Something about free-living zooxanthellae and its role in coral reefs, to instruct our knowledge:There is a legion of free-living zooxanthellae on coral reefs, which are bi-flagellated, like any dinoflagellate of the genus Symbiodinium sp, and swim like any dinoflagellate. But the zooxanthelae that enter the host lose the flagellum, and when they are expelled, they can no longer swim while, perhaps, it does not recompose the flagellum. Ciliates swim well and catch them easily, such as collecting eggs.
The process of expulsion of zooxanthellae from coral tissues (coral bleaching) is a physiological and coral dependent process, since they are trapped in their tissues. This only occurs if the coral is alive. If the coral dies without expelling the zooxanthellae, they will remain trapped in their necrotic tissues and also die, trapped there.
When RTN occurs, zooxanthelae are not expelled and whitening of coral tissues does not occur! So much so that the rags of coral tissue, which detach from the skeleton, remain colorful.
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(PDF) Free-living zooxanthellae: Behavior, ecology and importance on the coral reef
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