The effects of symbiotic state on heterotrophic feeding in the temperate anemone Anthopleura elegantissima

The temperate sea anemone, Anthopleura elegantissima is facultatively symbiotic with at least two distinct algae: zooxanthellae (Symbiodinium muscatinei) and zoochlorellae (Elliptochloris marina). Symbiotic A. elegantissima potentially receive excess photosynthate from their algal partners, which supplements heterotrophic feeding. But asymbiotic individuals must rely solely on heterotrophic food sources. We predicted that asymbiotic A. elegantissima, due to their lack of algal symbionts, would have a more effective heterotrophic feeding strategy. Symbiotic and asymbiotic A. elegantissima were collected from the field and heterotrophic feeding features were measured (i.e., anemone morphology, tentacle adhesive force, nematocyte sensitivity, cnida size, cnida density, ingestion time, digestion time and absorption efficiency). The anemones were then exposed to natural sunlight or shaded conditions for three weeks and the feeding features were again compared. Few aspects of heterotrophic feeding in A. elegantissima were affected by symbiotic state. Asymbiotic anemones had the largest nematocysts immediately after collection, but were not more efficient predators. We found the greatest nematocyte sensitivity in anemones hosting zooxanthellae, suggesting a greater nutritional need for anemones in this symbiotic state. Though sunlight appeared to increase digestion rate in all anemones, irradiance also had negative effects. Anemones exposed to sunlight had lower cnida densities and smaller spirocysts. Sunlight also appeared to reduce cnidocyte function in asymbiotic individuals. Our results show that symbiotic state has little effect on heterotrophic feeding in A. elegantissima, suggesting that the symbiotic algae may contribute little to the host anemones' daily nutritional requirement and that nutrition in A. elegantissima may be obtained primarily through heterotrophy.