Corals are marine invertebrates in the class Anthozoa of the phylum Cnidaria, the phylum that also includes jellyfish and sea anemones. Some coral species live in relatively cold water, but the large majority grow in warm, tropical oceans. Corals can live on their own, but usually live in colonies of up to thousands of individual coral animals, called polyps, which are all genetically identical clones of one another.
Even though they cover less than 1% of the ocean floor, coral reefs are home to an enormous variety of marine life. They provide habitat for over 25% of all known marine species, including fish, crustaceans, mollusks, and many others. Coral reefs are also important breeding grounds for many commercially important fish species, such as tuna and snapper. In addition to their ecological importance, coral reefs also provide a range of benefits to humans, such as coastal protection, tourism, and food security.
This is because corals grow very slowly, at a rate of only a few millimeters per year. Over time, the combined growth of many generations of coral animals can form huge structures like coral reefs. In fact, some of the largest coral reefs in the world are over 10,000 years old! This long lifespan makes coral reefs some of the most valuable and ancient ecosystems on our planet. Unfortunately, many coral reefs are now under threat from factors such as climate change, overfishing, and pollution.
Scientists call this bioluminescence, or the ability of living things to create their own light. Some species of algae and mushrooms also have a similar ability. This is made possible by a chemical reaction that takes place within the coral's tissues. Although the purpose of bioluminescence in corals isn't completely understood, scientists believe that it may play a role in attracting prey, scaring away predators, or communicating with other coral individuals.
Corals use small, venomous tentacles with special stinging cells known as cnidocytes to paralyze and consume small marine animals. A typical coral's diet might consist of zooplankton, shrimp larvae, or even tiny fish.
We generally picture coral as beautiful reefs of brightly-colored limestone structures. In fact, however, the coral animals, or polyps, are mostly see-through. The bright reds, oranges, and pinks we're used to seeing come from an algae called zooxanthellae that live inside of the corals. This is a great example of symbiosis, where the corals offer protection to the algae and in turn absorb some of the nutrients the algae produce through photosynthesis.
