[貼圖] How Bubble-Rafting Snails Evolved

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Snail Surfer
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."



Male Hitchhiker?
A large female snail in the Recluzia cf. jehennei species preys upon a Portuguese man-of-war while perched on a raft of mucus bubbles. A tiny snail of the same species clings to the underside of the female's float.
Churchill was studying Recluzia snails when she noticed tiny hitchhikers attached to the females'  rafts—an observation never made previously by scientists, she said.  Though too small to determine the hitchhikers' genders, Churchill and  colleagues suggest the small snails are dwarf males, which attach to a  female once they find her in the open ocean.

The  snails are hermaphrodites, which means that these males will eventually  become female, make their own rafts, and float away.
"All  of them begin life as a larva in the water column, then metamorphose  into a juvenile, then become male, then become female," Churchill said.
Though  it may seem like a "weird lifestyle to us," such gender-switching is  common among snails, added Churchill, whose research was partially  funded by the National Geographic Society's Committee on Research and Exploration.





Jellyfish Dinner
A bubble-rafting violet snail feeds on a Portuguese man-of-war in Hawaii.
Churchill and colleagues have a theory for how the snails' bottom-dwelling ancestors took to rafting.
They  hypothesize that at some point, a female living near the coast may have  been briefly carried—along with her egg mass—by waves. This led to  temporary periods of rafting. Eventually this ancestor lineage evolved  the ability to create bubbles with their mucus and make rafts on  purpose.

"Obviously,  the ability to add bubbles probably didn't evolve overnight," Churchill  emphasized. "It would have been over some period of time, and  eventually that lineage became successful at adding many bubbles  together to create a float."

The  evolutionary transition from bottom-dwelling to bubble-rafting gave  snails access to new food sources at the surface, where they are  relatively free from competition, Churchill said.



Egg-Free Float
A female violet snail, Janthina janthina, is the most common species of bubble rafter.
J. janthina is also the only bubble-rafting species in which females brood their  young inside their bodies instead of laying egg capsules on their  floats, Churchill noted.
"Scientists think this may be an adaptation to living at the ocean surface, because Janthina janthina's float is more buoyant and not weighed down by egg capsules."



Snail Parasite
A female wentletrap and her egg mass sit atop their coral host while a dwarf male hangs out nearby.
Bottom-dwelling  relatives of the bubble-rafting snail, wentletraps have a peculiar way  of life, even among snails, Churchill noted. The animals are  ectoparasites, which means they live on-and eat-the skins of their  hosts, usually corals or anemones.

Females form their egg capsules by gluing the eggs together with a "blobby mass of mucus," Churchill said.



Ocean Attack
Aside from being hermaphroditic, the reproductive cycle of bubble-rafting snails (pictured, a Janthina janthina snail eats a jellyfish) is still somewhat of a mystery.
Scientists  do know that males must find females to deposit their sperm, "which is  pretty amazing if you consider they're drifting on the surface of the  ocean," Churchill said.
The females likely then lay egg capsules on their floats, and the eggs eventually hatch into free-swimming larvae.



Leaving a Mark
Violet  snail species can secrete purple dye—pictured staining a person's  fingers—which may help them defend against predators, Churchill said.
"Scientists  really aren't sure about the function of the purple dye," she said.  "It's been observed in [bubble rafters and wentletraps] when they are  disturbed, and suspected as a response to disturbances."
Big, "beautiful" clumps of bubble-rafting snail shells often wash up on beaches worldwide, she added.

"The cool thing about these snails is almost anyone on the globe can find them—they're pretty much everywhere."