since the concentration of free-linging endosymbionts is quite low, they may also be supplied indirectly to the coral by ingestion of fecal material released by corallivores and of zooplankton prey containing zooxanthellae;
p.309; in ....
Paulay G.; 1997; Diversity and Distribution of Reef Organisms; in Birkeland C. (ed.) Life and Death of Coral Reefs; Chapman and Hall London - UK;