Invertebrates
Sea Anemone
Actiniaria · also called sea anemones, anemone
A sea anemone looks like a flower and is nothing of the sort. It is an animal — a single large polyp, a close cousin of coral and jellyfish — that grips a rock with its base, waves a crown of venom-tipped tentacles to catch fish and plankton, and, if a predatory sea star comes too close, can pull up its base and crawl, somersault, or even swim away. Some live past a hundred.
Overview
Sea anemones are the order Actiniaria, around 1,100 species, in every ocean from tropical reefs to polar shores and from tide pools to the deep sea. Each is a cnidarian built as a hollow polyp — a muscular column, a gripping basal disc, and rings of hollow tentacles around a mouth that is also the anus. They are carnivores, and many also host symbiotic algae that share sugars, as reef corals do.
Stings, and moving
The tentacles carry nematocysts — coiled, barbed, venom-tipped threads that fire on contact. Most cannot pierce human skin; a few tropical species sting painfully. Despite looking rooted, an anemone can glide, somersault, or swim to escape danger. Many clone themselves by splitting, and where two clones meet, boundary anemones slap each other with special stinging structures in slow territorial wars.
The clownfish
Around ten large tropical anemones host anemonefish, which a special mucus coat protects from the sting. In return the fish defends and cleans the anemone, drops food scraps, and improves water flow — a partnership that benefits both.
Status
Not Evaluated and mostly common, but some large tropical host anemones have declined locally from bleaching in marine heatwaves (they can lose their algae and starve, like corals) and from aquarium-trade collection, since slow-growing host anemones are removed faster than they recover.
Sea Anemone: common questions
Is a sea anemone a plant or an animal?
It is definitely an animal, despite the flower-like name and appearance. Sea anemones belong to the phylum Cnidaria, alongside corals, jellyfish, and hydroids, and share the defining cnidarian feature, stinging cells called nematocysts. An anemone has no roots, no leaves, and no ability to photosynthesise on its own; it is a predator that catches and digests live prey in a gut cavity. The confusion comes from the fact that many anemones stay in one place and that some house symbiotic algae in their tissues, which do photosynthesise and share sugars with the anemone, in the same way reef-building corals do.
Can a sea anemone sting a person?
Most cannot, or only produce a faint stickiness or mild tingle, because their nematocyst threads are too short and weak to pierce human skin, which is why you can usually touch a rock-pool anemone safely. A minority of tropical species are more potent. The hell's fire anemone and a few others can cause a burning sting, blistering, and lasting welts, and handling anemones and then touching the eyes or mouth can transfer venom. The general rule on a reef or in a tide pool is to look but not touch, both for your safety and because the oils and salts on human skin can harm the animal.
How does the clownfish live in a stinging anemone?
The anemonefish is coated in a thick layer of mucus that either lacks the chemical signals that trigger the anemone's stinging cells or actively mimics the anemone's own surface chemistry, so the anemone treats the fish as part of itself and does not fire at it. A fish introduced to a new anemone acclimates over hours or days, brushing against the tentacles a little at a time and getting stung at first while its mucus coat adjusts. Once established, the fish gains a stinging fortress that keeps predators away, and the anemone gains a bodyguard that drives off anemone-eating fish, plus cleaning, extra food from the fish's scraps and waste, and better water circulation from its constant movement.
What is the difference between a sea anemone and a coral?
They are close relatives in the class Anthozoa, and a coral polyp looks essentially like a tiny anemone. The key differences are the skeleton and colonial life. A reef-building coral secretes a hard calcium carbonate skeleton and lives as a colony of many small connected polyps that build the reef structure together. A sea anemone is a single, usually large, soft-bodied polyp with no hard skeleton, and while some anemones form clonal patches, they do not build a shared mineral structure. Anemones are also generally more mobile and are found in a much wider range of habitats, from the deep sea to polar shores, than reef corals, which are largely restricted to warm, clear, shallow water.