Coral reefs need to grow just below the sea surface so that the corals’ symbiotic photosynthetic algae can absorb sunlight. If they are submerged too deep, the ecosystem wastes away without solar energy to make food. In a healthy ecosystem, this delicate balance is achieved by a constant and often overlooked tug-of-war. As corals build their skeletons up toward the sea surface, other organisms—mollusks, worms, and sponges—bore into and erode the skeletons to create shelters.