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. The symbionts benefit the corals because up to 50% of the organic compounds they produce are used as food by the polyps. The oxygen byproduct of photosynthesis and the additional energy derived from sugars produced by zooxanthallae 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.