A massive mounding stony coral. It forms the small patches of hard coral that exist in this otherwise bare rock archipelago.
In calm water it forms helmet-shaped or hemispherical mounds up to 6 m high and wide, among the largest single coral colonies anywhere. Growth is very slow, as little as a centimetre a year, so a colony that size is centuries old. The cups are very small and closely packed, with polyps about a millimetre across, which gives the surface a finely pitted, almost velvety look. Usually mustard-yellow, ochre, olive-brown or pale green, with no ridges or knobs, and often bearing Christmas tree worms and the scars of boring animals.
Two closely related species cannot be reliably separated from it in the field, and it is better to say so than to offer a false mark: the group needs skeletal microstructure. Treat a field identification as provisional.
It is the coral that builds the massive reef structure in the eastern Pacific, where the branching cauliflower corals do not. The polyps are mostly out by day. Pufferfish and parrotfish graze it heavily and the bite scars are easy to see. It lives down to about 30 m, slightly deeper than the cauliflower corals.
At Revillagigedo it has been recorded at Socorro between 3 and 21 m, at San Benedicto between 6 and 18, and at Clarión at 12. It is listed as near threatened.
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.
