Most of us have had the opportunity to dive along coral reefs, and those of us who haven’t have probably seen plenty of film footage featuring these incredible structures. Today, scientists know more than ever before about the lives of coral reefs; here, we share insight that is sure to give you a new appreciation for the world’s reefs.
What are Coral Reefs?
Coral reefs are built as living organisms, collectively known as corals, that deposit minerals which are made of limestone. As years, centuries, and millennia pass, these tiny deposits gradually accumulate, ultimately forming massive reefs that can extend for countless miles. There are millions of tiny organisms and thousands of tiny species which occupy coral reefs, but not all of them are builders; only a few actually produce the deposits which make up the limestone that forms reef structures.
These unique habitats support approximately twenty-five percent of all known marine organisms, and for this reason, they are not just essential to life on earth, they are also fascinating ecosystems.
Corals
Corals are often confused with rocks or plants, but the fact is that they are tiny animals that live together in extensive colonies. Each individual coral is called a polyp.
There are two types of coral: hard and soft. There are countless species of each, but all can be easily distinguished as they tend to grow in clusters that grow larger as time passes.
Hard corals have stout skeletons formed of calcium carbonate, which is a major component of limestone, and which forms into hard rocky structures as time passes. When diving a healthy reef, we don’t see the coral skeletons so much as we see the colors hermatype corals possess; these colors are provided courtesy of tiny commensal algae called zooxanthellae.
Soft corals often bear an even closer resemblance to plants than hard corals do. Instead of internal skeletons, they have stiff outer layers that support their internal structures. Some of these corals also host zooxanthellae.
Polyps
Coral polyps are spineless animals that grow to various sizes when mature. Polyps converge to form coral colonies which harden themselves by utilizing the limestone that is present within the water column. Equipped with outer shells as well as internal skeletons, polyps are usually nocturnal, extending their tentacles to feed after the sun goes down.
Coral Habitats
When you think of coral reefs, you probably envision clear blue water, dream about warm temperatures, and think about brightly colored marine life. While it’s true that coral reefs are prevalent throughout the Caribbean, the Red Sea, the Arabian Gulf, and the tropical Indian and Pacific oceans, as well as in areas such as Florida and Japan, reefs are being discovered in deep sea environments and in colder areas as well. Researchers estimate that there are approximately 110,000 square miles of coral reef throughout the world, however this estimation is one that’s hotly debated, as so much ocean territory remains unexplored. As for soft corals, which can thrive in cold water where most hard corals grow very slowly if at all, they can be found worldwide – even under ice.
The Origins of Coral
Accepted geologic theory is that coral reefs began to form more than 200 million years ago. Almost all the world’s existing reefs – those which are now underwater – are believed to be between five and ten thousand years old. Many mountains, cliffs, and rock formations now visible on land were once submerged coral reefs; these yield clues about oceans that disappeared long ago.
Living reefs are much like underwater cities, teeming with all sorts of life. Within the reef structure, various coral colonies thrive, and within each colony, there may be millions of tiny individual animals. Though the main reef structure is formed by hard corals, algae, sediment, and plants contribute to reef formation, often becoming compressed as they decompose after death.
There are four basic reef categories:
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Fringing Reefs – Fringing reefs are common, and are typically found surrounding islands and lying just offshore from continental coastlines. They are normally separated from the shorelines by narrow, shallow lagoons. Some famous fringing reefs include those found in the Bahamas and Hawaii.
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Barrier Reefs – Barrier reefs are also located along coastlines, but deep, wide lagoons normally separate them from the land masses they protect. Some famous barrier reefs include Australia’s Great Barrier Reef, Belize Barrier Reef, and the Florida Barrier Reef.
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Atolls – An atoll is a coral ring that forms a sheltered lagoon that may or may not hold an internal land mass. Many of the world’s atolls are located far away from continental shores, and were created as island land masses eroded or were submerged as sea levels rose. When fringing reefs around submerged land masses keep growing, they ultimately form coral atolls with inner lagoons. Some of the world’s most famous atolls include Lighthouse Reef in Belize, Bikini Atoll in the Marshall Islands, and Tubbataha Reef in the Philippines.
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Patch Reefs – Patch Reefs are smaller than other types of reefs, and grow in isolation, sometimes emerging from an island’s base or from a continental shelf. They often appear between fringing and barrier reefs, and though they rarely reach the water’s surface, they sometimes grow to impressive sizes. You’ll find plenty of patch reefs in Florida, Australia, Belize, the Caribbean, and other dive destinations.
How Corals Reproduce
Reefs use a number of reproduction methods; some have males and females that spawn, some are hermaphroditic and produce both sperm and eggs, and others, including elkhorn and boulder corals, are gonochoric, producing only one gender of coral in which some colony’s members produce only sperm and some colony’s members produce only eggs.
Larvae are produced in two ways: in some species, larvae are fertilized inside polyps and then released into the water, and in others, the larvae are formed when free-floating sperm and egg join in the water column.
After larvae are formed, they float to the water’s surface, where they can remain for several weeks. After partially maturing, they descend, ultimately sticking to hard surfaces and fully mature into adult coral polyps.
Whether naturally formed or supported by a man-made structure such as a sunken ship, a pier, reef balls, or simple debris, coral reefs protect shorelines from erosion, provide homes for countless marine animals, and provide divers with a variety of sites to explore. These are just a few of coral’s functions; they also help to regulate the planet’s ecosystem and much more. By caring for our environment and using caution when exploring coral reefs, each of us can help to ensure that these intriguing life forms continue to thrive.