Green fluorescence from cnidarian hosts attracts symbiotic algae

Significance Reef-building corals cannot survive without symbiotic algae, Symbiodinium , on which they depend for most of their energy. Most coral species gain symbionts from the environment early in life, and possibly after bleaching (i.e., the loss of symbionts in response to stress). However, Symbiodinium density on coral reefs is very low. Although it has long been hypothesized that corals must be able to attract free-living Symbiodinium , such a mechanism has yet to be identified. Here, we use a series of experiments to demonstrate that corals attract Symbiodinium using their endogenous GFP-related green fluorescence, revealing a biological signaling mechanism that underlies the success of this symbioses that is the building block of coral reef ecosystems.