Potential mechanisms for the regulation of host-symbiont biomass in cnidarian-alga symbiosis. 1, expulsion of symbiont cells in either detached whole host cells or pinched-off portions of host cells (i.e., aposomes). 2, expulsion of symbiont cells either via active exocytosis or as a result of host cell apoptosis. 3, intracellular degradation of the symbiont, as a result of programmed cell death of the symbiont, reengagement of the phagosomal maturation process in the host, or autophagic digestion of the symbiont by the host cell. 4, control of progression through the symbiont cell cycle by the host. Potential mechanisms for the regulation of host-symbiont biomass in cnidarian-alga symbiosis. 1, expulsion of symbiont cells in either detached whole host cells or pinched-off portions of host cells (i.e., aposomes). 2, expulsion of symbiont cells either via active exocytosis or as a result of host cell apoptosis. 3, intracellular degradation of the symbiont, as a result of programmed cell death of the symbiont, reengagement of the phagosomal maturation process in the host, or autophagic digestion of the symbiont by the host cell. 4, control of progression through the symbiont cell cycle by the host. G0, G1, S, G2, and M are the phases of the eukaryotic cell cycle (see Appendix 2), with G1 often being the longest phase and M always being the shortest (as generalized in the schematic). The host may render the intracellular environment unfavorable or signal to the symbiont in such a way that the cell cycle does not, for example, pass through the G1/S checkpoint; in this case, the cell could enter the G0 resting state. 5, control of host cell proliferation by the symbiont. Simon K. Davy et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05014-11