Snails and How They Get Around
How do snails migrate? Inch by inch or something more drastic? The geological rise of the Central American Isthmus separated the Pacific and the Atlantic oceans about 3 million years ago, creating a formidable barrier to dispersal for marine species. 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, lives in mangrove forests that hug the coast of Baja down to Panama, and the other, the Atlantic horn snail, resides in similar intertidal habitats along coasts from Texas to Panama.
The studied snail (Cerithioidea) is a very diverse superfamily. Its species can be found worldwide mainly in tropic and subtropic seas on rocky intertidal shores, seagrass beds and algal fronds, but also in estuarine and freshwater habitats. The freshwater species are found on all continents, except Antarctica. They are dominant members of mangrove forests, estuarine mudflats, fast-flowing rivers and placid lakes.
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? American scientist Simpson's proposed that terrestrial species can win sweepstakes routes, whereby highly improbable dispersal events result in colonization across geographical barriers. The present study suggests that marine species may also breach land barriers.
The horn snails apparently 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.
For further information: http://news.sciencemag.org/sciencenow/2011/09/did-flying-snails-cross-mexico.html?rss=1
Photo: http://www.junglewalk.com/photos/snail-pictures-I9602.htm; USDA Scott Bauer