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Kelp Raft Carries Marine Stowaways Hundreds of Kilometers

14 September 2010 7:03 pm
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Ceridwen Fraser

Seafaring seaweed. This bull kelp carried tiny crustaceans and other invertebrates 400 kilometers before landing on a beach in New Zealand.

You're not from around here, are you? That's what marine biologist Ceridwen Fraser thought when she spotted a piece of seaweed encrusted with large barnacles on a New Zealand beach in 2009. Turns out she was right. Genetic data now reveal that the kelp came from islands hundreds of kilometers to the south. And it carried hitchhikers—at least 10 types of marine invertebrates. The find represents the first direct evidence that whole communities of organisms can make long ocean voyages floating on organic rafts.

For more than a century, scientists have pondered how plants and animals colonize remote islands and hop between landmasses. (How did lemurs get to Madagascar, for example?) Charles Darwin suggested that floating vegetation could carry seeds long distances over the water. But direct evidence has been hard to come by. Scientists have observed brittle stars, anemones, and crustaceans riding rafts of seaweed in the open ocean, but few have witnessed them making landfall. Even less evidence exists for vertebrates riding rafts. A notable exception occurred in 1995, when a hurricane swept 15 iguanas off a Caribbean island. They traveled more than 300 kilometers on a tangled mass of trees before landing on a different island.

Jonathan Waters, a biologist at the University of Otago in New Zealand, suspected that a type of seaweed called bull kelp might be able to transport marine organisms like the trees had carried the iguanas. Kelp anchors itself to the seafloor with a hollow, rootlike structure called a holdfast that houses a diversity of marine organisms, such as worms, sponges, and crabs. When the kelp breaks off and floats away, presumably the organisms go with it.

Last year, when Fraser, a postdoctoral researcher working with Waters, spotted that kelp on the beach, she noticed that it carried some rather large goose barnacles. These crustaceans begin growing on seaweed after it begins to float. Because the barnacles grow at a known rate, scientists can use them to estimate how long the kelp has been at sea. More time floating likely means a longer journey. Most kelp Fraser sees on the beach has tiny barnacles or none at all. So finding a piece with larger barnacles was "really exciting," she says. By May 2010, the researchers had located five more kelp specimens with large barnacles. The team estimated that these kelp rafts had been at sea a minimum of 22 to 67 days.

To determine the kelp's origin, the researchers sequenced two of its genes. The new specimens genetically matched kelp from two island groups—the Snare Islands and the Auckland Islands, 390 and 600 kilometers distant from St. Clair beach on New Zealand's South Island, respectively. Given wind patterns and ocean currents, the kelp could have easily made the journey in the time it was estimated to have been at sea, the researchers will report online tomorrow in the Proceedings of the Royal Society B.

When the researchers dissected the holdfasts, they found a tiny teeming ecosystem that included 10 species of marine invertebrates: two tiny crustaceans, a sea spider, six species of mollusks, and a sea star. A genetic analysis of several of the crustacean hitchhikers revealed that they also came from Auckland Island, confirming the kelp results.

This is the first study to use genetic analysis to pinpoint the source of a raft carrying live organisms, says Kenneth Sytsma, a botanist at the University of Wisconsin, Madison. He says it will pave the way for more studies that look at the genetics of not only rafts but also seeds or spider eggs.

What the paper doesn't do is demonstrate that the organisms found inside the kelp could establish themselves in New Zealand, says Ellen Censky, a biologist at the Milwaukee Public Museum in Wisconsin and one of the researchers who reported the iguana landing.

Still, says Anne Yoder, an evolutionary biologist at Duke University in Durham, North Carolina, "it is another piece of evidence in the growing pile" that suggests rafting plays an important role in animal dispersal. She adds that much of the evidence in support of rafting has been circumstantial. This study, however, provides a clear demonstration that a single raft can carry several unrelated organisms across long distances, she says. "I definitely buy it."