Most reef-building corals have a mutually beneficial relationship with a microscopic unicellular algae called zooxanthellae that lives within the cells of the coral's gastrodermis. As much as 90 percent of the organic material the algae manufacture photosynthetically is transferred to the host coral tissue. Due to the need for sunlight to conduct photosynthesis, this type of energy production happens during daylight hours. more…
Capture of Prey
While reef-building corals may get up to 90 percent of the energy they need from their symbiotic algae, most corals also capture and consume live prey. Feeding usually occurs at night. [a] During feeding, a coral polyp will extend its tentacles out from its body and wave them in the water current where they encounter small fish, zooplankton, bacterioplankton, or other food particles. A coral's prey ranges in size from nearly microscopic zooplankton to small fish, depending on the size of the coral polyps. The surface of each tentacle has thousands of stinging cells called nematocysts. When small prey float or swim past, the tentacles fire these stinging cells, stunning or killing the prey before passing it to the mouth. Once food is digested, waste is expelled through the same opening. [a]; [b] SCUBA divers who have gone on a night dive may have had the chance to observe this feeding method; often the beam of their dive lights will attract zooplankton and other prey, which nearby coral colonies quickly consume.
In addition to capturing prey with their tentacles, many corals also collect very fine dissolved organic particles in mucous filaments which they then draw into their mouths. [c] These filaments originate in the mesenteries within the polyp's stomach. more…