Two female bubble-rafting violet snails, Janthina exigua. Scientists believe the bubble float evolved from an ancestral egg mass. Egg capsules are attached to the underside of the floats. Credit: Denis RiekANN ARBOR—It’s “Waterworld” snail style: Ocean-dwelling snails that spend most of their lives floating upside down, attached to rafts of mucus bubbles.

Scientists have known about the snails’ peculiar lifestyle since the 1600s, but they’ve wondered how the rafting habit evolved. What, exactly, were the step-by-step adaptations along the way?

University of Michigan graduate student Celia Churchill and coauthors believe they’ve found the answer to that intriguing question. In a paper published in the Oct. 11 issue of Current Biology, they show that bubble rafting evolved by way of modified egg masses.

The bubble-rafting snails, members of the family Janthinidae, secrete mucus from their “foot,” a broad, muscular organ at the base of the snail’s body. But instead of using slime to get around or to communicate chemically, as other types of snails do, they trap air inside quick-setting mucus to make bubbles that glom together and form rafts on which the snails spend the rest of their lives.

“We had a pretty good idea that that janthinids evolved from snails that live on the sea floor,” Churchill said. The question was, which specific group of snails gave rise to the janthinids, and how did the janthinid lineage make the transition from bottom dwellers to surface surfers?

A rare bubble-rafting brown janthina snail, Recluzia cf. jehennei. Scientists believe the bubble float evolved from an ancestral egg mass. Credit: Denis RiekTo find the answer, Churchill and coauthors first sequenced DNA from janthinids and other snail families thought to be closely related to them and used techniques of molecular phylogenetics to identify the ancestral lineage. They discovered that the rafting snails are descendents of sea-floor snails called wentletraps that parasitize corals and sea anemones. The researchers then asked which specific habits of wentletraps might have morphed over time into raft-building.

“We thought of two possibilities,” said Churchill, who did the work under the direction of Diarmaid