Scientists predict – and witness – evolution during 30-year experiment on marine snails

Snails on a small rocky islet evolved before the eyes of scientists. Marine snails were reintroduced after a toxic algae bloom eliminated them from the bay. Although the researchers intentionally introduced a separate population of the same snail species, it evolved to bear a striking resemblance to the population that went extinct more than 30 years earlier.

The study, led by researchers from the Institute of Science and Technology Austria (ISTA) and the Norwegian University of the North, is published in Scientific advances.

The year is 1988. The Koster Archipelago, a group of islands off the west coast of Sweden near the border with Norway, is hit by a particularly dense toxic algae bloom, wiping out populations of marine snails. But why would anyone care about the fate of a group of snails on a three-meter-square rock in the open sea? It turns out that this event would open up the possibility of predicting and seeing evolution unfold before our eyes.

Previously, the islands and their small intertidal rocky islets supported dense and diverse populations of marine snails of the species Littorina saxatilis. While snail populations on the larger islands, some of which were reduced to less than 1 percent, recovered within two to four years, several skerries do not appear to be able to recover from this blow.

Marine ecologist Kerstin Johannesson of the University of Gothenburg, Sweden, saw a unique opportunity. In 1992, she reintroduced L. saxatilis snails to their lost habitat, launching an experiment that would have far-reaching implications more than 30 years later.

It enabled an international collaboration led by researchers from the Institute of Science and Technology Austria (ISTA), the University of the North, Norway, the University of Gothenburg, Sweden, and the University of Sheffield, UK, to predict and witness ongoing developments.

Wavy snails and crab snails

L. saxatilis is a common species of marine snail found throughout the coasts of the North Atlantic, where different populations have evolved traits adapted to their environment. These traits include size, shell shape, shell color, and behavior.

The differences in these traits are particularly striking between the so-called Crab and Wave ecotypes. These snails evolved several times in different locations, either in environments exposed to crab predation or on rocks exposed to waves, away from crabs.

Wavy snails are generally small and have a thin shell with specific colors and patterns, a large, rounded opening, and bold behavior. Crab snails, on the other hand, are surprisingly larger, have thicker, unpatterned shells, and a smaller, more elongated opening. Crab snails also behave more cautiously in their predator-dominated environment.

Sweden’s Koster Archipelago is home to these two different types of L. saxatilis snails, often neighbors on the same island or separated only by a few hundred meters across the sea. Before the toxic algal bloom of 1988, the snails of the Waves inhabited the skerries, while nearby shores were home to both crab and wave snails. This close spatial proximity would prove crucial.

Rediscover old traits

Seeing that the skerries’ wave snail population had been completely wiped out due to toxic algae, Johannesson decided in 1992 to reintroduce snails into one of these skerries, but of the crab ecotype. With one to two generations per year, she rightly expected the crab snails to adapt to their new environment before scientists’ eyes.

“Our colleagues have already seen evidence of snail adaptation during the first decade of the experiment,” says Diego Garcia Castillo, a graduate student in the Barton group at ISTA and one of the leading authors. the study.

“Over the 30 years of the experiment, we have been able to reliably predict what the snails will look like and which genetic regions will be involved. The transformation has been both rapid and dramatic,” he adds.

However, snails did not develop these characteristics entirely from scratch. Co-corresponding author Anja Marie Westram, a former postdoctoral fellow at ISTA and currently a researcher at the University of the North, explains: “Some of the genetic diversity was already available in the starting crab population, but with little prevalence. conditions in the recent past. Snails’ access to a large gene pool led to this rapid evolution.

Diversity is the key to adaptation

The team looked at three aspects over years of experience: the snail phenotype, individual genetic variabilities, and larger genetic changes affecting entire regions of chromosomes called “chromosomal inversions.”

During the first generations, researchers witnessed an interesting phenomenon called “phenotypic plasticity.” Very soon after being transplanted, the snails changed their shape to adapt to their new environment. But the population also quickly began to change genetically. Researchers were able to predict the extent and direction of genetic changes, particularly for chromosomal inversions.

They showed that the rapid and spectacular transformation of the snails was perhaps due to two complementary processes. Rapid selection of traits already present at low frequency in the transplanted crab snail population and gene flow from neighboring wave snails that could have simply rafted 160 meters to reach the skerry.

Evolution in the face of pollution and climate change

In theory, scientists know that a species with high enough genetic variation can adapt more quickly to change. However, few studies aimed to experiment with evolution over time in nature.

“This work allows us to take a closer look at repeated evolution and predict how a population might evolve traits that evolved separately in the past under similar conditions,” says Garcia Castillo.

The team now wants to discover how species can adapt to modern environmental challenges such as pollution and climate change.

“Not all species have access to large gene pools and the evolution of new traits from scratch is extremely slow. Adaptation is very complex and our planet also faces complex changes with episodes of weather extremes, rapidly advancing climate change, pollution and new pests,” Westram says. She hopes this work will stimulate further research into the maintenance of species with vast and diverse genetic makeup.

“Perhaps this research helps convince people to protect a range of natural habitats so that species don’t lose their genetic variation,” concludes Westram.

Today, the snails that Johannesson brought to the skerry in 1992 have grown to a thriving population of around 1,000 individuals.