Sea snails evolved defenses against invasive predators

Over the last 20 years, the Gulf of Maine has turned into a hotspot for invasive species, many of which arrive in ship ballast water.  These invaders – and the changing environmental conditions that favor them – have reshaped coastal ecosystems, including tidal snail populations.

A recent study has examined how two common snail species (Nucella lapillus and Littorina obtusata) have evolved in response to a major predator: the invasive green crab (Carcinus maenas). 

The key finding is that snails have developed thicker shells, reflecting a shift from short-term, plastic defenses to more durable, genetic adaptations.

The research was co-authored by Geoffrey Trussell, an evolutionary biologist at Northeastern University’s Marine Science Center and his collaborator James Corbett.

Rapid invasion by a voracious predator

Originally thriving in more southern waters, green crabs have steadily moved north up the Atlantic coast. They feed aggressively on tidal snails, among other prey, and their recent establishment in the Gulf of Maine has placed new pressure on native species. 

“Lots of invasive species have arrived on our shores, mostly through ship ballast,” Trussell said. “So you have this confluence of significant environmental changes.”

By comparing snail data collected 20 years apart – first in 1998, when green crabs were still new arrivals, and again in 2018 – the researchers documented a striking shift. Snail shells had become not only thicker, but also less able to adapt flexibly to varying predator threats.

The nature of snail defenses

Previously, many tidal snails displayed phenotypic plasticity: they would grow thicker shells if they detected “risk cues” from predators in their environment, such as chemical signals and hormones emitted by crabs. 

In lab tests, snails reared in water near green crabs ended up with a protective, thicker armor compared to those in predator-free tanks.

As green crabs grew more common, this on-demand defense began to shift toward permanent genetic changes. Over generations, snails started producing thicker shells without relying as much on situational cues. While effective at deterring predators, it comes at a cost. 

“Back in the early days, if a snail had a thick shell, they had very little soft body mass,” Trussell noted. “If they had a thin shell, they had a lot of body mass.”

Because snails require adequate body mass to carry out vital functions – particularly reproduction – the move toward bigger, heavier shells may reduce their reproductive output.

Decline in behavioral flexibility

In ecology, “plasticity” refers to an organism’s ability to adjust traits in real time based on its environment. In the 1990s, snails in the Gulf of Maine possessed more of this short-term adaptability. 

Two decades later, genetic evolution is overshadowing plasticity, suggesting that constant predator pressure and environmental change have steered snails toward more fixed physical traits.

“That body mass has direct effects on things like how many offspring they’re able to produce,” Trussell said. “So that [decreased] behavioral plasticity has huge consequences, not only for how these [snail] communities are structured, but also potentially how their ecosystem functions.”

Complexity of snail defenses 

This study underscores the significance of multi-decade ecological monitoring. 

“This work highlights the value of going back and revisiting experiments and populations to understand how things have changed,” Trussell explained. 

He noted that the new round of experiments repeated the same procedures – and sometimes used the same containers – he relied on in graduate school.

The new data also reveal unexpected complexity within snail species. In some cases, individuals from the same species and sex, collected in the same location, displayed notably different levels of UV reflectance or shell thickness.

This discovery hints at underlying genetic variability and the intricate interplay between environment and heredity.

Looking toward the future

Trussell and Corbett plan to repeat the study in 2028, extending the project’s timeline to 30 years. They aim to explore how snail defenses and population structures continue to shift, as both climate change and the green crab invasion persist.

By documenting how these tidal snails evolve, the research offers a broader window into how coastal systems transform under invasive pressure and environmental change. 

As certain traits become genetically “locked in,” they reshape the dynamics of predator-prey interactions – while also reminding ecologists that prolonged, in-depth observations are often the only way to grasp the full evolution of ecosystems in a changing world.

The study is published in the journal Science Advances.

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