Pigs may never fly, but in the Gulf of Mexico, snails take to the skies. Or at least they used to. A new study suggests that two species of marine snails may have traveled between the Atlantic and Pacific oceans—all in the belly of a bird.
The two species are known as horn snails, and both look a bit like tiny black party hats. One, the Pacific horn snail (Cerithideopsis californica), lives in mangrove forests that hug the coast of Baja down to Panama, and the other, the Atlantic horn snail (C. pliculosa), resides in similar intertidal habitats along coasts from Texas to Panama. "Every time you take a step [in their habitats], you may be stepping on hundreds of them," says Peter Marko, a biogeographer at Clemson University in South Carolina.
The two snails used to be the same species, crossing between the Pacific and Atlantic oceans via a narrow strip of water that once separated North America from Central America. But about 3 million years ago, a land bridge sprang up near modern-day Panama, and the single species split in two, which scientists thought were separated for good.
The new study reveals that the two species continued to intermingle long after the land bridge formed. When researchers analyzed the DNA of 29 populations of horn snails, they dug up a surprising find. Genes from the Pacific Ocean snail had invaded the Atlantic Ocean snail and vice versa, hinting that no land bridge could keep the sister snails apart. Study co-author Mark Torchin, an ecologist at the Smithsonian Tropical Research Institute in Panama, and colleagues concluded that the mollusks had mixed and matched at least twice. A few Pacific snails colonized the Atlantic Ocean nearly 1 million years ago, and Atlantic snails traveled west just over 70,000 years ago, the group reports online today in the Proceedings of the Royal Society B.
How did they get across this land bridge? In the 1940s and '50s, the American scientist George Gaylord Simpson argued that small mammals may have colonized Madagascar by riding logs and other floating debris from Africa. "We kind of flipped that hypothesis around and looked at whether marine organisms crossed over land to colonize new oceans," Torchin says.
The horn snails, he suspects, found their own life rafts: shore birds. Although the group can't prove that the scenario is true, they think it could have gone like this: Nearly 1 million years ago, a wading heron gobbled a basking Pacific horn snail, shell and all. Luckily for this intrepid explorer, armored invertebrates can survive for days in the bellies of shore birds. Snug inside its unsuspecting taxi, the snail soared high above what was likely Mexico before being excreted in the Atlantic Ocean—a journey of about 200 kilometers or more. About 70,000 years ago, the researchers think, one or more Atlantic horn snails took their wagons west in the same way.
Such chance events, no matter how rare, can have big consequences for animal populations, says study co-author Osamu Miura, now of Kochi University in Japan. When the Pacific snails splashed down into the Atlantic, he suspects, they could easily have brought with them brand-new genes that helped the Atlantic snails fight off disease.
"It's certainly possible that snails could get carried over a narrow land bridge like this aided by birds," says Marko, who was not involved in the study. But he's not convinced. To date the rare migration events, the Smithsonian team assumed that many of the Atlantic and Pacific snails had continued to breed right up until the rise of the Panama land bridge. Many other marine species, however, split millions of years before that, unable to cross the shallow waters around modern-day Central America. If the snails were one of those early-dividing species, then the team's dating could be off, he says. But even with that error, the snail migration from the Atlantic to the Pacific "still looks like it's pretty recent," he says. So the bird-taxi scenario still has wings.