Corals are stimulated to expand their polyps to feed by a number of influences. Temperature, the time of the day, the oxygen content of the water, water movement, the presence of food or sediment are the influencing factors. Coral polyps will expand and contract throughout the day and night, depending upon how much food is readily available. The Zooplankton contained in the coral polyps releases the amino acids glycine, glutathione and proline, which appear to induce a feeding response in most corals.
Polyps Feed Until Their Appetite Has Been Satisfied
The amount of food required depends upon the individual species, metabolic requirements and the amount of food available. In the wild, corals seem to feed more actively at night rather than during the day. As the sun sets and the reef becomes darker, plankton rises from the reef, providing a food-rich environment for the polyps. This is the major food source for most corals.
As the sun sets on the reef and the zooxanthellae reduce their photosynthetic activity (which is used for fueling a high rate of calcification for growth), there isn't much left for the corals to do but eat.
Corals Are Adaptable
It seems that corals are very adaptable animals. When introduced into an aquarium, where the entire food availability routine is reversed due to the fact that most aquarists feed their tanks during the day (when the tank is lit), most corals change or at least modify their feeding habits.
Corals which normally grow in the lower light (deeper) areas of the reef depend much more heavily on active feeding than do shallower water corals. Shallower depth (high light) corals need to actively feed on zooplankton for most of their nitrogen uptake since the zooxanthellae are already photosynthesizing sufficient carbon and transporting it to the coral animals.
Corals which normally retract their polyps during the day may leave them open longer in low light conditions in the aquarium in order to meet their carbon requirements.
Although some corals actively feed on available plankton during the daylight hours, a majority of corals open their polyps, extend their tentacles with their stinging nematocysts and actively feed during the night.
The Importance of a Corals Environment
Ideally, the environment in which aquarium corals are kept should be as close to those on the reef as possible. A number of coral aquarists have found that feeding their corals at night with the assistance of an automatic feeder and running their mechanical filters and skimmers through timers which turn them off during coral feeding hours has provided good results.
The Complete Coral Food Recipe, inspired by the "Homemade Coral Ration" from "Aquarium Corals - Selection, Husbandry, and Natural History", by Eric H. Borneman, provides an ingredient list and preparation instructions for a nutritious, freezable meal for corals.