Fig 1.

The six phases of symbiosis establishment and persistence in cnidarian-algal symbiosis. 1, initial surface contact between the algal symbiont and cnidarian host cell; 2, symbiont engulfment by the host cell; 3, dynamic sorting of the symbionts (now enclosed by a membrane of host origin), leading either to rejection of the symbiont (dashed arrow) or acceptance; 4, proliferation of the symbiont via cell division within the host tissues; 5, dynamic stability, where the symbiont population is maintained at a steady density; and 6, symbiosis dysfunction and breakdown (for example, in response to environmental stress). For simplicity, not all the possible cellular events are represented here; for more detailed descriptions of these events, see the relevant sections in the text.