English text
Corals look like plants or rocks, but they are animals.
They stay attached to the ocean floor and cannot move to find food.
Unlike plants, corals cannot make their own food, so they catch small particles from the water.
A coral colony is made of hundreds or thousands of tiny animals called polyps.
Each polyp is soft, and most are no thicker than a coin.
Each reef-building coral polyp makes a hard limestone skeleton.
Over time, the growth and death of polyps build the foundations of coral reefs.
Because of this, reefs are the largest structures made by living things on Earth.
Most corals host microscopic algae inside their tissue.
The algae use the coral's waste for photosynthesis and give oxygen and useful nutrients back.
This kind of partnership, in which both sides benefit, is called symbiosis.
It has lasted for roughly twenty-five million years.
Stress from changes in temperature, light or nutrients can make corals expel their algae.
Without the algae, the coral turns white, which is known as bleaching.
A bleached coral is not dead, but it is weaker and more likely to die.
Rising ocean temperatures caused by climate change make bleaching a growing concern.