Bolder snails grow stronger shells

Bold snails are built to be tough.

A close look at bold snails’ shells reveals that they are rounder, thicker and more bite-resistant than shy snails’ shells. This finding, published online April 22 in Biology Letters, shows that within a species, bolder individuals can build bodies with better-developed predator defenses. These individual differences may have a genetic foundation, the scientists suggest.

“The idea that individuals can have body structures that match their behavior type is really interesting,” says Simon Lailvaux, a biologist at the University of New Orleans. “Findings like these expose how little we know about variations among individuals within a species.”

Radix balthica snails adapt quickly to stress from predators or other features of their environment. This flexibility leads to a lot of variation among individual snails, says study coauthor Johan Ahlgren, an ecologist at Lund University in Sweden. In other studies, he and colleagues have observed that bold snails have a higher survival rate when exposed experimentally to predator fish and that all snails from ponds with predator fish are bold.

Whether bold snails grow stronger shells or stronger shells make snails bolder is not yet clear.

“You need to be bold to get all the resources to pay for these shells,” Ahlgren says. “At the same time, if you are a bold snail that needs to take risks to pay for your high metabolism, a protective shell will keep you safer in risky habitats.”

In the new study, Ahlgren and colleagues collected snail eggs from fish-free ponds, raised the snails and did not expose them to any type of stress. “We still found a lot of variation in shell shape,” Ahlgren says. “One thing that could explain the shell shape was the boldness of the snails.”

Ahlgren and colleagues tapped the snails’ shells with tweezers and then timed how long it took for their heads to reemerge. Snails that popped their head out again in 10 seconds or less were considered bold. Snails that took 15 seconds or longer were considered shy. Statistical analyses comparing images of shell shape with personality showed that bolder snails had rounder shells with wider body whorls and larger body openings. These features distribute the force of a predator’s bite over a wider area, making the shell tougher to shatter. “Snails that invest in a rounder shell also invest in a thicker shell, something that makes them even stronger,” Ahlgren says.

Snails in this study, however, would not need to spend the extra energy to grow stronger shells, because they were never exposed to predators. Yet some snails still did it anyway. Ahlgren and colleagues argue that predation pressure could be written into the genetics of the snails, with boldness acting as a key driver of shell shape.

Lailvaux is cautious about this genetic perspective. He suggests that detailed experiments to breed snails and reconstruct evolutionary trajectories of boldness and shell shape are needed to establish a genetic association.