In reality, most corals are heterotropic organisms, depending at least partly on actively feeding on (with their thousands of stinging tentacles) or absorbing nutrients from the surrounding water as well as autotrophic nutrition. Most corals are mixotrophic in that they utilize both both autorophic and heterotrophic methods of feeding to survive.
Corals are said to be unlike any other critter in that such a large portion of their body is devoted to capturing food. With their huge number of tentacles containing endless stinging cells, no wonder C.M. Younge in 1931 stated, "When an animal possesses an organ or set of organs which perform certain functions with perfect efficiency, it can be taken as axiomatic that such organs are used."
If you think of the world's oceans as "The Big Soup", you would be right. The oceans are the personification of the term "food chain". Everything from microscopic bacteria to the fishes in the sea is there. The zooanthellae contained in coral polyps provide a wide range of materials needed by the coral, but coral feeding must supply the balance of the nutrients and vitamins required for metabolism and growth.
It is now believed that symbiotic (autotrophic) corals, as a group, obtain from 20 - 50% of their food from heterotrophic feeding on plankton and dissolved organic materials. Some hard corals can obtain 200 - 300% of their basic energy needs from heterotrophic feeding while most meet more than 100% of their needs in this manner. Octocorals and Zoanthids can meet 10 - 100% of their needs in this manner. Not only is intake in excess of basic energy needs required for reproduction and growth, it also increases the respiration rate of the entire colony, leading to an even greater increase in growth rates.
Exactly What Do Corals Eat?, the next article in this series will delve into which natural foods corals dine on.