Mastigias have evolved to rely on a distinctive partnership for their nutrition. They have a symbiotic relationship with single-celled marine algae. Inside the tissues of Mastigias’ frilly arms live millions of single-celled algae called zooxanthellae, which use solar energy to convert carbon dioxide and solar energy into carbohydrates that the jellyfish use for energy.
The lake-bound jellyfish migrate across the lake and back during the day, swimming continuously. Scientists believe this is for two reasons: to keep their zooxanthellae in the light, and to avoid the shadows that form at the lake edges, where Mastigias’ predators (bottom-living sea anemones) live. At night, the Mastigias stop moving horizontally and swim up and down in the lake, away from the edges. Scientists have suggested that the nightly trips might provide the algae with essential nutrients available only in deeper water.
http://www.whoi.edu/oceanus/viewArticle.do?id=102729§ionid=1021
