Green fluorescence from cnidarian hosts attracts symbiotic algae.

Reef-building corals thrive in nutrient-poor marine environments because of an obligate symbiosis with photosynthetic dinoflagellates of the genus Symbiosis is established in most corals through the uptake of from the environment. Corals are sessile for most of their life history, whereas free-living are motile; hence, a mechanism to attract would greatly increase the probability of encounter between host and symbiont. Here, we examined whether corals can attract free-living motile by their green fluorescence, emitted by the excitation of endogenous GFP by purple-blue light. We found that have positive and negative phototaxis toward weak green and strong purple-blue light, respectively. Under light conditions that cause corals to emit green fluorescence, (e.g., strong blue light), were attracted toward live coral fragments. were also attracted toward an artificial green fluorescence dye with similar excitation and emission spectra to coral-GFP. In the field, more were found in traps painted with a green fluorescence dye than in controls. Our results revealed a biological signaling mechanism between the coral host and its potential symbionts.