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What Causes Coral Bleaching

Coral is a collection of living, breathing organisms called polyps. These polyps clump together and form a colony. Polyps are very similar to anemones and have a small mouth and many tiny tentacles. Each polyp secretes a cap of limestone around itself as a skeleton. As it grows it divides and the coral expands. Polyps catch their food by injecting small creatures with the stinging cells on their tentacles. The tentacles then bring the food into the stomach to digest. Coral comes in many different colors and types.

What is Coral Bleaching?
Coral or polyps live in symbiosis with another organism known as zooxanthellae. Zooxanthellae is an algae that escapes being eaten by living inside of the polyps tissue. The zooxanthellae is what give the coral its color. In return for a safe place to live the coral gets up to 90% of its food from the zooxanthellae. Coral bleaching is when the polyps expel the zooxanthellae living inside of it. This causes the polyps’ tissue to become transparent and the white limestone skeleton to show through giving the coral a white or bleached look. Most corals begin to starve after they have expelled their zooxanthellae and they die.

What Causes Coral to Become Bleached?
Coral are very sensitive to certain factors within their environments. This include things such as water temperature, ultraviolent radiation, salinity, opacity and nutrient quantities. Coral bleaching occurs when the environment that is necessary for the coral to sustain its zooxanthellae colony changes in some way or no longer exists. Some of the triggers for the expulsion of the zooxanthellae include:

  • Increase in the water temperature (sometimes a decrease will trigger a release also)
  • Increase in ultraviolent radiation (from hole in the ozone layer or lower sea levels)
  • If the water pH becomes too acidic
  • Starvation due to less food in the area
  • Change in salt levels in the water
  • Disease
  • Damage (by cyanide fishing or people walking on the reef)

Temperature change is the most common cause of coral bleaching and widespread bleaching has been noted over the last century. Many marine scientists blame global warming for coral bleaching as it not only warms the water and increase overall temperatures, but adds more water making the salinity level drop.

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