Reef-building corals (Scleractinia)

node · earth · Tropical and subtropical shallow seas, broadly 30 degrees N to 30 degrees S

Colonial cnidarians of the order Scleractinia whose individual polyps — soft, tentacled, a few millimetres across — secrete an external skeleton of aragonite, a crystalline form of calcium carbonate. The colony is the accumulation of thousands of such skeletons; the reef is the accumulation of colonies over centuries to millennia, cemented and bored and re-cemented. Only the zooxanthellate (hermatypic) scleractinians build reefs, and they do it under a narrow set of conditions: clear water, low nutrients, salinity near oceanic, temperatures broadly between about 18 and 30 degrees Celsius, and enough light to run photosynthesis in the tissue. That light requirement, not depth pressure, is why the reef-builders are concentrated in the top few tens of metres. Azooxanthellate scleractinians exist in cold and deep water down to thousands of metres, but they build slowly and are not the subject of this Journey.