The Great star coral (Montastraea cavernosa) is a colonial stony coral found in the Caribbean seas. It forms massive boulders and sometimes develops into plates. Polyps are the size of a person's thumb and fully extend at night.
This coral occasionally has a fluorescent red or orange color during daytime; it has been found recently that this color is due to phycoerythrin, a cyanobacterial protein. It appears that, in addition to symbiotic zooxanthella, this coral harbors endocellular symbiotic cyanobacteria, possibly to help it fix nitrogen.