More about Bleaching

[Photo showing bleaching in Reunion courtesy of Jean-Pascal Quod,]

This provides a bit more in-depth discussion about coral whitening, which can occur for more reasons than just warm temperatures.
[Text adapted from and courtesy of]

Sometimes corals can appear whitened, but be suffering from other problems.

Corals can appear white due to a number of causes.  Bleaching, disease, and being eaten by predators can all turn corals white.
Here’s some guidance on how to tell the different possibilities apart.
Don’t worry if you have a hard time telling the difference; reporting disturbed corals is important whatever the cause.

Is it bleaching, disease, or eating by predators?


  • Gradual color loss
  • Begins on upper surface
  • More than one colony affected
  • Tissue intact
  • Bleached coral is not always white


  • Disease progresses across the colony
  • White or black band between the dead coral and the live coral
  • Tissue death at the interface between the disease and the live coral

Drupella (snails eating):

  • Jagged edges between live and dead coral
  • Coral eaten in patches
  • Snails often nearby new white scars

COTS (crown of thorns starfish eating):

  • Abrupt interface between live and dead tissue
  • COTS tend to prefer plate and branching corals
  • COTS scars are larger than Drupella scars

Bleaching can be caused by a variety of stresses, not just warm temperatures.

Bleaching can be caused by other stresses besides warm water, including freshwater from rain or rivers, sedimentation, and pollution.
Also, bleaching doesn’t always leave corals white– other colors like brilliant yellow, red, purple, and other colors due to plant pigments can occur when corals lose their algae symbionts.  Widespread bleaching over large areas is, however, generally caused by overly warm water.

While it might not always be easy to distinguish all the different threats impacting corals, just reporting whether the corals appear healthy or unhealthy can help direct attention to areas which are under stress and can use more monitoring.