Colonial polyps joined by a shared mat, filling gaps between corals and covering dead rubble.
Colonial anthozoans without a hard skeleton, the individual polyps a centimetre or two across, each with a flat oral disc ringed by two circles of short tentacles, and all of them joined at the base by a common sheet of tissue that spreads over the substrate. A colony reads as a low mat or a carpet of small buttons rather than as a group of separate animals. Colour runs green, brown, cream, pink and grey, often with a contrasting disc.
Many species stiffen the body wall by incorporating sand grains and sponge spicules taken from the surroundings, which makes the colony gritty and tough and lets it hold ground that softer animals cannot. Zoanthids grow over dead coral, rubble, rock, dead shell and debris, and they are among the first things to cover a surface that a storm or a bleaching event has cleared.
The separations worth holding are these: an anemone is a single large polyp, a corallimorph is a single flat disc with the tentacles reduced to knobs, and a soft coral has eight pinnate tentacles per polyp on a fleshy stalk. A zoanthid polyp has two rings of simple tentacles and a mat joining it to its neighbours. Recorded from the shallows to about 25 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
