Other Environmental Themes

Dance of the Jellyfish

Just before dawn, millions of Mastigias jellyfish bob aimlessly in the western portion of a lake in Palau—an archipelago lying in the southeastern edge of the Philippine Sea. When the sun rises, the motions become much more systematic as the jellyfish move en masse toward the lake's eastern rim. They swim for an hour or more, propelled by nonstop contractions of their "swimming bells," until they reach their destination, stopping before the shadows cast by tree branches. The jellyfish follow the sun as it moves westward, swimming back toward the western shore, again stopping before the shadowy edge. What's behind this strange ritual?

The jellyfish move in this curious fashion—making a daily roundtrip migration from the western part of the lake to the east and back—to increase their exposure to sunlight in order to benefit the symbiotic algae called zooxanthellae living inside their bodies. Ever the perfect host, the jellyfish even rotate during daylight hours to provide equal exposure to all members of the algae community. In this mutualistic arrangement, the algae are ferried around all day to bask in the sun, avoiding shadows that would limit their productivity. At night, the jellyfish descend to the lake's depths to fertilize the algae cells with key nutrients. In return, the jellyfish get a free lunch (and breakfast and dinner), eating the products of the zooxanthellae photosynthesis.

It was not always this way. At one time, Mastigias used to prey upon zooplankton, capturing the tiny creatures with their stinging cells. But the former atoll became an isolated saltwater lake tens of millions of years ago when the island's reefs were pushed upward by colliding tectonic plates. Trapped inside a lake, the Mastigias exhausted its normal food supply, thus becoming unable to sustain a predatory lifestyle. To survive, the jellyfish began to cultivate its food from within. Over time, its stinging cells lost their punch, and the transition from predator to farmer—albeit one with odd travel habits—became complete.