are the world's primary reef-builders. Ahermatypic corals are either colonial or solitary and are found in all regions of the ocean and do not build reefs. Some live in tropical waters but some inhabit temperate seas, polar waters, or live at great depths, from the photic zone down to about . Ecology Scleractinians fall into one of two main categories: Reef-forming or hermatypic corals, which mostly contain zooxanthellae; Non-reef-forming or ahermatypic corals, which mostly do not contain zooxanthellae In reef-forming corals, the endodermal cells are usually replete with symbiotic unicellular dinoflagellates known as zooxanthellae. There are sometimes as many as five million cells of these per of coral tissue. Up to 50% of organic compounds produced by symbionts are used as food by polyps. The oxygen byproduct of photosynthesis and the additional energy derived from sugars produced by zooxanthellae enable these corals to grow at a rate up to three times faster than similar species without symbionts. These corals typically grow in shallow, well-lit, warm water with moderate to brisk turbulence and abundant oxygen, and prefer firm, non-muddy surfaces on which to settle. Most stony corals extend their tentacles to feed on zooplankton, but those with larger polyps take correspondingly larger prey, including various invertebrates and even small fish. In addition to capturing prey in this way, many stony corals also produce mucus films they can move over their bodies using cilia; these trap small organic particles which are then pulled towards and into the mouth. In a few stony corals, this is the primary method of feeding, and the tentacles are reduced or absent, an example being Acropora acuminata. Caribbean stony corals are generally nocturnal, with the polyps retracting into their skeletons during the day, thus maximising the exposure of the zooxanthellae to the light, but in the Indo-Pacific region, many species feed by day and night. Non-zooxanthellate corals are usually not reef-formers; they can be found most abundantly beneath about of water. They thrive at much colder temperatures and can live in total darkness, deriving their energy from the capture of plankton and suspended organic particles. The growth rates of most species of non-zooxanthellate corals are significantly slower than those of their counterparts, and the typical structure for these corals is less calcified and more susceptible to mechanical damage than that of zooxanthellate corals. Scleratinians were previously believed to be obligatory hosts of another group of barnacles, the