What organism in the sea is animal, plant, and mineral all in one? One that also produces slime and has group sex? A hint: these creatures are small, but collectively create some of the largest biologic structures on the planet. The answer? Reef-building corals. They feed on zooplankton with tentacles the animal. They have symbiotic algae living within their tissues that photosynthesize—the plant. And they build a skeleton of calcium carbonate—the mineral. Reef-building corals are the engineers of a complex undersea topography that is the foundation for one of the most diverse and beautiful ecosystems on Earth. More than four thousand species of fishes have been found on coral reefs, encompassing perhaps 25 percent of all documented types of fishes in the ocean. Add to that, more than eight hundred types of coral along with hundreds of crustaceans, mollusks, sponges, and other invertebrate species. Mix in the microbes, and the diversity housed within a coral reef may be beyond compare. And it all depends on the architecture built by a diminutive but amazing creature—the coral polyp.
Reef-building corals are most commonly found in the relatively warm, shallow, and clear water of the tropics and subtropics. A few corals are solitary, but most live as colonies of small, interconnected animals, called polyps. They are simple creatures, essentially a ring of tentacles surrounding a mouth and stomach. The coral polyp looks very much like its relative the sea anemone, or a bit like an upside-down version of its other cousin, the jellyfish. In a colony, coral polyps are connected to one another by tissue that allows them to share food.