Anthopleura elegantissima, the aggregating anemone, is one of the most abundant intertidal sea anemones on the Pacific coast of North America, forming dense carpets of clonal individuals on rocky shores from Alaska to Baja California. It is a facultative photosymbiont associating with either zoochlorellae or zooxanthellae depending on light levels, making it a tractable model for studying photosymbiosis flexibility. The species reproduces asexually through clonal division, forming genetically identical clumps that compete aggressively with neighboring genets. Its genome has been sequenced, revealing extensive immune gene diversity and gene families associated with symbiosis regulation.