About Corals

The Biology of Corals

Where are corals found?  Corals can be found throughout the oceans, from deep, cold waters to shallow, tropical waters.  Shallow coral reefs have optimal growth rates in warm water ranging from 70–85° F (21–29° C). Coral reefs can be found at depths exceeding 91 m (300 ft), but reef-building corals generally grow best at depths shallower than 70 m (230 ft). The most prolific reefs occupy depths of 18–27 m (60–90 ft), though many of these shallow reefs have been degraded. Corals also need salt water to survive, so they also grow poorly near river openings with fresh water runoff. Other factors influencing coral distribution are availability of hard-bottom substrate, the availability of food such as plankton, and the presence of species that help control macroalgae, like urchins and herbivorous fish.

Coral Distribution Map


Basic ecology.  Corals are in fact animals (not a plant!) that fall under the phylum Cnidaria and the class Anthozoa. They are relatives of jellyfish and anemones.  Corals can exist as individual polyps, or in colonies and communities that contain hundreds to hundreds of thousands of polyps. For example, brain corals consist of colonies of many individual polyps; each individual polyp averages 1-3 mm in diameter. Corals can be divided into two groups: hard coral and soft coral. Hard corals, also known as stony coral, produce a rigid skeleton made of calcium carbonate (CaCO3) in crystal form called aragonite, with reef-building capabilities. Alternatively, soft corals, including sea fans, do not produce a rigid calcium carbonate skeleton and do not form reefs, though they may be present in a reef ecosystem.

Feeding: 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. In addition to the symbiotic relationship with algae, most corals capture and consume live prey ranging from microscopic zooplankton to small fish, depending on coral size. Using its tentacles that extend outside it body, the coral with utilize it nematocysts, or stinging cells, to stun and kill  its prey before passing it to its mouth. Once the food has been digested, the waste is expelled from the same opening.

Reproduction: Corals are unique in that they are capable of reproducing both sexually and asexually. Sexual reproduction is the more common method and can be performed in two ways: broadcast spawning or brooding. Broadcast spawning consists of both male and female coral expelling massive amounts of gametes (eggs and sperm) into the water column during synchronized events. Brooding is similar to broadcast spawning, except only the male gametes are released into the water column. Coral sperm is negatively buoyant once released and hopefully will be carried by ocean currents to female coral where they will fertilize the egg cells of the female coral.


Coral Spawning