Coral Reefs



Life History


Study Site






Works Cited

A coral reef community (image from

            Coral Reefs are the dominant ecosystem in shallow tropical waters. They provide the foundation for the community of various invertebrates and fishes which make this habitat their home.

            Corals are small colonial animals in the phylum Cnidaria. They are divided into two sub groups: hard corals or Anthozoans, and soft corals or Gorgonians (octocorals). Corals are made up of many, many polyps which are modified for different functions like reproduction and feeding. The key feature of corals is that they are in a symbiosis with tiny dinoflagellates called zooxanthelle. These tiny creatures actually live inside of the corals and are photosynthetic algae which produce food (organic compounds) for the coral while the coral provides them with a place to live, as well as carbon dioxide, some nutrients, protection and access to light (Thomas). Without this key interaction, the entire coral reef community would not be possible, because the tropical waters are too nutrient-poor to support life. Thus, millions of tiny zooxanthelle produce key nutrients and give them to the corals, which are then distributed throughout the community through nutrient cycling.

            Zooxanthelle aren’t the only photosynthetic organisms in coral reefs, though. An abundance of seaweeds and coralline algae also grow here for herbivorous fish like the Rainbow Parrotfish to consume. (You can read more about this on the Herbivory page).



           Coral Reefs grow in generally shallow waters because they depend on the zooxanthelle for food. In order to make food by photosynthesis these tiny algae need access to sunlight. Thus, the coral reef community is restricted to depths that light can penetrate. In the ocean this is called the photic zone. Even deep water reefs do not extend far below 75 meters, and most reefs are located in shallower waters from 1 to 15 meters deep.

            There are many species of fish that make their home in the coral reef habitat and fill many different niches within it. Many of these, like the Parrotfishes, are herbivorous and graze upon the algae that grow amongst the reef. They are of key importance because if they did not keep the growth of algae in check, it would eventually get too long and smother the corals. There are also predatory fishes which inhabit the reef. Perhaps the one which inspires the most fear is the Great Barracuda. When seen snorkeling they seem like the bullies in the neighborhood, fierce and menacing. Indeed, they are known for their aggressive behavior, but as fish they are simply doing their job by filling their role in the community.


            Coral Reefs are a very dynamic environment, but they are also very sensitive. They grow only in warm tropical waters (Bermuda being the northernmost location which supports them). However, the water cannot become too warm or it will begin to impact the community negatively. With the advent of global warming, there has been a marked rise in the amount of disease seen on coral reefs all over the world (Harvell et. al. 1999). Coral bleaching has also increased in frequency over the last few years. Without adequate protection, these precious communities could be lost forever. It is up to everyone to be conscious of the decisions they make in their everyday lives and how that can impact the world around them. One day these communities may vanish and all that will remain are pictures and stories.