Coral Reef
A coral reef is an underwater ecosystem built by colonies of tiny marine animals called coral polyps, which secrete calcium carbonate (CaCO₃) to form hard, branching skeletons that accumulate over thousands of years into complex three-dimensional reef structures. Coral reefs require warm (23–29°C), clear, shallow, sunlit waters with normal marine salinity, as the polyps depend on photosynthetic algae called zooxanthellae living symbiotically within their tissues. Coral reefs cover less than 1% of the ocean floor yet support approximately 25% of all known marine species, making them the most biodiverse marine ecosystems on Earth.
Types of Coral Reefs and Their Structural Characteristics
| Reef Type | Location | Structure | Example | Formation Time |
|---|---|---|---|---|
| Fringing reef | Adjacent to shore | No lagoon, attached to coast | Red Sea reefs | Thousands of years |
| Barrier reef | Offshore, parallel to coast | Deep lagoon between reef and coast | Great Barrier Reef | Hundreds of thousands of years |
| Atoll | Open ocean, ring-shaped | Surrounds central lagoon, no island | Maldives atolls | Millions of years (volcanic subsidence) |
| Patch reef | Shallow lagoon floor | Isolated circular mound | Florida Keys patches | Hundreds to thousands of years |
| Deep-water coral | Below photic zone | No zooxanthellae, cold water | Norwegian fjord reefs | Thousands of years |
Interactive Tools
NOAA Coral Reef Information System
Comprehensive database and educational resources on coral reef ecosystems and monitoring
Open ToolGoogle Earth: Great Barrier Reef
Satellite imagery of the Great Barrier Reef and other major reef systems worldwide
Open ToolKhan Academy: Coral Reefs
Lesson on coral reef ecology, biodiversity, and threats from climate change and bleaching
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Marine Ecosystem
A marine ecosystem is a community of living organisms — including phytoplankton, zooplankton, fish, marine mammals, and benthic organisms — interacting with each other and their physical environment (water, light, temperature, salinity, nutrients) in the ocean. Marine ecosystems range from shallow coastal zones and estuaries to the open pelagic ocean, deep-sea hydrothermal vents, and polar seas, with each zone characterized by distinct biological communities adapted to its conditions. These ecosystems provide critical services including oxygen production (phytoplankton generate ~50% of Earth's oxygen), carbon sequestration, food security for billions of people, and climate regulation.
Ocean Salinity
Ocean salinity is the concentration of dissolved salts in seawater, primarily sodium chloride (NaCl), along with chloride, sulfate, magnesium, calcium, and potassium ions. Average ocean salinity is approximately 35 parts per thousand (ppt) or 35 g of salt per kilogram of seawater, though it varies regionally due to evaporation, precipitation, river input, sea ice formation, and melting. Salinity directly affects seawater density and is a key driver of thermohaline circulation, marine organism physiology, and the freezing point of seawater.
Coastal Upwelling
Coastal upwelling is an oceanographic phenomenon in which wind-driven surface water is pushed away from the coast, causing cold, nutrient-rich water from deeper ocean layers to rise and replace it at the surface. This process is driven by the combined effects of prevailing winds blowing parallel to the coastline and the Coriolis effect, which deflects the surface water offshore — a process described by Ekman transport. Coastal upwelling regions are among the most biologically productive ocean areas on Earth, supporting major fisheries such as those off Peru, California, and West Africa.
From Arabic "qural" (small stones), which passed through Portuguese "coral" and Old French into English. The reef building process was first scientifically described by Charles Darwin in his 1842 work "The Structure and Distribution of Coral Reefs," in which he correctly proposed the theory of atoll formation through volcanic island subsidence.