In Vitro Phagocytosis of Different Dinoflagellate Species by Coral Cells.

Coral-dinoflagellate symbiosis is a unique biological phenomenon, in which animal cells engulf single-celled photosynthetic algae and maintain them in their cytoplasm mutualistically. Studies are needed to reveal the complex mechanisms involved in symbiotic processes, but it is difficult to answer these questions using intact corals. To tackle these issues, our previous studies established an in vitro system of symbiosis between cells of the scleractinian coral and the dinoflagellate , and showed that corals direct phagocytosis, while algae are likely engulfed by coral cells passively. Several genera of the family Symbiodiniaceae can establish symbioses with corals, but the symbiotic ratio differs depending on the dinoflagellate clades involved. To understand possible causes of these differences, this study examined whether cultured coral cells show phagocytotic activity with various dinoflagellate strains similar to those shown by intact . We found that (a) larvae incorporate and , but not , and very few , (b) cultured coral cells engulfed all four species but the ratio of engulfment was significantly higher with and than and , (c) cultured coral cells also phagocytosed inorganic latex beads differently than they do dinoflagellates . It is likely that cultured coral cells preferentially phagocytose and , suggesting that specific molecular mechanisms involved in initiation of symbiosis should be investigated in the future.