Isolated saltwater lakes on remote islands of the South Pacific serve as the stage for a surprising and beautiful daily dance. Spotted jellies or golden medusae (Mastigias papua) migrate west-to-east across their lakes every day, in huge, spectacular densities. Why? Like some other jellies, Mastigias harbor symbiotic algae in their tissues. These algae (in the group Zooxanthellae) are related to those found in …