Common tidal snails living off the Gulf of Maine have been forced to adapt to predatory pressure from invasive green crabs by growing thicker shells, faster – a response mechanism, it is feared, that could hinder their ability to reproduce as they lose what is known as behavioural plasticity.

Over the last two decades, the Gulf of Maine has become something of a hotspot for invasive species from across the world, arriving in their droves – according to local marine biologists and researchers – mostly through ship ballast.

Among the most successful of these invasive species contributing to what scientists have described as a “confluence of significant environmental changes” has been the predatory green crab, a small but colourful crustacean that has – over the last 20 years – surged northwards from the mid-Atlantic coast to dine out on the local delicacy.

Its meal of choice, of course: the common tidal snail.

In response to this influx, however, sea snails have evolved a defence mechanism to help keep the hungry green crab at, by evolving a thicker shell.

It’s all been laid bare in a groundbreaking study conducted by Dr Geoffrey Trussel, an evolutionary biologist and professor at Northeastern University’s Marine Science Centre in Nahant, Massachusetts which – commenced when he was a Ph.D student in the late ‘90s – spans the decades to monitor and document the scale of change.

The results of this decades long study have now been published in the academic journal, Science Advances in which Dr Trussell and his collaborator and co-author James Corbett record not only the rapid rate at which these sea snails have evolved, but the crisis they now face as a result of it.

The first study – documenting the responsive behaviours of two types of common sea snail, Nucella lapillus and Littorina obtusata – took place in 1998, when green crabs were first arriving in the Gulf of Maine. The second took place 20 years later, in 2018.

Over that time, the researchers found that the snails were growing shells ‘significantly more rigid in defense’. This would have been the ideal solution for a snail population facing – quite literally – the jaws of death. Ideal, were it not for the severe impingement the response mechanism was to have on their levels of adaptive ‘plasticity.’

Plasticity “is the ability to produce within generation change in a trait that enhances your ability to survive,” explained Dr Trussell. In the 1990s, snails in the Gulf of Maine possessed more of this short-term adaptability. 

Two decades on, however, and genetic evolution has overshadowed that plasticity, suggesting that constant pressure from predatory species – like the green crab – have steered snails towards more fixed physical traits. And that is an issue for snails.

Snails require adequate body mass to carry out vital functions, particularly reproduction but also shedding their shells for new ones. Snails with a thick shell tend to display very little soft body mass; something that may reduce their ability to shed a shell as well as their reproductive output.

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

With the advance of the green crab in recent decades, the researchers write, the snail’s “on-demand model of adaptation” – its plasticity – has “given way to more permanent evolution.”

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

For Dr Trussell, the fascination and the investigation won’t end here. He plans to make his return to the research once again, in 2028, to mark the third decade of the study.

“We’re going to keep looking at how this progresses,” he said.

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