What Is a Coral Reef?

Coral reefs are among the most biodiverse ecosystems on Earth — often called the "rainforests of the sea." Despite covering less than one percent of the ocean floor, they provide habitat, food, and shelter for an estimated quarter of all marine species. But what exactly is coral, and how does a reef form?

Coral is not a plant or a rock — it's an animal. Each individual coral is a tiny creature called a polyp, related to jellyfish and sea anemones. Polyps secrete calcium carbonate (limestone) to build hard exoskeletons, and over centuries and millennia, these structures accumulate to form the complex architecture we recognise as a reef.

The Partnership That Makes It All Work: Zooxanthellae

The secret to coral's success is a remarkable symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the coral's tissue and perform photosynthesis, providing the coral with up to 90% of its energy needs. In return, the coral offers the algae a protected environment and access to nutrients.

This partnership is also coral's greatest vulnerability — because it is the zooxanthellae that give coral its vivid colour. When stressed, coral expels these algae, turning ghostly white — a process known as coral bleaching.

The Structure of a Reef

Reefs are not uniform structures. They're divided into distinct zones, each with its own community of species:

  • Reef flat: Shallow, sometimes exposed at low tide. Dominated by tough, encrusting corals and algae.
  • Fore reef (reef slope): The seaward face, rich in coral diversity and fish life. This is the zone most divers explore.
  • Reef crest: The shallowest, most wave-exposed zone. Robust, wave-resistant coral species dominate here.
  • Back reef / lagoon: Calmer, sheltered water. Often home to seagrass beds and juvenile fish.

Threats Facing Coral Reefs Today

Coral reefs face a convergence of pressures, many human-caused:

Ocean Warming

Rising sea temperatures are the leading cause of mass coral bleaching events. When water temperatures exceed a coral's tolerance threshold — even by 1–2°C for an extended period — bleaching occurs. Repeated bleaching events give reefs insufficient time to recover.

Ocean Acidification

The ocean absorbs a significant portion of atmospheric CO₂. As CO₂ levels rise, seawater becomes more acidic, making it harder for corals to build their calcium carbonate skeletons. Over time, existing reef structures can begin to dissolve faster than they are built.

Overfishing and Destructive Fishing Practices

Removing too many herbivorous fish (such as parrotfish) allows algae to overgrow and smother corals. Blast fishing and cyanide fishing cause direct physical destruction to reef structures.

Pollution and Runoff

Agricultural runoff carrying fertilisers and sediment can trigger algal blooms that block sunlight and deplete oxygen. Plastic pollution and sewage discharge also degrade reef health.

Signs of Hope

Despite the threats, coral reefs are resilient when given the chance to recover. Marine protected areas (MPAs), coral gardening programmes, and reduced local stressors have all shown positive results. Some reef restoration projects are experimenting with coral strains that are more heat-tolerant — a potential adaptation tool for the future.

The most impactful thing individuals can do is support sustainable seafood choices, reduce carbon footprints, and practice responsible reef tourism: never touch the coral, never drop anchor on a reef, and choose operators who follow established snorkelling and diving guidelines.