Coral are marine invertebrates with outreaching stems known as polyps. Coral polyps have a stem-like appearance that cause people to believe corals are plants. However, these magnificent creatures are more closely related to animals as their leaflike tentacles feed into the mouth, which as at the center. Coral polyps are attached to an exoskeleton made of calcium-carbonate. Coral relies heavily on zooxanthellae, a type of algae that inhabits the see-through polyps and colours them. However coral requires photosynthesised nutrients as well as food that it has caught. When water temperature rises the zooxanthellae becomes poisonous to coral and has to be expelled. Unless the coral belongs to the Lohelia pertusa species(a cold water coral that does not require zooxanthellae) it will starve and possibly die if the temperature does not lower.
Natural coral threats are as follows :
The crown-of-thorns starfish is a predatory creature that feeds of coral.
When in large numbers they can wipe out entire reefs easily. This is called
a Crown-of-Thorns outbreak. Scientifically, an outbreak occurs when there
are thirty adults per hectare. Outbreaks have recently been increasing.
A widely believed hypothesis is that phytoplankton (microscopic organisms
which are mostly single celled plants) causes Crown-of-Thorn outbreaks
because agricultural land runoff (Water leaving farms for variable reasons
.The water generally contains chemicals that assist plant growth)
feeds the phytoplankton which are a food source for Crown-of-Thorns larvae.