Evolutionary biologist Geoffrey Trussell has tracked plasticity in the shells of sea snails in the Gulf of Maine for 30 years. Invasive green crabs have shown up in droves during that time, forcing rapid changes.
Over the past two decades, the Gulf of Maine has become a popular landing spot for invasive species from across the world, says Geoffrey Trussell, an evolutionary biologist and professor at Northeastern University’s Marine Science Center in Nahant, Massachusetts.
“Lots of invasive species have arrived on our shores, mostly through ship ballast,” he explains. “So you have this confluence of significant environmental changes.”
Trussell has witnessed those changes on the ground — very, very low ground. Starting when he was a Ph.D. student in the 1990s, he has monitored the evolution of two common species of sea snails living off Maine’s coast, tracking how they have responded to changes in that environment and the resulting influx of predators. Among the most successful of these are predatory green crabs — small, brightly colored crustaceans that have surged north from the mid-Atlantic coast over the past few decades and love to feast on tidal snails.
In a recent paper published in the academic journal Science Advances, Trussell and collaborator James Corbett document how the snails (Nucella lapillus and Littorina obtusata) have evolved in response. In brief: they’ve grown thicker shells.
The paper documents two sets of field experiments, conducted 20 years apart, measuring the thickness and plasticity in samples of tidal snails. The first study took place in 1998, when green crabs were first arriving in the snails’ Gulf of Maine territory, and the second in 2018.
Over that time, the researchers found, shells grew significantly more rigid in defense. And there was something else: The more recent cohort of snails showed reduced levels of adaptive “plasticity” in comparison to their predecessors.
Plasticity “is the ability to produce within generation change in a trait that enhances your ability to survive,” Trussell explains. In lab tests, snails raised in water with green crabs nearby — signaled to the snails through “risk cues” like smell and hormones — grew thicker shells than those raised without the looming threat of the predators.
“In the late 1990s, the degree of shell thickness plasticity in two [Littorina obtusata] populations coincided with their respective contact history,” the paper reads. “Plasticity was substantially greater in the northern versus southern population. This geographic pattern suggests that selection has shaped trait plasticity — longer contact history in the south favored the evolution of more fixed levels of defense, whereas the more recent and variable green crab invasion history at higher latitudes through the late 1990s favored greater plasticity.”
As green crabs have moved north in recent decades, though, that more on-demand model of adaptation has given way to more permanent evolution — the thicker snails’ shells became fixed genetic traits over generations. This increased fortification comes with downsides for the snails, including decreased body mass and more difficulty shedding their shells for new ones.
It’s a tricky development for the Gulf of Maine snails, Trussell says, because shedding shells is already more difficult in northern waters, which, while warming, are much colder than they are further south.
“Back in the early days, if a snail had a thick shell, they had very little soft body mass,” Trussell says. “If they had a thin shell, they had a lot of body mass. So in addition to the shells being thicker, body mass is plummeting.”“That body mass has direct effects on things like how many offspring they’re able to produce,” he adds. “So that [decreased] behavioral plasticity has huge consequences, not only for how these [snail] communities are structured, but also potentially how their ecosystem functions.”
In addition to serving as a linchpin for understanding the dynamics of broad, climate-driven changes in complex ecosystems like the Gulf of Maine, the two-decade study underscores the possibilities of long-running longitudinal research, Trussell argues. In the snail study and others, the full scope of the interplay between environmental and species evolution would be impossible to grasp without the benefit of time.
“This work highlights the value of going back and revisiting experiments and populations to understand how things have changed,” he says. “This experiment is a complete repeat of work I did as a Ph.D. student, down to the containers I used to raise the critters.”And it’s not over: the experiment will run again in 2028 to mark its third decade. “We’re going to keep looking at how this progresses.”