Coral - Anatomy and Feeding Behavior

WHAT IS A CORAL?
Coral colonies are composed of many tiny, cup-shaped animals called polyps.Polyps, which are related to jellyfish, can be as large as a saucer or smaller than the head of a pin. Millions of polyps working together eventually form the framework of coral reefs.


THE CORAL POLYP
The coral polyp is the living animal. Radially symmetrical, the polyp is basically a sack with a stomach, and a mouth surrounded by retractable, stinging tentacles. A polyp is generally just a few millimeters in diameter and a few centimeters in length. Their body is composed of a layer of outer epithelium and an inner jellylike tissue called the mesogea.

Coelenteron - The "stomach" where food is ingested and waste is expelled. The stomach closes at the base of the polyp, where the epithelial cells produce an exoskeleton called the basal plate or calicle.

Calicles - Calicles (Latin for "little cups") grow vertically around the polyp and create a "shell" into which the polyp can completely contract into, tentacles and all, whenever they are stressed by predators or the environment.

Corallite - A hard calcium carbonate shell which protects each polyp

Tentacles - Lined with stinging cells called nematocysts, the polyp's tentacles are located at the tips of the calicles. The nematocysts carry venom which is rapidly released in response to physical contact with another organism.

Polyps generally live in a coral "head," a compact colony of many genetically identical individuals. These colonies grow from a single "founder" individual that then divides repeatedly. Over a period of time, the colony of polyps creates the large calcareous skeleton that is most people picture when they think of coral. Colonies of hard corals can eventually become part of a reef, which has an upper surface exposed to light and contains hundreds of polyps.


WHAT DO CORALS EAT?
Coral polyps are generally nocturnal feeders and can feed on any of a variety of small organisms, including anything from microscopic plankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts (sometimes called 'cnidocysts'). 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.

Many corals also form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Through the process of photosynthesis, the zooxanthellae provide energy for the coral. They also aid in calcification. The algae, on the other hand, benefit by getting a safe place to live and a food source in the form of the polyp's carbon dioxide and nitrogenous waste.

The bright colors of tropical coral are primarily derived from pigments in zooxanthellae algae!