Jellyfish Lake, Palau: courtesy of the short bus crew

FLOATING in a silent world the color of milky green tea, I am enveloped by an undulating horde of 10 million jellyfish, some the size of cantaloupes, others the size of apples and a few no bigger than blueberries. All dance the two-step jelly ballet: pulse in, pulse out; pulse in, pulse out. Their simple rhythm is as soothing and vital as a heartbeat.

These jellies are found only in this 12-acre pocket of seawater. Known to tourists as Jellyfish Lake and to locals as Ongeiml Tketau, it is one of about 70 marine, or saltwater, lakes in the Republic of Palau, an island nation 550 miles east of the Philippines. Though the lakes remain connected to the sea through fissures in the islands porous limestone, they are shielded from wind and wave by high ridges covered in exuberant foliage. Sounds of the sea are muted by the music of the jungle: buzzing insects, chattering fairy terns, the eerie coo of the Micronesian pigeon. The sky is crisscrossed by fruit bats the size of hawks.

Palaus sheltered marine lakes are tiny seas imprisoned in terra firma. Five of the lakes contain unique jellyfish, each varying from its neighbors and their common ancestor in a dramatic example of the origin of species. Charles Darwin, of course, used island residents as models of his revolutionary theory. One species of bird, isolated on bits of land surrounded by water, radiated into a remarkable variety of new forms. The same forces work on marine species isolated in bits of water surrounded by land. If Darwin had stepped ashore in Palau instead of the Galápagos, the icon of evolution might be not Darwins finches, but Darwins jellyfishes.
Rise of the Jellies
Laura Martin and Lori Bell of the Coral Reef Research Foundation (CRRF), founded in 1991 by marine biologists to study and improve the protection of coral reefs, live the fantasy of every kid who ever dreamed of becoming a marine biologist. The CRRF compound has labs downstairs, an apartment upstairs, aquarium tanks in the courtyard and a boat at the backyard dock.

Lots of people have heard about the lake jellies, but there are so many misconceptions, Martin says. Again and again we hear, After millions of years of isolation, they evolved into stingless farmers of algae. None of that is accurate.

Palaus 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 Palaus 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.
Palaus first marine lake formed just 12,000 to 15,000 years ago after the last ice age ended and sea levels rose. Palaus rock islands were limestone peaks riddled with erosion-carved channels, fissures and depressions. Seawater seeping through the limestone transformed the largest depressions into marine lakes and swept in the larvae of spotted jellyfish and other sea creatures. In a mere moment of evolutionary time, the landlocked jellyfish radiated into five different subspecies, each attuned to its own isolated island of seawater. The jellies in the deepest lakes, which filled first and are therefore the oldest, diverged the most from their lagoon-living ancestor.

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