Sea anemonesActiniaria

It looks like a flower and it is an animal, closely related to jellyfish and corals.

An anemone is a single polyp: a muscular column stuck to the rock, a ring of tentacles at the top, and a mouth in the middle. That is the whole body plan. A coral is the same polyp in the hundreds or thousands, sharing a stone skeleton they build together; a jellyfish is the same body upside down, drifting rather than attached. Take a jellyfish, turn it over, glue it to the reef and remove the swimming, and you have an anemone.

The tentacles are loaded. Each one carries thousands of stinging capsules, each holding a coiled thread under enormous internal pressure, and when the trigger is touched the capsule fires in well under a millisecond with an acceleration of millions of times gravity. It is one of the fastest movements known in any living cell. A prawn brushing a tentacle is harpooned in hundreds of places at once, held, and folded into the mouth. Most anemones cannot hurt human skin; a few can.

The famous exception is a deal. Clownfish live inside anemones that sting everything else, protected by a mucus layer that stops the capsules from firing, and they pay rent: they chase off the butterflyfish that eat anemone tentacles, they drop scraps, and their movement circulates water through the animal. Neither partner does well alone. Many anemones also farm algae inside their own tissue, which photosynthesizes and feeds them sugars, so a large anemone in sunlight is partly a predator and partly a garden.

One more thing worth knowing: nobody has established a natural lifespan for them. Individual anemones have been kept for over eighty years without showing signs of aging, and the ones on the reef simply keep going, splitting themselves in two when it suits. A large anemone with clownfish in it may well have been in that exact spot longer than anyone diving on it has been alive.