A free-living coral that a worm living inside it carries across the sediment.
A small free-living stony coral, one or two corallites on a rounded skeleton up to about 25 mm across, pale grey, orange-brown or greenish, lying loose on soft sediment and attached to nothing. Heterocyathus shares the size, the habitat and the same partnership and is separated on skeletal detail rather than in the water, so a record from a dive covers both. The free-living mushroom corals are also unattached but are flatter, disc-shaped and very much larger.
The larva settles on a small gastropod shell and grows over and around it. A sipunculan worm — Aspidosiphon corallicola — occupies the shell cavity inside the coral and keeps a canal open to the outside, and it is the worm that moves the whole assembly, dragging it over the sediment and keeping it from being buried. The coral gains mobility and a defence against burial; the worm gains a shelter that grows with it and that it never has to leave for a larger one. On suitable soft bottoms densities of up to three hundred individuals per square metre have been recorded, so where one is found there are usually many.
The animal only looks like a pebble until it moves, and it moves slowly. A coral head sitting on open silt with a clean track behind it is the thing to look for.
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.
