Preferential expulsion of dividing algal cells as a mechanism for regulating algal-cnidarian symbiosis

A wide range of both intrinsic and environmental factors can influence the population dynamics of algae in symbiosis with marine cnidarians. The prese nt study shows that loss of algae by expulsion from cnidarian hosts is one of the primary regulators of symbiont population density. Because there is a significant linear correlation between the rate of algal expulsion and th e rate of algal division, factors that increase division rates (e.g., eleva ted temperature) also increase expulsion rates. Additionally, H-3-thymidine is taken up to a greater extent by algae destined to be expelled than by a lgae retained in the host cnidarians. Taken together, data for rates of exp ulsion, rates of division at different temperatures, and uptake of 3H-thymi dine suggest that dividing algal cells are preferentially expelled from the ir hosts. The preferential expulsion of dividing cells may be a mechanism f or regulation of algal population density, where the rate of expulsion of a lgae may be an inverse function of the ability of host cells to accommodate new algal daughter cells. This kind of regulation is present in some cnida rian species (e.g., Aiptasia pulchella, Pocillopora damicornis), but not in all (e.g., Montipora verrucosa, Porites compressa, and Fungia scutaria).