Together, a colony of coral polyps lays down layer upon layer of stony skeleton. Over thousands of years, they can create a whole reef.
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All corals are part of the Phylum Cnidaria, which roughly means “stinging animals.” Within this category is a diverse group that also includes sea anemones, jellies, soft corals, sea fans, and many others.
Stony corals are what most people probably think of when they hear the word “coral” – they are hard, reef-building critters that mostly live in clear, tropical water.
Although corals may look like plants, they are actually animals. Usually, they are very small and live together in colonies. Each individual is called a polyp, and it looks like a small bud with tentacles. In the close up photo below, you can see how the surface of a piece of coral is actually covered in many little individual polyps.
Many coral polyps live together in one colony; at night they put out their tentacles to feed. Photo by Nick Hobgood via Flickr, Creative Commons License.
Each polyp is home to another organism: cells of algae called zooxanthellae, or zoox, for short. Zoox perform photosynthesis, creating energy from sunlight like plants do, and share it with the coral. In return the coral give the zoox a safe place to live. This relationship, where the two organisms help each other, is called a symbiotic relationship. The zoox can only perform photosynthesis during the day, when the sun is out, so at night the corals feed by sticking their tentacles out into the current to catch food.
Each polyp sits in a little cup of calcium carbonate, a skeleton that it secretes underneath its soft body. Together, a colony of coral polyps lays down layer upon layer of stony skeleton. Many generations of polyps can slowly build up a coral head. Over thousands of years, they can create a whole reef.
Polyps sit on a skeleton of calcium carbonate. Here you can compare what the coral looks like when it is alive (left), and what a coral skeleton looks like without polyps (right). Photos by Jan Messersmith and Peter Nijenhuis via Flickr, Creative Commons License.
Coral reefs have many important roles. The structure they provide in the ocean creates a habitat that many other animals call home. Most of the ocean’s diversity can be found in coral reefs. Reefs are important to people as well – for example, they provide us with food, and they protect shorelines from storm damage.
However, coral reefs are also fragile and face many threats. Right now, more than half of the world’s reefs are threatened. Most corals require clear water so they can get sunlight, so runoff from land that makes the water murky puts them at risk. They also are in danger due to ocean warming and acidification. If the water gets too warm, it causes coral bleaching (the corals eject their zoox, which they need to survive, and turn white), and if the ocean becomes more acidic, it will dissolve their calcium carbonate skeletons. Overfishing is a danger as well: herbivorous fish eat seaweed, which competes with coral for space on the reef. Without those fish, the seaweed can overgrow the coral.
Stony corals create reefs, providing a habitat for other animals. Photo by Jim Maragos/U.S. Fish and Wildlife Service via Flickr, Creative Commons License.
Luckily, coral reefs are also resilient. If a reef is given protection, for example by a Marine Protected Area, evidence shows the ecosystem can fix itself, but it’s going to require that we all lend our support.