Bleached corals are not dead, but are more at risk of starvation and disease. Rising ocean temperatures caused by climate change is the primary cause of coral bleaching.
A temperature increase of just one degree Celsius for only four weeks can trigger bleaching. Changes in water quality, increased sun exposure and extreme low tides can also cause corals to bleach. A relative of jellyfish and sea anemones, corals are invertebrates that belong to a large group of animals called Cnidaria. They are known as colonial organisms, which means many individuals live and grow while connected to one another.
Each individual coral is made of a polyp — a clear, tube-shaped structure attached to a reef at one end and open at the other. The open end has a mouth that is surrounded by a ring of tentacles. Hard corals extract calcium from seawater to make limestone outer skeletons, which form coral reefs. Soft corals tend to be feathery in appearance such as sea fans and sea feathers. This algae, called zooxanthellae pronounced zo-UH-zan-thuh-lay , helps corals by removing waste and using it to produce food in a process known as photosynthesis.
When corals are stressed, they expel the zooxanthellae that live inside their tissues. Without the algae to provide colour, corals appear transparent and reveal their white skeletons.
Bleaching occurs when corals are under stress. Duration: 3 minutes Grade Level: Appropriate for all ages; content connects to NGSS standards for grades 5 and above Shallow reefs are particularly sensitive to increasing temperatures. Immersive Article: Inside Coral Bleaching. Read the article. How does water temperature affect the health of coral reefs? Go to lesson. Connections to the Next Generation Science Standards.
Disciplinary Core Ideas MS-LS Considering the relationship between coral cells and zooxanthellae, how can you explain the pattern of interaction among these organisms? Crosscutting Concepts Stability and Change. Related Resources. Coral Reefs and Climate Change. Help for Endangered Corals. During and the Great Barrier Reef suffered its worst mass bleaching event for years, wiping out two thirds of its coral.
Scientists are worried that the reef hasn't had enough time to recover from this last bleaching, so that's why this current event is particularly bad news. So just what is coral bleaching, and what is being done to stop it? It might look like a plant, but coral is actually an animal! Coral, or polyops as they're also called, are closely related to creatures like sea anemones and jellyfish.
They have a hard, protective limestone skeleton called a calicle, which they use to help anchor them to the sea floor. They often live in groups called colonies, and can form reefs when lots of colonies join together. Many types of coral have a special relationship with tiny plant algae called zooxanthellae - that live in the coral. Bleaching happens when sea temperatures get too high or low, causing the algae to get 'stressed out' and leave the coral.
This then turns the coral white and, with its main source of food gone, it is left very vulnerable. Some reefs also provide a home to species that can't be found anywhere else on Earth. Coral reefs also help to protect against coastal erosion which can damage cliffs and beaches.
Creatures and fish living on the coral provide food and income for local communities. The mining and burning of coal releases carbon pollution into the air, which is heating our planet and warming our oceans. If we continue to pollute the air and the ocean with carbon emissions at our high rate, coral reefs around the world will face a catastrophic future in coming decades — in our lifetime. The stunning colours in corals come from a marine algae called zooxanthellae, which live inside their tissues.
This algae provides the corals with an easy food supply thanks to photosynthesis, which gives the corals energy, allowing them to grow and reproduce. When corals get stressed, from things such as heat or pollution, they react by expelling this algae, leaving a ghostly, transparent skeleton behind.
Some corals can feed themselves, but without the zooxanthellae most corals starve. In some instances corals can recover from bleaching. If conditions return to normal, and stay that way corals can regain their algae, return to their bright colours and survive. However prolonged warmer temperatures and other stressors, like poor water quality, can leave the living coral in a weakened state. It can struggle to regrow, reproduce and resist disease — so is very vulnerable to coral diseases and mortality.
It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently. If we continue burning fossil fuels at our current rate, severe bleaching events are likely to hit reefs annually by the middle of the century.
This would be devastating for coral reefs as they would have no chance to recover.
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