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Unlock This CourseCoral reefs are some of the most diverse and productive ecosystems on Earth. Often called the "rainforests of the sea," these underwater structures are built by tiny marine animals called coral polyps. Despite covering less than 1% of the ocean floor, coral reefs support about 25% of all marine species.
Key Definitions:
Coral reefs form when coral polyps attach to hard surfaces on the sea floor. Over thousands of years, these tiny animals build up layers of calcium carbonate, creating the massive structures we see today. The process requires warm, clear, shallow water with plenty of sunlight for the zooxanthellae to photosynthesise.
There are three main types of coral reefs, each with distinct characteristics and formation processes. Understanding these types helps us appreciate the diversity of coral reef ecosystems around the world.
These reefs grow directly from the shore with no lagoon separating them from land. They're the most common type and are found in shallow waters close to coastlines. Examples include reefs around the Red Sea and parts of the Caribbean.
These reefs are separated from the shore by a lagoon. They form parallel to coastlines but are further offshore. The Great Barrier Reef in Australia is the world's largest barrier reef system, stretching over 2,300 kilometres.
Ring-shaped reefs that surround a central lagoon. They typically form around volcanic islands that have sunk below sea level over time. The Maldives and many Pacific islands are examples of atolls.
Coral reefs have distinct zones, each with different characteristics and species. Understanding these zones helps explain the incredible biodiversity found in reef ecosystems.
Fore reef: The seaward side facing the open ocean, with strong currents and waves. Large, robust corals dominate here.
Reef crest: The shallowest part, often exposed at low tide. Only the hardiest corals survive the wave action.
Back reef: The sheltered lagoon side with calmer waters. More delicate coral species and seagrass beds thrive here.
Coral reefs support an extraordinary variety of life. This biodiversity exists because reefs provide numerous ecological niches - different spaces and roles that species can occupy.
The complex three-dimensional structure of coral reefs creates countless hiding places, feeding areas and breeding grounds for marine organisms. From tiny plankton to large sharks, every level of the food web is represented.
Over 4,000 fish species live on coral reefs. These include herbivores like parrotfish that eat algae, carnivores like groupers that hunt smaller fish and cleaners like wrasse that remove parasites from other fish.
Countless invertebrates call reefs home, including sea urchins, starfish, crabs, shrimp and molluscs. Many have evolved bright colours and unique shapes to survive in this competitive environment.
Seagrasses and various algae provide food and shelter. The zooxanthellae living inside corals are crucial primary producers, converting sunlight into energy through photosynthesis.
The Great Barrier Reef off Australia's northeast coast is the world's largest coral reef system. It contains over 2,900 individual reefs and 900 islands, covering 344,400 square kilometres. Home to 1,500 fish species, 400 coral species and 4,000 mollusc species, it demonstrates the incredible biodiversity possible in coral reef ecosystems. The reef supports both marine life and human communities through tourism and fishing industries.
Coral reefs are built on partnerships. The most important relationship is between coral polyps and zooxanthellae, but many other symbiotic relationships exist throughout the reef ecosystem.
These relationships have evolved over millions of years and are essential for reef survival and growth.
Zooxanthellae live inside coral tissues and provide up to 90% of the coral's energy through photosynthesis. In return, corals provide the algae with protection and nutrients. This partnership allows reefs to thrive in nutrient-poor tropical waters.
Cleaner fish like wrasse remove parasites and dead tissue from larger fish. The cleaners get food whilst the larger fish stay healthy. These "cleaning stations" are busy hubs of reef activity.
Coral reefs are very sensitive to environmental conditions. They require specific physical and chemical conditions to survive and grow, which explains their limited global distribution.
Understanding these requirements helps explain why coral reefs are found only in certain parts of the world's oceans.
Corals need warm water between 18-30°C. Most reefs grow in waters around 23-25°C. Water that's too hot or too cold can cause coral bleaching and death.
Shallow, clear water allows sunlight to reach the zooxanthellae for photosynthesis. Most reef-building corals grow in water less than 50 metres deep.
Corals need clean, clear water with low nutrient levels. Pollution and sediment can block sunlight and damage coral tissues.
The Caribbean contains about 9% of the world's coral reefs, including the Mesoamerican Reef System - the second-largest barrier reef. These reefs support over 65 coral species and 500 fish species. However, Caribbean reefs have lost over 50% of their coral cover since the 1980s due to climate change, pollution and overfishing, making them a critical case study for reef conservation efforts.
Coral reefs face numerous threats from both natural and human activities. Understanding these threats is crucial for developing effective conservation strategies.
Climate change is the biggest threat, but local human activities also cause significant damage to reef ecosystems worldwide.
Rising sea temperatures cause coral bleaching, where stressed corals expel their zooxanthellae and turn white. Ocean acidification, caused by increased CO2 absorption, makes it harder for corals to build their calcium carbonate skeletons.
Coastal development, pollution from agriculture and sewage, overfishing and destructive fishing practices all damage reef ecosystems. Tourism, whilst economically important, can also cause physical damage if not properly managed.
Coral reefs provide enormous benefits to both marine ecosystems and human communities. Protecting these valuable ecosystems requires global cooperation and local action.
The value of coral reefs extends far beyond their beauty, providing essential services to both marine life and human societies.
Reefs act as natural barriers, reducing wave energy by up to 97% and protecting coastlines from erosion and storm damage. This saves billions in coastal defence costs.
Over 500 million people depend on reefs for food and income through fishing and tourism. Reefs provide protein for many coastal communities, especially in developing countries.
Many reef organisms produce compounds used in medicines. Scientists continue to discover new drugs from reef species, including treatments for cancer and other diseases.