Coral reef

Coral reefs are among the most biologically rich ecosystems on the planet, characterised by reef-building corals that form extensive structures in tropical and subtropical seas. These reefs are composed of colonies of coral polyps held together by calcium carbonate, forming rigid frameworks that support complex ecological communities. Occupying a minute fraction of the global ocean surface yet providing habitat for a vast proportion of marine life, coral reefs play a crucial role in biodiversity, coastal protection and global biogeochemical processes.
Corals belong to the class Anthozoa within the phylum Cnidaria, a group that includes jellyfish and sea anemones. Unlike their soft-bodied relatives, stony corals secrete hard exoskeletons of aragonite, enabling the construction of enduring reef structures. Reefs thrive in shallow, sunlit, low-nutrient waters, where conditions favour both coral growth and the photosynthetic activity of the symbiotic algae that live within coral tissues.

Biological Characteristics

Reef-building corals consist of colonies of small polyps that reproduce and grow continuously. These polyps obtain nutrients through a combination of filter feeding and symbiosis with zooxanthellae, microscopic algae that provide energy through photosynthesis. The secretion of calcium carbonate creates the exoskeletons that accumulate over time to form reefs.
Coral reefs are sometimes described as the ‘rainforests of the sea’ due to their exceptional diversity. Although they occupy less than 0.1 per cent of the world’s oceans, they support at least a quarter of all marine species. Organisms inhabiting reefs include fish, molluscs, crustaceans, echinoderms, sponges, tunicates and various cnidarians. Their complex architecture provides niches for feeding, breeding and shelter, promoting high levels of ecological interaction.
Reefs are most commonly found in warm, shallow seas but may also occur in deeper or colder waters on a smaller scale. Deep-sea and cold-water coral reefs form under low-light conditions, relying exclusively on filter feeding rather than symbiosis.

Environmental Conditions and Threats

Reef development is optimal in waters that are warm, clear, sunlit and agitated, with low nutrient concentrations. High nutrient levels favour algae and plankton that outcompete corals for space and light. Modern shallow-water reefs are under increasing threat from multiple environmental stressors, including:

• Nutrient pollution, particularly nitrogen and phosphorus from agricultural runoff and wastewater.
• Ocean warming, which can induce coral bleaching by disrupting the coral–algae symbiosis.
• Ocean acidification, reducing the availability of carbonate ions required for skeleton formation.
• Overfishing, including destructive practices such as blast fishing, cyanide fishing and damaging spearfishing.
• Chemical pollution, including substances found in some sunscreens.
• Unsustainable land use, leading to sedimentation and contamination from coastal development.

Global assessments suggest that shallow tropical reefs have declined by approximately 50 per cent since the mid-twentieth century. Their loss threatens not only biodiversity but also the livelihoods of millions of people who depend on them for fisheries, tourism and coastal protection.

Ecosystem Services and Economic Value

Coral reefs deliver extensive ecosystem services. They act as natural barriers that protect coastlines from storms and erosion, support fisheries through nursery habitats and provide recreational and tourism opportunities. Economic valuations vary widely, with global estimates ranging from tens of billions to trillions of US dollars annually. The high economic value reflects their ecological importance and their contribution to human welfare.

Formation and Growth

Modern reefs largely formed after the Last Glacial Period, when rising sea levels flooded continental shelves. Most existing reefs are less than 10,000 years old. Reefs grow upwards and outwards through the accumulation of coral skeletons and other calcareous organisms such as coralline algae. Growth is fastest in warm, clear waters with abundant sunlight; vertical growth is limited by light availability and reefs cannot grow above sea level.
Reefs also occur around oceanic islands and atolls. Many such islands are volcanic in origin, while others arise from tectonic uplift. The nature of the underlying substrate and the history of sea-level change strongly influence reef morphology.

Darwin’s Atoll Formation Theory

Charles Darwin proposed a widely accepted theory explaining the formation of atolls during his voyage on HMS Beagle. He suggested that atolls develop through three stages:

1. Fringing reef formation around a volcanic island.
2. Barrier reef formation as the island gradually subsides.
3. Atoll formation once the island sinks below sea level, leaving a ring-shaped reef surrounding a central lagoon.

Darwin hypothesised that beneath each atoll lies the submerged remains of the original volcanic island. Subsequent geological and drilling studies have supported this conclusion. His theory relies on the limited depth range in which coral polyps can survive; if the seafloor subsides too quickly or rises above sea level, coral growth cannot keep pace, resulting in coral drowning or subaerial exposure.

The Great Barrier Reef as an Example

The Great Barrier Reef illustrates reef formation on a continental shelf rather than a volcanic island. Around 20,000 years ago, sea level was significantly lower. As the climate warmed, sea level rose, gradually inundating the coastal plain. Coral communities colonised submerged hills, forming patch reefs, cays and eventually extensive reef structures. Sea level has remained relatively stable for the last 6,000 years, allowing the living reef to develop into the world’s largest barrier reef system.

Reef Material and Geological History

Reefs consist predominantly of calcium carbonate in the form of aragonite secreted by corals. Other components, such as shell fragments and coralline algae, enhance their structural resilience. In geological history, reef-building has been dominated by different organisms during different periods. Calcareous algae and archaeocyathans formed the earliest reefs in the Cambrian. Rugose corals contributed significantly during the Palaeozoic, and rudist bivalves formed major reefs during the Late Cretaceous. Modern reefs are primarily constructed by scleractinian corals.
Reef skeletons preserve environmental information. The oxygen isotopic composition of coral aragonite is widely used to infer past sea-surface temperatures and salinity, making reefs important archives for palaeoclimatic research.

Types of Coral Reefs

Since Darwin’s classification, additional reef types have been identified. Common large-scale reef forms include:

• Fringing reefs, directly attached to shorelines or separated by shallow lagoons; the most widespread type.
• Barrier reefs, separated from land by deeper lagoons and often extending for long distances.
• Atolls, ring-shaped reefs enclosing central lagoons, typically developing around subsiding volcanic islands.
• Table or platform reefs, broad flat reefs often found on continental shelves.
• Bank reefs and patch reefs, irregular formations that do not fit strict definitions but contribute to regional reef landscapes.