These facts about coral reefs are presented in conjunction with the USGS Coral Reef Project.
What is a reef?
The word “reef” holds different meanings for everyone. 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?
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.
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.
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.
Related Content
Learn about the USGS Coral Reef Project's current studies by topic, and its study locations, listed below.
Coral Reef Project
Coral Reef Project: Kwajalein Island
Coral Reef Project: Roi-Namur Island
Coral Reef Project: Molokaʻi
Coral Reef Project: Maui
Coral Reef Project: Kauaʻi
Coral Reef Project: Oʻahu
Coral Reef Project: Tutuila
Coral Reef Project: Guam
Coral Reef Project: Hawaiʻi
Coral Reef Project: Lānaʻi
Coral Reef Project: Kahoʻolawe
Sea-Level Rise and Climate Change Impacts to Reefs
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- Science
Learn about the USGS Coral Reef Project's current studies by topic, and its study locations, listed below.
Coral Reef Project
Explore the fascinating undersea world of coral reefs. Learn how we map, monitor, and model coral reefs so we can better understand, protect, and preserve our Nation's reefs.Filter Total Items: 16Coral Reef Project: Kwajalein Island
The USGS is working to provide a better understanding of how spatially-varying atoll morphology and coral cover interact with changes in water level to affect the propagation of waves of different heights and wavelengths across atoll reefs.Coral Reef Project: Roi-Namur Island
The USGS is working to provide a better understanding of how spatially-varying atoll morphology and coral cover interact with changes in water level to affect the propagation of waves of different heights and wavelengths across atoll reefs.Coral Reef Project: Molokaʻi
As part of the USGS Coral Reef Project, recent USGS work on Molokaʻi includes looking into the coral record to find clues to past sedimentation events.Coral Reef Project: Maui
As part of USGS Coral Reef Project studies, the USGS has been heavily involved in efforts to improve the health and resilience of Maui's coral reef system, bringing expertise in mapping, circulation and sediment studies, and seismic surveys.Coral Reef Project: Kauaʻi
As part of the USGS Coral Reef Project, the USGS is working on the island of Kauaʻi to identify circulation patterns and a sediment budget for Hanalei Bay to help determine any effects to the coastal marine ecosystem.Coral Reef Project: Oʻahu
As part of the Coral Reef Project, the USGS is working closely with other local groups to investigate poor water quality issues in Maunalua Bay on the southeast coast of Oʻahu.Coral Reef Project: Tutuila
As part of the USGS Coral Reef Project, the USGS is working on Tutuila with its partners: 1.) in Faga’alu Bay to better understand the impacts of land-based pollutants on the coral reefs, and 2.) on the north coast to help determine the effects of circulation on nearshore waters.Coral Reef Project: Guam
As part of the USGS Coral Reef Project, the USGS is working on the west coast of Guam to help determine the effects of sedimentation in nearshore waters.Coral Reef Project: Hawaiʻi
As part of the USGS Coral Reef Project, the USGS is working on the Kona (west) coast of Hawaiʻi to evaluate geologic resources at two historical parks.Coral Reef Project: Lānaʻi
Little is actually known about the overall extent, biodiversity, morphology, or health of Lānaʻi’s reef tracks. As part of the USGS Coral Reef Project, the USGS collected high-resolution swath bathymetry data and underwater video and photography in early 2013 in order to develop high-resolution seafloor habitat and reef morphology maps of the coral reef systems off of eastern Lānaʻi.Coral Reef Project: Kahoʻolawe
As part of the USGS Coral Reef Project, the USGS is trying to better understand how nearshore processes impact the deeper, scattered coral reef communities of Kahoʻolawe.Sea-Level Rise and Climate Change Impacts to Reefs
Learn how the USGS studies sea-level rise and climate change impacts to coral reefs. - Multimedia
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