Pat Lohmann recently traveled to the tiny Western Pacific island nation of Palau to locate coral reefs with Anne Cohen, a scientist at Woods Hole Oceanographic Institution (WHOI). On a free day, Lohmann, a scientist emeritus at WHOI and an expert diver, headed straight to see one of Palau's natural wonders, Ongeim’l Tketau, also known as Jellyfish Lake, where he trained his high-definition video camera on the thousands of jellyfish undulating around him.
“In some ways, you see more in the video than we did when we were in the lake,” Lohmann said. “You can slow it down, you can see it all.”
This landlocked saltwater lake is home to millions of globular jellyfish ranging from minuscule to basketball-size. Every day, like clockwork, they all swim in a mass back and forth across the quarter-mile-long lake. Tourists don snorkels and swim among the pulsing blobs.
The lake’s jellyfish are mostly Mastigias whose ancestors came into the lake from the nearby ocean. Scientists think that rising sea levels thousands of years ago allowed seawater to fill the steep-sided marine lake, one of several in Palau. It’s a little like a giant natural aquarium, with similar conditions to the nearby ocean, but on a smaller scale and with far less variety of fauna. The lake is connected to the ocean only by fissures in the surrounding limestone, which keeps the lake isolated and the animals trapped.
Lohmann said he had read a lot about the jellyfish and watched low-resolution video of them on YouTube, so he knew what to expect of the lake. “The unexpected part was that it was harder to get to than I thought. It’s a sinkhole on an island, so you can’t just walk in from the beach. You end up going up over a steep rim and then back down again,” over rocks slick with tropical growth, he said. “And you’re dressed for swimming, not for hiking.”
But being among these extraordinary jellyfish was worth the hike, he said.
Most jellyfish feed by capturing small animal plankton with specialized stinging cells on their long tentacles. (Fortunately for Lohmann and the other snorkelers, the lake’s denizens, while they can deliver a mild sting, lack the powerful stinging ability that makes many jellyfish unpleasant swimming companions.) Mastigias can also capture and feed on tiny animals, but they have another option.
Mastigias have evolved to rely on a distinctive partnership for their nutrition. They have a symbiotic relationship with single-celled marine algae. Inside the tissues of Mastigias’ frilly arms live millions of single-celled algae called zooxanthellae, which use solar energy to convert carbon dioxide and solar energy into carbohydrates that the jellyfish use for energy.
The lake-bound jellyfish migrate across the lake and back during the day, swimming continuously. Scientists believe this is for two reasons: to keep their zooxanthellae in the light, and to avoid the shadows that form at the lake edges, where Mastigias’ predators (bottom-living sea anemones) live. At night, the Mastigias stop moving horizontally and swim up and down in the lake, away from the edges. Scientists have suggested that the nightly trips might provide the algae with essential nutrients available only in deeper water.
Studying Mastigias’ movementsin Jellyfish Lake, Kakani Katija, now a postdoctoral scholar at WHOI, has proposed that swimming jellies, moving up and down en masse, could play an ecologically significant, previously unsuspected role in mixing stratified layers of water within the ocean. Her work was published in 2009 in the journal Nature.
"There's no ecosystem like Jellyfish Lake in the world," she said. "This is a great place to see how the jellyfish affected mixing in the lake, because there's much less wind and tidal motion affecting the water than in the ocean, so you can isolate the effect of the jellies."
WHOI Director of Research Larry Madin (whose doctoral adviser William Hamner was one of the first to describe the Mastigias migration) studies jellyfish and other gelatinous ocean animals, from the Antarctic to the tropics.
"These Mastigias are beautiful and hypnotic," he said, “and they also offer a remarkable lesson in how animals and ecosystems adapt and evolve.” Interestingly, the Mastigias in each of Palau’s five jellyfish lakes, though the same species, all look different from each other and from Mastigias in the nearby ocean. The jellyfish in all five lakes have zooxanthellae, but each lake's jellyfish may depend on its symbiotic algae for nutrition to a different degree.
Madin participates in an international group called the Jellyfish Blooms Working Group, which studies the possible global expansion of jellyfish blooms. He encourages anyone interested in jellyfish and their kin to check out the group's pages at www.jellywatch.org.