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Migrating sea turtles carry entire worlds on their backs—ones teeming with life forms small enough to fit between grains of sand. Life on such a scale is easy to overlook, but these miniature communities hold clues that could help protect the living islands they call home.
It’s been known for years that sea turtles harbor metropolises composed, in part, of such visible organisms as barnacles, algae, and tiny crustaceans. But new research on loggerhead turtles in the Gulf of Mexico suggests the populations they ferry are far more diverse than scientists ever imagined.
Loggerheads transport a vast array of meiofauna, a group of animals that are bigger than bacteria but still too small to see with the unaided eye. Creatures on the meiofauna roster, including mud dragons and water bears, range from under a millimeter to just 20 micrometers in length. That means 100 of the smallest meiofauna could cuddle comfortably on a pinhead. A swimming reptile, by comparison, provides plenty of real estate.
Jeroen Ingels, a marine ecologist at Florida State University and lead author of the study, says his team found an average of 33,000 hitchhikers per turtle, with one loggerhead hosting nearly 150,000 passengers. “The numbers were a shock,” he says. “We expected to find thousands, but not hundreds of thousands.”
More surprising still was the scope of different species, particularly among nematodes—worm-like animals found in sediments the world over. Nearly 7,000 nematodes representing 111 genera were found on the turtles.
“We’d expect the shells to be dominated by certain species that are well-adapted to this kind of lifestyle,” says Ingels. So, to discover the same variety as might turn up in a bustling seafloor was extraordinary, he says. “It means there are so many microhabitats and niches on the back of this turtle. [They allow] all these species to be there in fully functioning communities.”
Exactly how meiofauna come to live on large marine life remains something of a mystery. But in this case, the turtles’ feeding habits hint at an answer. Some loggerheads are sediment miners that dig into the seabed in search of clams, crabs, and other invertebrates. They kick up sand—and the meiofauna within—in the process.
This activity may help explain how some of Earth’s tiniest inhabitants—which seldom move more than a meter in their lifetimes—have come to be found all over the world. Conversely, studying meiofauna may also help scientists gain a better understanding of turtles’ own travels.
Every seabed has a signature assemblage of meiofaunal life. If those communities could be mapped out, they would provide a reference for scientists researching where a sea turtle has been—essentially providing an alternative to tagging and tracking.
Combining this method with existing techniques could help researchers better connect the dots between where sea turtles go to forage and where they go to lay their eggs—an insight that would be of huge importance to sea turtle researchers globally, says Kathryn Wedemeyer-Strombel, a conservation scientist with the Eastern Pacific Hawksbill Initiative who was not involved in the study.
Not only could robust meiofauna mapping help identify critical sea turtle habitat, but because nematodes and their small neighbors are sensitive to environmental changes, their whereabouts can serve as an indicator of the health of marine sediments and the larger ocean ecosystem.