A female violet snail, Janthina exigua, hangs from a float of homemade mucus.
Scientists have long observed snails "surfing" the oceans on such rafts, which can serve as flotation devices, egg-storage areas, and platforms for young snails.
But it was unknown how the family of fewer than ten bubble-rafting species evolved their odd lifestyles, said Celia Churchill, a Ph.D. student at the University of Michigan in Ann Arbor.
Churchill had already suspected that bubble rafters evolved from bottom-dwelling snails that produce mucus-filled egg masses. To pinpoint the rafting snails' closest relatives, the team sequenced DNA from bubble-rafting species and other potential "sister families," using molecular techniques to piece together an ancestral family tree.
The results revealed that bubble rafters descended from a bottom-dwelling snail called the wentletrap, which still exists today.
Both snail groups secrete mucus from their feet-muscular organs at the bases of their bodies. But instead of making egg masses, the bubble rafters use the quick-hardening mucus to create rafts with the "consistency of bubble wrap," said Churchill, whose new study appeared recently in the journal Current Biology.
"You can pop it if you get a fresh one."
(See "How Snails Walk on Water Is a Small Miracle.")
Pictures: How Bubble-Rafting Snails Evolved
Scientists have cracked an evolutionary mystery: How did some snails come to "surf" the oceans on mucus-bubble rafts?