Light intensity as a factor in the regulation of the density of symbiotic zooxanthellae in Aiptasia tagetes (Coelenterata, Anthozoa)

Experiments designed to investigate the effect of different levels of illumination on the density of symbiotic zooxanthellae in the anemone Aiptasia tagetes (Duch. & Mich.) are described.
The anemones are found to regulate the densities of their zooxanthella populations to fixed levels which are dependent upon ambient light intensity.
Regulation is continuous and results from the interaction of increase in algal numbers, growth of host tissues, population of new host tissues by symbionts and extrusion of zooxanthellae by the host.
The nature of the process by which zooxanthellae are selected for extrusion is discussed and a scheme is outlined indicating the pathways through which the level of the algal population in Aiptasia , and other coelenterates, is believed to be controlled.