The pirate's best friend on a long sea voyage: rum. But for sailing snails, it's mucus. Shelled critters belonging to the family Janthinidae, the purple or violet snails, traverse tropical waters clinging to rafts that look like inflatable pool loungers. Now, a new evolutionary study of these gastropods suggests that they long ago developed their floatation devices from some of their reproductive tissues: strings of eggs stuck together with goo.
Bubble-rafting snails, including the brightly-colored common purple snail (Janthina janthina), bob upside-down right below the surface in oceans across the globe, waiting for floating prey that includes stinging jellyfish known as Portuguese man-of-wars. When a potential meal sails by, janthinids extend their sharp mouths like cannons and rasp off chunks from the gelatinous animals. But like free-spirited pirates, the snails, which grow to about 3 centimeters long, travel wherever the waters and winds take them. Scientists have known that the often-long flotation aids contain mucus, says Celia Churchill, an ecologist at the University of Michigan, Ann Arbor. Both male and female bubble-rafters secrete thick goo from their feet, and then fold the mucus to form air-trapping pockets. "If you're into mucus, it's a fascinating mucus," says Churchill. "It's almost like bubble wrap."
But researchers had few clues to how such rafts evolved. To get an idea, Churchill and her colleagues looked at differences in DNA between janthinids and their close relatives to reconstruct the snails' evolutionary tree. They concluded that bubble-rafters branched off at an unknown time from another group of living snails called epitoniids or wentletraps. These snails live at the bottom of the sea, not the top, munching on corals and anemones. But when they reproduce, females exude long trains of developing eggs embedded in webs of sand and mucus. These nets bear the hallmarks of life rafts in the making, but the group couldn't be sure.
Then the team stumbled on the specimen of a very rare Australian snail belonging to the genus Recluzia. Various physical traits seemed to mark these beige-colored invertebrate as a middle step in the evolution of janthinids from wentletraps. Only female Recluzia construct rafts, which also happen to contain viable eggs at different stages of development, suggesting that the flotation devices might double as reproductive structures , the group reports today in Current Biology. The bubble rafts of most female -- but not male -- janthinids also contain eggs capable of hatching into larvae, further evidence that the structures evolved from the wentletraps' egg nets, Churchill says.
Churchill and colleagues suspect the snails' evolution may have gone like this: From time to time, bottom-dwelling wentletrap ancestors accidentally trapped small bubbles in their egg nets, and then floated to the surface until those bubbles popped. Because those high waters abounded in edible jellyfish, some wentletrap ancestors began to capitalize on making more bubbles, eventually floating exclusively. Males also learned to fabricate their own, egg-free life rafts.
Those first snails to bob to the surface faced rough waters, says Carole Hickman, an evolutionary morphologist at the University of California, Berkeley. To begin with, sunlight beats down relentlessly on floating animals, drying them out. Studies of such harsh-living animals may give scientists new insight into how animals survive tough environments, Hickman says: "Particularly at a time of global warming and climate change, we need to look at things living in extreme environments, … how they have evolved."
David Lindberg, an evolutionary biologist also at Berkeley, agrees that the team highlighted a fascinating clutch of animals: The janthinids "are an incredibly extravagant group with an extraordinary evolutionary history." And, he says, Churchill and colleagues completed one of the trickier feats in evolutionary biology: demonstrating how a seemingly unwieldy trait might have evolved in steps over time. "You could have all that diversity," Lindberg says. "But if you can't order it, you really are just telling stories."