Before we start, let's consider how reefs grow. The main organism that constructs modern reefs, the coral, includes a number of species belonging to the Cnidaria, a phylum of organisms that uses stinging cells to capture their prey. Modern corals are colonial structures of millions of individual polyps that grow primarily in shallow and clear tropical and subtropical waters, restricted to these areas by light levels and temperatures as well as by nutrients.
Both types of coral reproduce both sexually and asexually. Asexual reproduction involves simple cell division, or budding, and takes place within the colony, whereas sexual reproduction involves the release of gametes into seawater. This is an amazing process that for many species happens once a year timed by the lunar cycle. The fertilized egg forms a larval planula that settles before forming a new colony.
Video: Coral Growth (00:58)
Large reef structures including fringing and barrier reefs, as well as atolls, represent the growth of these colonies over many thousands or millions of years.
Evolution of an Atoll
Examples of modern corals living in symbiosis with dinoflagellate algae called Zooxanthellae
The algae live within the coral polyp and receive CO2 from the polyp that it requires for photosynthesis. Through photosynthesis, the Zooxanthellae convert the CO2 to O2 and provide this vital gas to the coral as well as crucial nutrients. Reefs are made up of much more than corals and their algal symbionts. Other organisms, including coralline algae (algae that secrete high magnesium calcite and conduct photosynthesis on their own), are active framework builders in modern reefs. Today, sponges, sea anemones, sea urchins, a diverse array of fish, and many other organisms live within reefs, some playing a vital role in building reefs and keeping them healthy and others taking advantage of their decline.