Coral Organisms

Many corals as well as other cnidarian groups such as sea anemones (e.g. Aiptasia), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Aiptasia, while considered a pest among coral reef aquarium hobbyists, serves as a valuable model organism in the study of cnidarian-algal symbiosis. Typically a polyp harbors one species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification. The algae benefit from a safe environment, and consume the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often drives the coral to eject the algae. Mass ejections are known as coral bleaching, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as GFPs (green fluorescent protein). Ejection increases the polyp's chances of surviving short-term stress—they can regain algae at a later time. If the stressful conditions persist, the polyp eventually dies.
Feeding
Polyps feed on a variety of small organisms, from microscopic plankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts. The tentacles then contract to bring the prey into the stomach. Once digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle.