Reef-building corals establish an endosymbiotic relationship with dinoflagellate in the family Symbiodiniaceae, which is commonly called zooxanthella. Zooxanthellae can still grow outside the host corals as other dinoflagellates. In general, the seawater surrounding coral reefs is heterotrophic and not suitable to sustain photosynthetic organisms, including zooxanthellae. Therefore, the cell density of zooxanthellae in seawater is low. Within host cells, zooxanthella lose their motility, becoming immotile. Although they lose their motility, the host provides sufficient nutrients, such as nitrogen and phosphorus, with the zooxanthellae providing photosynthetically produced energy, such as sucrose, in return. The mutual symbiotic relationship between corals and zooxanthellae allow them both to adapt to the heterotrophic environment of coral reefs. Zooxanthellae within coral are major producers in coral reef ecosystems, which are one of the most biologically enriched environments on our planet. Thereby, the healthy coral-zooxanthellae symbiotic relationship is important for all organisms in coral reefs. Unfortunately, we are losing corals worldwide due to coral beaching induced by increases in seawater temperature. Coral bleaching occurs mainly through the expulsion of zooxanthellae from host corals. The expelled zooxanthellae are still alive and remain free-living in seawater until they find a new coral host. I will explain the unique characteristics of zooxanthellae, and outline difference in their lifestyles in inside and outside the host corals.