Columns and mounds that are the main reef-builder of the mid-depth Caribbean reef. Slow-growing, so a large head is centuries old.
Heads reach several metres across; the complex this species belongs to reaches about 5 m in diameter and 2 m high. The individual cups are small, 2 to 3 mm, and flattened. The colony is massive and forms stacked lobes and columns with smooth even surfaces and living tissue on the tops of the columns. Brown to yellowish-brown.
The two species it was separated from in the 1990s are the confusion and are worth learning. One has an irregular bumpy surface with unevenly sized cups; the other is less heavily calcified with more uniform cups, forming smooth mounds and skirted plates with vertical keels. This is the one that makes discrete columns. Depth helps as well: this species is the shallowest of the three.
It historically dominated Caribbean lagoons and upper reef slopes as a framework builder. The polyps are mostly withdrawn by day. It lives between the surface and about 80 m.
It is listed as endangered, is threatened under the US Endangered Species Act, and is highly susceptible to stony coral tissue loss disease, dying in the first wave of any outbreak.
Two naming notes. The genus changed in 2012, when it moved out of Montastraea. And the common names in this group are not settled: NOAA uses “boulder star coral” for one of the sibling species and calls this one the lobed star coral, where dive literature uses boulder star coral for this species or for the whole complex.
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.
