Stromatolites, such as the ones shown above, are living fossils. They built the first reefs on Earth. Today, stromatolites live in stressed marine environments. These are from Shark's Bay World Heritage Site in Australia.
What is a reef?
The word "reef" has different meanings to different people. To a mariner, the term "reef" applies to shallowly submerged navigational hazards. To a surfer, a reef is an undersea obstruction that can make waves (and surfboards) break. Scientists generally restrict the definition of a reef to rigid biological constructions.
The Earth's first reef-building organisms were photosynthesizing cyanobacteria living about 3.5 billion years ago. From fossil remains, it is known that a variety of organisms have constructed reefs, including bivalves (clams and oysters), bryozoans (coral-like animals), and sponges. Corals have been found in fossil reefs as old as 500 million years, but corals similar to the modern colonial varieties have constructed reefs "only" during the last 60 million years.
What is a coral reef?
The hard skeleton of coral is formed by the secretion of calcium carbonate by the polyp. Polyps live in the coralium; they gather food particles with tentacles and feed from sugars produced by photosynthesizing zooxanthele.
Corals are animals related to jellyfish and anemones. Solitary and colonial corals catch plankton and suspended food particles with arm-like tentacles, which feed a centrally located mouth. Coral reefs are formed by huge colonies of corals that secrete hard calcareous (aragonite) exoskeletons that give them structural rigidity. These colonial "hard corals" may form elaborate finger-shaped, branching, or mound-shaped structures, and can create masses of limestone that stretch for tens or even hundreds of miles.
Most hard corals also host symbiotic algae, a long-standing and successful partnership. These algae provide them with an additional food source through photosynthesis.
When corals are stressed, they expel these algal symbionts through a process known as coral bleaching. Corals also face serious risk of diseases; black band, white band, and yellow band diseases have been reported from many localities. Hawaiian corals, however, have been relatively free from disease, but the first case of black band disease was reported in 1994.
How do coral reefs form?
Corals have a wide distribution in the world's oceans, but the varieties that form reefs typically are restricted to relatively shallow, warm tropical waters between 30° north and south latitudes. Clean, clear water with the right amount of nutrients is essential to their health. After initial colonization of a hard substrate and given suitable conditions for coral growth, an individual larval "spatfall" gives rise to a colony.
Given enough time, coral colonies become thickets. As coral thickets build upward on the skeletal remains of older colonies, a reef is established. Today, richly diverse coral reefs are found along tropical coastlines, on the margins of volcanic islands, and as isolated coral atolls.
Australia's Great Barrier Reef is the world's largest. It stretches approximately 1,250 miles (2,000 km) along the Pacific coast.
Coral reefs are dynamic, evolving through time into different forms. During his voyages on the HMS Beagle, Charles Darwin first recognized the progressive development of coral reefs on volcanic islands. Volcanic islands subside; that is, they have a tendency to cool, condense, and sink through time. As they subside, tropical coral reefs grow upward along their margins.
The Hawaiian Islands-Emperor Seamounts chain is a classic example of this process. Active volcanic islands are found at the southeast end of the chain. Beyond Kauaʻi, however, islands are subsiding slowly and coral reefs have developed around the volcanic cores of islands. French Frigate Shoals, Midway, and Necker Islands are dominantly coral limestone accumulations, but deep sediment cores have revealed the volcanic origins of these islands.
The animation above shows the progressive development of fringing coral reefs into barrier reefs and ultimately into atolls.
This coral atoll in the South Pacific shows a well developed central lagoon and emergent reef crest. Active coral growth is on the margins of the island.
Why are coral reefs in peril?
Coral reefs are sensitive indicators of the health of marine environments. Yet coral reefs are in decline in many parts of he world. It is estimated that 30% will be destroyed or seriously degraded in the next ten years.
The causes of reef degradation are many. They are being stressed and killed by a variety of local human activities such as grounding of ships, improperly placed anchorages, destructive fishing practices, such as dynamiting or cyanide poisoning, and simply overfishing, which disrupts the balance of these fragile ecosystems.
Pollution and sediment runoff from land are major causes of stress, and even human activities conducted at great distance through warming and pollution can affect coral sustainability.
As coral reefs become stressed, they also are more susceptible to viral and bacterial infections, such as black, white, and yellow band diseases. It is critically important to better understand the role of natural processes and the impact that human activities may have on coral reef health.