Corals are colonial animals that resemble humans in one respect: they’re great engineers. Corals construct reefs by excreting calcium carbonate. Hundreds of billions of individual corals working at this project, generation after generation, create these enormous structures. And these structures are crucial to marine life. _________. And water that’s low in nutrients should be, and generally is, low in life. But coral reefs are full of life; the density and diversity of life on a healthy reef may be greater even than in a rain forest.