An encrusting fire coral that takes the shape of whatever it grows over - dead coral, rock, an old anchor - rather than standing up in plates. The surface is smooth to lumpy, mustard yellow through tan to greenish-brown, sometimes pushed into short stubby lobes a few centimetres high, with a pale margin where the colony is spreading. There are no polyp cups to see: the genus name means "thousand pores", and the openings its feeding and stinging polyps emerge from are barely visible without a lens.
That smoothness is exactly what makes it dangerous. A blade of fire coral is obvious and avoidable; a crust that has taken over a dead coral head reads as bare rock, and divers steady themselves on it. The sting comes through on contact and raises the same burning welt as its upright relatives. It lives on reef crests, upper slopes and lagoons from the shallows to about 25 m (80 ft), catching zooplankton with those stinging cells and drawing the rest of its energy from symbiotic algae in its tissues. Colonies spawn into open water and also spread by fragmentation, a broken piece cementing itself and growing on. Like all Millepora it is a hydroid rather than a true coral, and it builds a limestone skeleton anyway - which is why it carries a CITES Appendix II listing, applied across every reef-builder with a skeleton to control the curio trade rather than because this animal is at risk.
- smooth mustard-colored crust following the shape of whatever it covers
- knobby or lumpy surface, sometimes with short stubby lobes
- pale edge where the colony is still spreading
- no visible polyp cups
