Distribution patterns and nutritional contributions of algal symbionts in the sea anemone Anthopleura xanthogrammica

The Pacific intertidal sea anemone Anthopleura xanthogrammica hosts 2 algal symbionts, zoochlorellae Elliptochloris marina and zooxanthellae Symbiodinium muscatinei, either alone or co-occurring.Previous studies have suggested that zoochlorellae and zooxanthellae represent 'cool' and 'warm' symbionts with respect to their field distributions, and that these symbionts may differ in their nutritional contributions to their host.We examined the seasonal distribution, density, and growth of these symbionts in A. xanthogrammica tentacles from tidepools and surge channels on the Olympic peninsula in Washington State, USA, measured temperature variation between these microhabitats, and estimated the contributions of zoochlorellae to A. xanthogrammica diet.Tentacles containing dense concentrations of zoochlorellae were found in both tidepools and surge channels at the lower intertidal limit of anemone occurrence.At the upper intertidal limit, tentacles containing primarily zoochlorellae were found in tidepools, and tentacles containing primarily zooxanthellae were found in surge channels.More extreme high tem peratures in the upper surge channel may limit the distribution of zoochlorellae and favor a higher proportion of zooxanthellae in this microhabitat.Despite pronounced seasonal fluctuations in temperature, symbiont composition, density, and dietary carbon sources remained remarkably con sistent.Stable isotope analysis showed that A. xanthogrammica received a greater proportion of dietary carbon from zoochlorellae (62-70%) than from heterotrophic feeding on Mytilus californianus mussels (31-38%).This study shows that dense concentrations of zoochlorellae are found in A. xanthogrammica tentacles in cooler microhabitats, and that this symbiont can contribute substantially to anemone nutrition.