The clonal anemone, Anthopleura elegantissima, is the dominant cnidarian in the intertidal zone of the Pacific Northwest. This anemone is unique because it is one of two temperate cnidarian species that harbors, intracellularly, two distinctly different algal photosymbionts: zooxanthellae (ZX) and zoochlorellae (ZC). The purpose of this investigation was to compare any differences in physiological patterns of ZX and ZC in A. elegantissima to determine the reason(s) for the maintenance of two such symbionts.

The effects of temperature, light intensity, season, and anemone size on algal production and respiration within anemones were investigated. Both photosynthesis and respiration were measured by a computer-controlled underwater respirometer equipped with a light sensor and oxygen electrodes to measure daily oxygen flux of the intact association.

The results of this study suggest that ZX are the "superior" symbionts since they display greater photosynthetic rates and presumably higher translocation rates. As temperature and light intensity increase, the productivity of ZX-bearing anemones increases faster than that of ZC-bearing anemones. The photosynthetic rates of ZX-bearing anemones were also higher than that of ZC- bearing anemones during much of the year. Regardless of algal species, summer productivity rates were higher than winter productivity rates due to a combination of higher light intensities and temperatures. Regardless of season, photosynthetic rates of algae in small anemones were higher than that of algae in large anemones.

Both environmental and biological parameters may contribute to and modulate the distribution of ZX- and ZC-bearing anemones within the intertidal zone. High light intensities, coupled with elevated temperatures, promotes ZX-anemone associations. In contrast, low light and temperature regimes provide ZC with a potentially higher competitive advantage due to their high intrinsic growth rate.