Coral reefs are some of the most diverse ecosystems in the world. Coral polyps, the animals primarily responsible for building reefs, can take many forms: large reef building colonies, graceful flowing fans, and even small, solitary organisms. Thousands of species of corals have been discovered; some live in warm, shallow, tropical seas and others in the cold, dark depths of the ocean.

Coral characteristics

Shallow water, reef-building corals have a symbiotic relationship with photosynthetic algae called zooxanthellae, which live in their tissues. The coral provides a protected environment and the compounds zooxanthellae need for photosynthesis. In return, the algae produce carbohydrates that the coral uses for food, as well as oxygen. The algae also help the coral remove waste. Since both partners benefit from association, this type of symbiosis is called mutualism.

Deep-sea corals live in much deeper or colder oceanic waters and lack zooxanthellae. Unlike their shallow water relatives, which rely heavily on photosynthesis to produce food, deep sea corals take in plankton and organic matter for much of their energy needs.

What are the main threats to coral reefs?

• Climate change: Corals cannot survive if the water temperature is too high. Climate change has already led to sharply increased rates of coral bleaching – killing vast areas of reef – and this is predicted to increase in frequency and severity in the coming decades. Scientists estimate that at current rates of ocean warming and reef decline, most of the world’s coral reefs could be lost in the next few decades.
• Overfishing: This affects the ecological balance of coral reef communities, warping the food chain and causing effects far beyond the directly overfished population.
• Unsustainable coastal development: Tourist resorts and other coastal infrastructure have been built directly on top of reefs or close enough to them to cause significant damage. The impacts of coastal development can vary widely, and cumulatively can put coral reef systems under considerable additional pressure. Some resorts and coastal developments empty their sewage or other wastes directly into water surrounding coral reefs.
• Pollution: Urban and industrial waste, plastics, sewage, agrochemicals, and oil pollution are poisoning reefs. These toxins are dumped directly into the ocean or carried by river systems from sources upstream. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae.

• Sedimentation: Erosion caused by construction (both along coasts and inland), mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can ‘smother’ corals by depriving them of the light needed to survive. The destruction of mangrove forests, which normally trap large amounts of sediment, is exacerbating the problem.
• Destructive fishing practices: These include cyanide fishing, blast or dynamite fishing, bottom trawling, and muro-ami (banging on the reef with sticks). Bottom-trawling is one of the greatest threats to cold-water coral reefs.

Other threats

• Careless tourism: Careless boating, diving, snorkeling, and fishing happens around the world, with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs.
• Coral mining: Live coral is removed from reefs for use as bricks, road-fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters who don’t know or don’t care about the longer term damage done, and harvested for the live rock trade.