Coral reefs are one of the most complex and colorful tropical ecosystems. Coral, a tiny animal that spends its life rooted in one place, is the foundation of this ecosystem. It builds the reef that supports the rich sea life around the shore. These massive reefs are home to as many different animals as the rain forests.
As large as reefs can get, they are built by tiny animals: coral polyps. Like all animals, corals eat, breathe and reproduce. As larvae, they even move. Each coral polyp has a skeleton, mouth, tentacles, gut and sex cells. Inside their bodies, most corals even house tiny food factories called zooxanthellae, single celled algae that live just under the clear skin of the polyp. These tiny algae and the coral have a symbiotic relationship needed for survival. The algae make food for the polyp using energy from the sun. The algae feed off the coral's waste. The coral gives the tiny algae oxygen and a safe place to live.
Coral polyps can also find food by sweeping the sea with their tentacles to capture zooplankton tiny animals and phytoplankton tiny plants that drift on ocean currents. Nematocysts or stinging barbs inside cells on a polyp's tentacles harpoon plankton, and then fine hairs and mucus carry it to the animal's mouth.
Coral polyps are fussy about where they live. To survive they need clean, clear ocean water with its salts and minerals, warm temperatures (but not too warm), currents to bring them their food, sunlight for the algae living inside and a solid place to grow. Unless these conditions are almost perfect, corals won't survive.
Some coral polyps, known as hard corals, build their own skeletons. Unlike people and other animals, their skeletons aren't inside their body. They actually secrete their skeleton and sit in it. Each coral polyp produces a protective cup or skeleton of calcium carbonate, a chemical compound the animal absorbs from seawater. Unlike our internal skeletons, made from living cells, coral skeletons are lifeless rocky deposits outside a polyp's body. Coral polyps are invertebrates, which means that they don't have an internal skeleton with a backbone.
Another type of coral is soft corals. These corals can live in deeper waters because most of them, but not all of them, don't have zooxanthellae. These corals are called soft corals because they don't secrete a hard skeleton. Most of these corals get support from sclerites or microscopic calcareous particlesalmost the same things sponges are made of. Slcerites are kind of like glass and keep the soft corals from flopping over.
Coral and algae bring life to barren seas. Crystal-clear tropical seas support little life. They offer few resources: mostly plankton and sunshine. But algae and corals make the most of those resources. Together, corals and algae concentrate and recycle nutrients, building reefs that teem with life.
Coral polyps clone themselves to build colonies. Over many generations, these slow-growing colonies form the basis of the reef. Like apartment buildings with many tenants, coral heads are colonies of thousands of tiny, individual polyp animals. Each colony starts as a single polyp, which splits to make two identical animals. The process repeats, and the colony grows.
Just like other animals, corals reproduce sexually. Because polyps can't move to meet mates though they release eggs and sperm into the water. Releasing of the eggs and sperm is often triggered by a full moon when the ocean waters begin to warm in the spring or summer. After releasing millions of brightly colored eggs and smoky-looking clouds of sperm, they unite and grow into tiny larvae. The larvae are left on their own to swim off and establish new colonies, often many miles from the parent reef. Some corals reproduce by brooding, fertilizing eggs inside of the polyp and releasing a fully formed larvae rather than an egg.
Besides forming new colonies through sexual reproduction, corals can form new colonies in other ways. Sometimes rough weather breaks off a piece from an existing colony. If that fragment resettles, it may continue to grow as a new colony. This process is called fragmentation. They can form new colonies through asexual reproduction or to make an exact copy without eggs and sperm. By budding, coral polyps divide to create new individuals. As more polyps bud, a colony grows.
Coral colonies create complex communities. Growing slowly over hundreds of years, hundreds of coral colonies form massive reefs. The different corals can coexist, as each exploits a different part of the reef. And reefs provide homes for a wide variety of fish, invertebrates and marine plants.
Other Reef Builders
Other organisms also play roles in building reefs. Working together, these plants and animals can build and maintain a reef for thousands of years. Frame builders, like coral, add their bodies to the reef, giving it shape and size. Sand and sediment fill in the gaps between coral heads. Cementers, like encrusting algae, hold this material in place. Sediment producers, like parrotfish who use a hard beak to bite off piece of coral, break down hard parts of the reef. Their waste becomes the sandy bottom. Bioeroders, like worms, decompose dead organisms, adding broken-down skeletons to the sediment.
Philippine coral reefs are among the most extensive, diverse, and abundant reefs in the world.
Why Are Reefs Vulnerable?
Reefs are vulnerable to damage from violent weather, changing environments and human interference. They need certain conditions to be present in order to survive. Given time, coral can recover from almost any disaster. However, if damaging conditions continue without relief, the reef system collapses.
People harm coral reefs either directly or indirectly. Air pollution, an example of an indirect method, causes the ozone layer to be destroyed. This causes global warming through a green house effect that traps warm air in our atmosphere and causes ocean temperatures to rise. If the ocean gets too hot, corals will bleach or their zooxanthellae leaves or dies. Without their algae, the corals will starve to death. In addition, without the algae, the coral has no color and the white of the skeleton shows through the transparent polyp bodies. People have been noticing coral bleaching since the turn of the century, but since the 1980s it has become increasingly worse.
People also destroy reefs in many direct ways. Dredging, sedimentation and excess nutrients from farming cause algae to grow out of control and overgrow coral reefs. Toxic chemical pollution such as oil spills, anchor damage, destructive fishing methods and collection of all the animals by divers are just some of the ways in which reefs are being destroyed around the world.
Natural disasters will damage reefs, as well. Typhoons and hurricanes are strong storms that can cause huge waves that rip away huge chunks of reef. But these disasters rarely strike the same place twice, so the reef will generally recover.
Many conservation organizations are working to promote awareness of coral reefs through education and trying to promote laws that protect coral reefs. The problems lie in areas where people break the laws because of a lack of enforcement. These groups are also working with fishers to change the methods in which they fish, replacing destructive methods with reef-friendly methods. They encourage governments and companies to cut down on pollution, both into the ocean and into the air. These groups work to educate people, encouraging them to visit coral reefs while being careful not to harm them. Some organizations are even building artificial reefs in places where living reefs have been destroyed.
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Old reefs have provided the land we live on. Living reefs provide us with protection from strong storms, slowing down the powerful waves heading toward shore. Coral reefs provide building materials for the homes of people around the world, and the sand and rubble they leave on the shores can be used to make concrete. Corals and the animals they house, especially sponges, provide natural chemical products that can be used in medicines. These rich ecosystems are also sources of protein from fish and other sea creatures like shrimp and lobsters. The coral reef ecosystem is rich in biodiversity and complexity. The reef ecosystem is valuable to humans in many ways - and in many ways in which we have not yet discovered.
NOTE: The following Web sites are not maintained by the John G. Shedd Aquarium and will open in a new browser window.
Hurricane and Typhoon information from NOAA:
Allen, G. R. 1997. Tropical Marine Life. North Claredon, Vermont: Periplus Editions Ltd. 64p. ISBN# 962-593-157-0.
Perrine, D. 1997. Mysteries of the Sea. Lincolnwood, Illinois: Publications International, Ltd. 312p. ISBN# 0-7853-2430-5.
Reader's Digest. 1984. Reader's Digest Book of the Great Barrier Reef. Sydney, Australia: Reader's Digest Services Pty Limited. 384p. ISBN# 0-949819-41-7.
Veron, J. 2000. Corals of the World. Volume 1. Townsville, Australia: Australian Institute of Marine Science. 463p. ISBN# 0-642-32236-8.
Asexual Reproduction - Reproduction that does not involve the union of egg and sperm; results in an exact copy of the original animal.
Bleaching - The process through which ocean waters warm, causing the coral's zooxanthellae to leave or die. Without their algae, the corals starve to death. Also, the algae are what give the corals their beautiful colors. Without the algae, the coral has no color and the white of the skeleton shows through the transparent polyp bodies.
Brooding - A form of sexual reproduction where the eggs are fertilized inside the coral polyp. The egg remains inside the polyp until it grows into fully formed larvae, which is released.
Budding - When a coral polyps divides to create a new individual. As more polyps bud, a colony grows.
Coral colony - A group of coral polyps that form a unit known as a coral head.
Fragmentation - The process through which a new coral colony forms when a piece breaks off an existing colony, resettles on a new area of the reef, and starts to form a new colony.
Hard coral - The members of the order Scleractinia, which secrete a heavy, external, calcareous skeleton, and many of which are primary contributors to the building of coral reefs.
Hurricane - A strong tropical cyclone that is caused by a low-pressure system over tropical or sub-tropical waters with organized convection or thunderstorm activity and a definite cyclonic surface wind circulation. This classification comes into effect when winds reach 74 mph (33 m/s). They are called a "hurricane" in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E. A hurricane is a typhoon, just on the other side of the world.
Invertebrate - An animal without a backbone.
Nematocyst - Stinging barbs like tiny harpoon-like stingers which deliver a paralyzing poison sting to stun or kill prey.
Phytoplankton - Microscopic plants that float at the mercy of the ocean currents.
Polyp - An animal that does not move because it is attached to a hard substrate; resembles a flower with the bottom of its stalk attached to a stable surface and tentacles and oral arms facing upward; reproduces asexually.
Sclerites - Microscopic calcareous particles.
Soft coral - A coral that is made of sclerites that lives in the deeper waters of the reef because it doesn't rely on sunlight for nutrients.
Spawn - The release of sex cells, eggs or sperm, into the water.
Symbiotic relationship - Symbiosis - In order to survive, certain creatures form partnerships with other kinds of animals or plants for feeding, housing or protecting one another.
Typhoon - A strong tropical cyclone that is caused by a low-pressure system over tropical or sub-tropical waters with organized convection or thunderstorm activity and a definite cyclonic surface wind circulation. This classification comes into effect when winds reach 74 mph (33 m/s). They are called a "typhoon" in the Northwest Pacific Ocean west of the dateline. A typhoon is a hurricane, just on the other side of the world.
Zooplankton - Microscopic animals that float at the mercy of the ocean currents and have weak swimming ability.
Zooxanthellae - Tiny single-celled algae called dinoflagellates that have a symbiotic relationship with an organism like the giant clam or corals. In exchange for a safe place to live under the organism's skin, the zooxanthellae provides its host with nutrients it makes using the sun.
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