Marine invertebrates that secret a calcium carbonate exoskeleton and live symbiotically with algae, with the algae providing nutrients like carbon and the coral nitrogen, phosphorous, and an abode. Corals are perforate (porous skeleton) or imperforate (solid skeleton). Colonial corals live in deep water, and reef-building corals in warm, shallow water where their zooxanthellae algae can receive sunlight. When corals die, their outer skeletons remain, growing the reefs layer upon layer (see Atoll). As of the second millennium, two fifths of the world’s coral had disappeared due to industrial pollution, and all of the remainder is under threat.