Zooxanthellae are mixotrophic dinoflagellates that live in symbiosis with scleractinians and other invertebrates. Depending on their characteristics, Zooxanthellae are divided into clades and they inhabit different types and areas of coral depending on environmental conditions in which they are located. There are some argues between scientists about how much Zooxanthellae actually benefit from the coral and whether their relationship is really mutualistic. Corals supply CO2 and other nutrients to Zooxanthellae, they constantly maintain in photic zone so they have enough sufficient light energy for photosynthesise. Also, corals offer them protection from grazers and UV damage by fluorescent protein (GFP) from the coral tissue. Despite these benefits, Zooxanthelle also lose a significant fraction (up to 90%) of their photosynthesis products and cannot grow within the host due to limited space as much as they could as a free stage. Expulsions from host or digestion are possible every time environmental conditions change even a little bit and if nitrogen conservation hypothesis is correct, supply of nutrients was limited by the host. Whichever of these hypotheses was correct, this symbiotic relation held for 200 million years and is one of the most important for the survival of coral reefs.