A Thicker Skin--for Snails

Intertidal whelks. Courtesy of Tim Edgell

Tuesday, June 10th, 2008--

European green crabs (Carcinus maenas) invaded the eastern seaboard of the U.S. in the 1800s. Since then, the snails living with these invasive crustaceans have developed thicker shells--presumably as crab-defense, researchers say. In a new study published in the journal Biology Letters, biologists report on what triggers the snails to thicken their shells.

Tim Edgell, a biologist at Bamfield Marine Sciences Centre in British Columbia and an author on the paper, collected intertidal whelks (Nucella lamellosa)—a type of snail--from an area of the northeast Pacific where invasive green crabs are still scarce. Those whelks had not been exposed to the invasive crabs before the experiment, although they had been exposed to the red rock crab (Cancer productus)--which is native to the area.

The whelks lived beside the aquariums containing green crabs--the water from the crab-containing aquarium overflowed into the whelk buckets. "So the snails are receiving chemical effluents that are coming off the crabs. Say if it pees in the water - the snails can smell that pee, but they are never in physical contact with the crab," Edgell says. (It is suspected that the chemical signals that snails detect are carried in crab urine, Edgell says.)

Edgell discovered that in 50 days, the whelks that were exposed to native crabs--the crabs they know--increased the thickness of their shells three-fold. However, the snails exposed to the invasive green crabs--which they had never encountered--showed no response. The snails did not appear to detect the invasive crabs as predators.

"It means that something about the smell of native crabs is different than the smell of these exotic crabs and the snails know that," Edgell explains. It also means that snails have evolved to differentiate between these crabs, Edgell says.

The process by which snails go from not recognizing green crabs to responding to them could happen either through evolution or association, Edgell says. If it's an evolutionary change, the snails that do respond to the new predator are favored and so their genes disperse more quickly. This type of genetic shift in the population "would likely take a long time," Edgell says.

The other possibility is that snails learn the invasive crabs are dangerous by association: when green crabs come to an area, they start killing snails--which also produces a smell. It is known that snails can recognize the scent of dying snails, says Edgell. Within a generation, snails could start to associate snail death with the new green crab smell. "By association they may learn to respond to the green crab cue," Edgell says.

Whatever drives the snail's shell defense, it may not be foolproof. The green crabs have adapted too. Edgell says if you look at snails on the east coast, where green crabs and snails have lived together for hundreds of years, "wherever you find a population of snails that have very thick shells, you also find green crabs with very large claws."

--Flora Lichtman