Don,
Most organisms that we call "corals" are symbiotic organims. Meaning two organisms living together communally to the benefit of both. Yes, corals are animals but they contain algae in their tissues. These algae (zooanthellae) are members of the dinoflagellate family to be exact. I'm not sure that the 98% number is accurate. They do get the majority of carbohydrates from the symbiotic algae, but still need amino acids and other nutrients from the water and particulate foods.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano