Working at a Snail's Pace

By Lori Bona Hunt

Zoology professor Elizabeth Boulding is answering some big questions about evolution with the help of some very tiny creatures.

For the past eight years, Boulding has been studying marine snails native to Vancouver Island's West Coast to determine if evolution could rescue a species from extinction, even when that species is small in number and forced to adapt relatively quickly.
"We wanted to investigate factors that determine whether a population will adapt to change in the environment without going extinct," she says. Boulding chose Littorina subrotundata - snails that grow to be about three to five millimetres in length - because they live in a rocky and wave-exposed region with few predators and have smooth, thin shells. She wanted to see if the snails would develop thicker shells to resist predators if there was a sudden "invasion."

But fooling with Mother Nature isn't easy. Boulding had to introduce a predator and convince it to stick around long enough to test her theory, then find a way to make sure the snails ventured into dangerous territory.

"We simulated an invasion with purple shore crabs," says Boulding. Five times the size of the snails, the crabs love to snack onthin-shelled creatures. "If the snails have thicker shells, it makes them less preferable prey," she says. "We knew the snails would need to almost double the thickness of their shells."

Boulding built "crab condos" out of cement to give the crabs refuge, making it possible for them to live in the wave-swept areas. "It took a few tries to come up with the exact living conditions - very cold and wet - that the crabs preferred."
She encouraged them to stay by ensuring their next meal was never far away. "These are lazy crabs. They won't leave their homes for long, so they only want to move about two metres from their condos."

Boulding tethered snails to fishing line with waterproof epoxy glue, making sure that some of the snails ended up right near the crabs' "front doors."

"We tethered them at different distances so we could make comparisons based on distance from the crab condos," she says.

Every May, Boulding makes the trip to Vancouver Island to release another 90 crabs and collects the empty snail shells, which are still attached to the fishing line. "You can take one look at the shell fragment and tell what ate it," she says. She brings the shells back to her Guelph laboratory, where they are carefully measured and studied.

She also estimates snail migration between species that have adapted to the crabs and those that have not using molecular markers called microsatellites. The microsatellites involve using DNA as biological tags on the snails. "High migration from nearby populations that are not adapted to crabs will prevent the snails from developing thicker shells," she says.

Based on her research, Boulding has developed a theoretical model to help her predict whether the snails will evolve enough to survive. This was an important step not only to her research, but also for others in her field because the model can be applied to any species.

"It isn't just for snails. Any time there's an invading species, you can use this model to predict the effect on a native species."

The model tells her that based on the findings thus far, the snails should have increased their shell thickness enough to survive, and more important, it should have become a genetic trait, rather than something developed in just one generation.

Boulding will have her first real chance to test her theory and model predictions next summer, when she should have a full 10 generations of snails to study and make comparisons.

"If we're lucky, we get two generations of snails a year, and you really need 10 generations to see results."

Boulding, who joined the University of Guelph in 1993, has published some of her findings from this project in academic journals, including Heredity, Proceeding of the Royal Society B, Marine Biology and the Journal of Experimental Marine Biology and Ecology.

Her research is funded in part by a Premier's Research Excellence Award and the Natural Sciences and Engineering Research Council of Canada.