Pictures: How Bubble-Rafting Snails Evolved

Male Hitchhiker?

Photograph courtesy Denis Riek

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. (The Society owns National Geographic News.)

(See "Cloned Fathers Mate With Insect Daughters-From Inside.")

Published October 19, 2011

Jellyfish Dinner

Photograph by David Fleetham, Alamy

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.

(See "Snails Survive Being Eaten by Birds—A Mystery.")

Published October 19, 2011

Egg-Free Float

Photograph courtesy Denis Riek

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."

(See "Pictures: Fire Ant Swarms Form Living Life Rafts.")

Published October 19, 2011

Snail Parasite

Photograph courtesy Adriaan Gittenberger

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.

(See "Pictures: U.S. Frogs Deformed by Parasite Infections.")

Published October 19, 2011

Ocean Attack

Photograph courtesy Denis Riek

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.

(See "Squid Males 'Bisexual'-Evolved Shot-in-the-Dark Mating Strategy.")

Published October 19, 2011

Leaving a Mark

Photograph by Suzanne Long, Alamy

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."

(See ocean pictures.)

Published October 19, 2011