Corals feed off the byproducts of a microscopic algae living within their own tissue, called zooxanthellae. The photosynthetic activity of the algae is vital to the
survival of the coral animals, which use the energy to extract calcium from the
seawater and build their calcareous skeletons. The zooxanthellae are protected
by the hard coral and obtain plant nutrients from the coral. What type of relationship is this?