Palau’s marine-lake jellyfish actually diverged very quickly from their common ancestor, the spotted jellyfish. Like other jellyfish, the spotted jellies are cnidarians, a scientific grouping that includes reef-building corals. The spotted jellyfish drift in Palau’s lagoon, zapping the occasional zooplankton with their stinging nettles and absorbing the sugary by-products of photosynthesizing algae living in their tissues.

Like many jelly species, the spotted jellyfish has a multi-stage life cycle. Adult males and females with the familiar bell-shaped bodies are called medusae, but you would not recognize very young jellyfish as jellyfish at all. After medusae release eggs and sperm into the water, fertilized eggs hatch as larvae that drift for a few days before attaching to solid objects, such as rocks. The larvae morph into polyps resembling tiny anemones. Polyps can bud off into more polyps or, when conditions are right, into new young medusae.
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the jellyfish do not “eat” algae. Like their lagoon ancestors, the jellyfish simply absorb their algae’s photosynthetic leftovers. The jellies get about three-fourths of their energy from algal excretions and the remainder from prey. In essence, the jellyfish are landlords that hunt a bit on the side.

The jellyfish-algae partnership did not originate in the lakes, either. Ancestral spotted jellyfish brought the arrangement with them. “Spotted jellyfish in the lagoon have basic behaviors that help ‘sun’ their algae,” Martin explains. “They move eastward in the morning. The lake jellies have adapted this migration to each individual lake. The most spectacular migration is in Jellyfish Lake.”

The jellies’ migration delicately balances time in the sun (to benefit their algae) and predator avoidance. The gelatinous masses of peanut-shaped Jellyfish Lake begin their day in the western basin. As the sun rises they pulsate eastward toward the rising sun—but not too far east, because the lakeshore is covered with jellyfish-eating anemones. The jellies stop swimming east when they hit the shade cast by mangrove trees lining the shore.
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At nightfall the jellies switch to a vertical migration. Jellyfish Lake reaches depths of 100 feet, but only the top 45 feet contain oxygen. The bottom is a toxic vat of hydrogen sulfide. Bacteria do a brisk business at the interface, metabolizing both the oxygen above and hydrogen sulfide below. Every night the jellies bob up and down from the surface to the bacterial layer. Besides helping the jellyfish stay in place, dipping down treats the jellies’ algae to a midnight snack of nutrients excreted by the microbial masses.