemone an animal?

Scientists sort living things by how they are built and how they live, not by how they look. By every important test, a sea anemone is an animal.

A sea anemone eats other organisms, which plants do not. It has muscle fibers that let it stretch, shrink, bend and quickly pull in its tentacles, and it has nerves that coordinate those movements. It develops from a fertilized egg into a tiny larva called a planula that swims with hair-like cilia before settling down. Its cells, like ours, have no cell walls.

The sea anemone body plan

A sea anemone is a single polyp, a soft tube closed at the bottom and open at the top. It has three main parts:

• Pedal disc: a sticky foot that holds the anemone to rock, shells or other hard surfaces.
• Column: the trunk, which can be smooth, warty or sticky enough to hold bits of gravel.
• Oral disc: the top, with a slit-shaped mouth in the middle surrounded by rings of tentacles.

The mouth leads to a single body cavity that works as a stomach. There is no separate exit, so waste goes out the same way food came in. Instead of a skeleton, the anemone keeps its shape by filling its body with seawater and holding it in, a system called a hydrostatic skeleton.

Sea anemones have no brain. Two simple nerve nets, one in the outer skin layer and one lining the gut, carry signals across the body. Jellyfish manage with a similar setup, as explained in Do jellyfish have brains?

Stinging cells: the cnidarian signature

The feature that most clearly marks a sea anemone as a cnidarian is its stinging cells, called cnidocytes. Each one holds a tiny capsule, a nematocyst, containing a coiled hollow thread. When prey touches the tentacle, the capsule fires and the thread shoots out like a harpoon, piercing the prey and injecting venom. The name of the whole group, Cnidaria, comes from a Greek word for nettle.

These cells let a sea anemone catch fish, shrimp and other moving animals, even though it can barely move itself. The way it does this is explained step by step in How do sea anemones eat?

What about the algae inside?

Some sea anemones are green, and many do get energy from sunlight, which adds to the confusion. They do it secondhand. Many species host single-celled algae, called zooxanthellae or zoochlorellae, inside their own cells. The algae photosynthesize and pass sugars to the anemone, and in return they get a safe, sunny home.

This partnership does not make the anemone a plant, any more than a cow becomes a plant by eating grass. Reef-building corals have the same arrangement, a topic covered in Is coral a plant or an animal? The large tropical magnificent sea anemone, famous as a clownfish home, gets much of its food from its algae but still catches prey with its tentacles.

Relatives: corals, jellyfish and hydras

Sea anemones belong to the phylum Cnidaria, along with jellyfish, corals, sea pens, hydras and the Portuguese man o’ war. Within it, they are anthozoans, a class that also includes corals. Anthozoans live their whole adult lives as polyps and never go through the swimming, bell-shaped medusa stage that jellyfish have.

There are more than 1,000 known species of sea anemone, which form the order Actiniaria. Most are only 1–5 cm (0.4–2 in) across, but a few giants, such as Mertens’ carpet anemone of Indo-Pacific reefs, can exceed 1 m (3.3 ft). Tube anemones, which live in tubes buried in sand, look similar but belong to a separate, only distantly related group.

Sea anemones you can see in Europe and North America

You do not need to dive a tropical reef to meet a sea anemone. Rock pools along the coasts of Europe and North America are full of them.

The beadlet anemone (Actinia equina) is one of the most common animals on European rocky shores. Out of water it looks like a red, brown or green blob of jelly up to about 5 cm (2 in) across. Underwater it opens up to 192 tentacles, and a ring of bright blue beads below them, called acrorhagi, is packed with stinging cells used in fights with neighboring anemones. One famous beadlet, nicknamed “Granny”, was collected on the coast of Scotland in 1828 and kept alive until 1887.

The snakelocks anemone (Anemonia viridis) of the Atlantic coasts of Europe and the Mediterranean has long green tentacles with purple tips that it rarely pulls in. On the Pacific coast of North America, from Alaska to California, the giant green anemone (Anthopleura xanthogrammica) lives in tide pools, where its crown of tentacles can reach about 25 cm (10 in) across.