Under the shimmering Caribbean Sea and Atlantic Ocean that surrounds Anguilla’s limestone shores lies a barrier that is thousands upon thousands of years old. The barrier, made up of the same material as the island on which we live, has protected Anguilla from centuries of hurricanes, storm damage, and ground seas. This barrier, though, is not only sturdy and old, it’s alive and it’s called a coral reef.

What is a coral and what are coral reefs?

Finger coral, Round Rock Bay, Anguilla (Photo by F. Mukhida)

Most corals are tropical creatures. They prefer warm, clear, and shallow water that has few nutrients in it. They are colonial animals that are related to jellyfish. Unlike jellyfish, though, that swim around and move about in the water column, corals are anchored to the seabed by calcium carbonate (limestone) cups called corallites. Each individual coral is called a polyp. By joining their limestone skeletons together and by sharing connective tissue, coral polyps are able to form coral colonies. These hard coral colonies form the reef structures that we see in Anguilla’s waters.

There are other types of corals too, called soft corals. These corals include the sea whips and sea fans that we see swaying with the water currents. These corals, though, don’t form reefs.

One of the main reasons that hard corals are able to form these huge reef structures is because of their special relationship with a type of algae called zooxanthellae. The algae live deep inside each polyp’s tissue. It is a symbiotic relationship – that is, the polyp and the algae need each other to survive.

The zooxanthellae produce food by capturing light energy from the sun that penetrates down through the water column and converting it into chemical energy through a process called photosynthesis. The polyp meanwhile uses its tentacles to catch plankton from the water. The algae create oxygen and sugar as by-products of photosynthesis and these by-products are very important to the coral as they provide another source of energy for the animal. The coral polyp, meanwhile, also produces waste material in the form of carbon dioxide and nitrogen. The zooxanthellae, in turn, consume the coral’s by-products. This coral-zooxanthellae arrangement, therefore, isn’t just useful, it’s actually necessary – neither organism could produce or find all that it needs to survive on its own.

Since both coral polyps and zooxanthellae are living creatures, they both go through the natural lifecycle of birth, reproduction, and death. But even at the end of their lives, their purpose isn’t over. After the coral has died, they form the base upon which new corals can grow. In fact, when exploring a coral reef, the only live part of the structure is on its surface, everything else below it may have been dead for days, months, years, or even centuries.

Sea fan (Photo by S. Wynne)

Helping to keep the live and dead parts of the reef together is another important organism – coralline algae. The algae, similar to coral polyps, also secrete limestone. This limestone acts as a cement or glue and helps keep the sand that settles on to the reef along with any coral fragments together and attaches them to the base below.

The reef, therefore, is constantly growing out sideways and, more importantly and particularly, upwards. This growth is essential especially today as sea levels rise since the zooxanthellae that live in the corals need light to photosynthesise; they can’t grow well in water deeper than 30 metres. If water levels rise too quickly, the reef may not be able to keep up with it. This means one thing for the reef: death.

Corals grow at different rates depending on their species (type) and their morphology (shape). Branching corals, such as elkhorn, can grow up to 10 centimetres a year while the rounded, boulder-type brain and star corals grow about 1 centimetre a year. In Anguilla’s waters, many of the giant elkhorn coral that used to help make up the reefs were broken after severe hurricanes in the 1990s. They haven’t made much of a recovery and now, the predominant coral types that we see are the brain and star corals along with the non-reef forming sea fans.

While corals are an important component of the reef (they provide most of its structure), many other organisms add to its diversity and complexity. In addition to the algae that grow all over it, sea slugs, sea cucumbers, fish (including parrotfish, damselfish, doctorfish, old wife, snappers, and groupers), crustaceans, lobsters, sea urchins, and even sea turtles are found living on it, in it, near it, and/or feeding on it. Thus, the coral reef and its associated care-takers are a food chain in itself.

Brittlestar and zoanthid on vase sponge (Photo by S. Wynne)

Some of the organisms (including worms and sponges) actually bury themselves into the reef. They may be seeking temporary protection from predators, an overnight (or day) resting spot, or sometimes permanent home. Some scientists suggest that up to 40 percent of a reef is actually holes and caves made by these creatures. Despite this “emptiness,” the reef is still strong. Part of the reason is because of the cementing action of the coralline algae.

With all of this action and all of these organisms using and living on the reefs, coral reefs are among some of the most productive ecosystems on the planet. Despite the amount of food that they produce and provide, though, coral reefs exist in some of the most nutrient-poor and least productive waters in the world. And they actually depend on this; they produce what they need and anything more would be deadly. This is why grey water and sewage (both of which have much phosphates and nitrates in it) can be so damaging to corals – they don’t need the nutrients and the extra tends to stimulate excessive algal growth. While some algae can be good for the reef (it provides food for many herbivorous fish), too much algae could suffocate the corals and kill them.

Fish in a coral reef crevice, Round Rock, Anguilla (Photo by F. Mukhida)

Research results show that the total amount of living matter on a healthy reef that is produced by the plants and algae that grow on it – the gross primary productivity (GPP) of the reef – can be up to 250 times more than that of the surrounding ocean. At the same time, even though the GPP may be high, the net productivity – what’s left over after use by the reef organisms – is exceedingly low. Almost everything that is produced by the reef ecosystem is used by the reef ecosystem. And this is one of the main reasons why the harvesting of coral reef resources, including snappers, parrotfish, and groups, can be so difficult to maintain and even balance, especially at commercial levels – there’s just not enough to go around.

…To be continued. Part 2. Coral Reefs: Taking an Ecosystem Approach to Use and Conservation.


Note: This article is supported by information from Alex Brylske’s article Coral Reefs: Fall of the Cities Beneath the Sea. His article can be found at www.dtmag.com/Stories/Marine%20Life/05-03-Feature.htm.