A coral reef is a diverse underwater ecosystem built and maintained by colonies of tiny marine animals called coral polyps. Coral reefs are among the most productive and biodiverse ecosystems on Earth, often referred to as the 'Rainforests of the Sea'.
Coral polyps are small, soft-bodied organisms belonging to Class Anthozoa, Phylum Cnidaria. Key features include:
Note: Unlike other cnidarians (e.g., jellyfish), Anthozoa exist only as polyps — there is no medusa stage.
Coral reefs depend critically on a mutualistic symbiosis between coral polyps and photosynthetic algae called Zooxanthellae (Symbiodinium spp.), which are dinoflagellates living within coral tissues.
| Partner | Benefit Received |
|---|---|
| Coral polyp | Up to 90% of energy needs supplied by algal photosynthesis; also receives |
| Zooxanthellae | Shelter inside coral tissue; and nutrients (nitrogen, phosphorus) from coral metabolism |
Zooxanthellae also give corals their characteristic brown/golden colour.
When corals are stressed (e.g., by elevated water temperature, pollution, or ocean acidification), they expel their zooxanthellae. Without the algae, corals turn white — a phenomenon called coral bleaching. Prolonged bleaching leads to coral death.
There are three main types of coral reefs, classified by their structure and relationship to land:
Despite covering less than 0.1% of the ocean floor, coral reefs:
| Threat | Effect |
|---|---|
| Climate change (rising sea temperatures) | Coral bleaching and death |
| Ocean acidification (increased ) | Dissolves skeletons; inhibits reef building |
| Pollution (agricultural runoff, sewage) | Algal overgrowth smothers corals |
| Overfishing | Disrupts ecological balance |
| Physical damage (destructive fishing, tourism) | Direct destruction of reef structure |
| Feature | Detail |
|---|---|
| Builders | Coral polyps (Class Anthozoa, Phylum Cnidaria) |
| Skeleton material | Calcium Carbonate () |
| Key symbiont | Zooxanthellae (dinoflagellate algae) |
| Types | Fringing, Barrier, Atoll |
| Biodiversity | ~25% of marine species in <0.1% of ocean area |
| Nickname | 'Rainforests of the Sea' |