Jellyfish plays an important role in ocean energy budgets.

If you were to snorkel just before dawn at the popular tropical Pacific destination Jellyfish Lake, you'd have lots of company: millions of golden jellyfish, known to scientists as Mastigias papua, mill around the western half of the lake, waiting for sunrise.

With the sun's first rays, Jellyfish Lake, located 550 miles east of the Philippines in the island nation of Palau, comes alive. As the sky brightens in the east, the golden jellies turn and swim toward a solar beacon.

The jellies need sunlight to sustain algae-like zooxanthellae within their tissues; the zooxanthellae in turn sustain the jellies.For several hours the jellyfish swim, the contractions of their bells never stopping, until they approach the eastern end of the lake.

Following the rising sun, the jellyfish are stopped, not by the edge of the lake, but by shadows cast by overhanging trees--which they meticulously avoid.Millions of jellyfish that started out in the west are now densely packed around the illuminated eastern rim of the lake. For a few hours around noon, they're stationary, basking in the mid-day sun directly overhead.

Later in the afternoon, the solar cycle--and jellyfish cycle--reverse, and the jellies swim westward.Eventually the jellyfish complete one round-trip migration from west to east and back, each day between sunrise and sunset.

What the jellies are doing, say marine scientists Michael Dawson of the University of California at Merced and John Dabiri of the California Institute of Technology, is "biomixing"--as they swim, they're churning and churning the waters and nutrients of the lake.

Jellyfish like Mastigias papua and the moon jelly Aurelia aurita use their body motion, Dawson and Dabiri have found, to generate water flow that transports small copepods within feeding range. "The 'underwater turbulence' the jellies create is being debated as a major player in ocean energy budgets," says Dabiri.

At night, the jellies descend into a bottom-layer of hydrogen sulfide. Although snorkeling in the surface waters of the lake is allowed, scuba diving is prohibited to avoid disturbing the jellyfish, and to reduce the risk of hydrogen sulfide poisoning.