A flat skeletonless disc of a polyp, related to the hard corals it sits among.
A single polyp spread into a flat circular disc with a mouth at the centre and a short column beneath, the disc anything from two to fifteen centimetres across. The tentacles are reduced: instead of the long tentacles of an anemone there are rows of short knobs, low warts or fine radiating lines over the surface of the disc, sometimes with a fringe of slightly longer ones at the margin. Colour runs green, brown, blue, violet and grey, and many fluoresce strongly under blue light.
The group sits between the hard corals and the anemones, and the practical difference from a hard coral is the absence of a calcareous skeleton: a corallimorph is soft all the way through, and a colony pulled by current moves as tissue rather than as stone. They spread by dividing, so the animals occur as sheets of near-identical discs covering dead coral, rubble, bottles and other debris in shallow silty water.
The name causes trouble. A mushroom coral is a free-living hard coral with a stony skeleton and radiating septa; a mushroom anemone has neither. The corallimorph decorator crab, Cyclocoeloma tuberculatum, carries small polyps of this group planted on its carapace, and finding the crab usually begins with recognising the corallimorphs on it. 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.
