Snails defy death and revive through evolution

Cast your mind back to the year 1988. The Koster archipelago, a cluster of islands off the Swedish west coast near Norway, suffered a harsh attack from a significant bloom of toxic algae. This occurrence left an indelible mark on the marine snail populations residing in these islands – driving an impact on their evolution, and pushing them to near extinction.

But why should this matter to anyone but marine biologists or environmentalists? Well, this unfortunate turn of events would provide an opportunity to observe the fascinating process of evolution in real-time.

Decades-long experiment with snails

The Koster islands and the encompassing rocky islets known as skerries were habitats for dense and diverse populations of marine snails, specifically the species Littorina saxatilis.

While the snail populations bounced back on the larger islands within two to four years, the populations on smaller skerries couldn’t recover from this catastrophic event.

Kerstin Johannesson, a marine ecologist at the University of Gothenburg, Sweden, saw an opportunity amidst this disaster. In 1992, she reintroduced the L. saxatilis snails to their original skerry habitat, thus initiating an experiment with profound implications that researchers could witness over three decades later.

Ultimately, the experiment enabled an international team led by the Institute of Science and Technology Austria (ISTA) to observe and predict evolution of snails in real-time.

Snails evolved to fit their environment

The marine snail species L. saxatilis is spread across the North Atlantic shores. Evolved to adapt to their environment, these snails exhibit differences in size, shell shape, shell color, and behavior.

The two distinct types that develop, most prominently known as the Crab- and Wave-ecotype, are particularly intriguing. The Wave snails tend to be smaller, with thin shells of unique colors and patterns, a large and rounded aperture, and bold behavior.

In contrast, the Crab snails are noticeably larger, possess thicker shells without patterns, a smaller and more elongated aperture, and exhibit wary behavior due to their predator-filled environment.

Remarkably, the Koster archipelago served as a home to both these types of L. saxatilis. Following the deadly algae bloom, Wave snails perished, leaving the shores desolate.

However, Crab snails were still abundant nearby. Capitalizing on this proximity, Johannesson decided to reintroduce the Crab snails to the wave snail’s old abode in 1992.

Snails reclaim lost traits through evolution

Johannesson’s decision turned out to be a masterstroke. With the snails churning out one to two generations every year, scientists could witness the Crab snails adapt to their new environment in real-time.

“Our colleagues saw evidence of the snails’ adaptation already within the first decade of the experiment,” said Diego Garcia Castillo, a graduate student in the Barton Group at ISTA and a leading author of the study.

“Over the experiment’s 30 years, we were able to predict robustly what the snails will look like and which genetic regions will be implicated. The transformation was both rapid and dramatic.”

But this evolution and transformation wasn’t a case of the snails developing their traits from scratch. Some of the genetic diversity was already available in the starting Crab population, though at low prevalence. These findings indicate that access to a large gene pool can hasten the process of evolution.

Genetic diversity in snail evolution

Over the duration of the experiment, the team examined the snails’ phenotype, individual gene variabilities, and larger genetic changes that affect entire regions of the chromosomes called “chromosomal inversions.”

They observed a rapid selection of traits already present at a low frequency in the transplanted Crab snail population, which may be further bolstered by gene flow from neighboring Wave snails.

Implications for the future

This research on snail evolution opens a window into understanding how species can adapt to modern environmental challenges such as pollution and climate change. Not all species have access to large gene pools, and evolving new traits from scratch is slow.

Anja Marie Westram, a former postdoc at ISTA, now a researcher at Nord University, expressed her hope for the future. “Perhaps this research helps convince people to protect a range of natural habitats so that species do not lose their genetic variation.”

Fast forward to the present day, and the snails Johannesson brought to the skerry in 1992 now have a thriving population of around 1,000 individuals. These humble snails, once victims of an environmental catastrophe, are now helping us understand the wonders of nature and evolution.

The study is published in the journal Science Advances.

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