ZoanthidsZoantharia

Zoanthids are colonial anemone relatives that carpet rock in mats of identical polyps, each a short column crowned by a ring of tentacles. Unlike sea anemones, which sit alone, the polyps share a fleshy sheet at the base and open and close together, so a colony behaves like one animal spread over the substrate.

Most species cement sand grains and sponge spicules into the body wall, which makes them gritty to the touch and tough enough to hold a spot in the surge zone that softer animals cannot. They feed on plankton and on what their symbiotic algae make for them, which is why they favour bright, shallow, wave-washed rock.

The group carries a warning worth knowing. Some zoanthids contain palytoxin, one of the most poisonous non-protein substances known, and cases of poisoning come from handling colonies with bare hands or from scrubbing them in a home aquarium. In the water they are harmless to look at and photograph, but they are not animals to touch.