The snail population in the Koster Islands, Sweden, is undergoing rapid evolutionary changes due to genetic diversity while facing environmental pressures.
In 1988, the Koster Islands (an archipelago off the west coast of Sweden near Norway) experienced a particularly dense bloom of toxic algae, devastating the marine snail populations. One might wonder why the fate of snails on a small, three-square-meter rock in the open sea matters so much. However, this event created a unique opportunity to predict and witness the evolutionary process unfolding before our eyes.
Historically, the islands and small coastal reefs were home to dense and diverse populations of the marine snail Littorina saxatilis. While snail populations on larger islands recovered within two to four years (at times some populations dropped below 1%), certain populations on the reefs continued to struggle to recover from the devastation.
Marine snails in Sweden.
A Breakthrough Experiment
Marine ecologist Kerstin Johannesson from the University of Gothenburg, Sweden, saw a unique opportunity. In 1992, she reintroduced the L. saxatilis snail back into their lost habitat and began an experiment with profound implications more than 30 years later. This later experiment involved international collaboration from researchers at the Institute of Science and Technology Austria (ISTA), Nord University, Norway, the University of Gothenburg, Sweden, and the University of Sheffield, UK, with the goal of observing the ongoing evolutionary process.
L. saxatilis is a common marine snail species found throughout the North Atlantic coast, where different populations evolve characteristics adapted to their environments. These characteristics include size, shell shape, shell color, and behavior. The differences among these prominent traits create two branches: wave snails and crab snails. Wave snails are typically smaller, have thin shells with distinctive colors and patterns, large round apertures, and often exhibit reckless behavior. In contrast, crab snails are larger, have thicker shells without patterns, smaller and longer apertures, and display more cautious behavior due to their environment often being preyed upon by predators.
The Koster Islands of Sweden are home to both types of L. saxatilis snails, often found side by side on the same island or just a few hundred meters apart at sea. Before the toxic algae bloom in 1988, wave snails inhabited the reefs, while the nearby coastline was a shared habitat for both crab and wave snails. This close spatial proximity would prove significant.
Noticing that the wave snail population on the reefs had been completely wiped out due to the toxic algae, Johannesson decided in 1992 to reintroduce crab snails into one of these reefs. With one to two generations per year, she was right to expect that the crab snails would adapt to the new environment right before the scientists’ eyes. Diego Garcia Castillo, a graduate student at the Barton Group at ISTA and one of the lead authors of the study, stated: “Our colleagues observed evidence of snail adaptation within the first decade of the experiment.” Castillo added, “Throughout the 30 years of the experiment, we have been able to accurately predict what the snails would look like and which genes would be affected. The transformation process has been swift and powerful.”
Rediscovering Evolutionary Traits
However, the snails did not evolve these traits from scratch. Co-author Anja Marie Westram, now a researcher at Nord University, explained: “Some genetic diversity was already present in the original crab snail population but at low frequencies. This is because the species had experienced similar conditions in the recent past. Access to a larger gene pool has fueled this rapid evolutionary process.”
The research team examined three aspects over the years of conducting the experiment: the phenotypes of the snails, the genetic variability of individual snails, and larger genetic changes affecting entire regions of chromosomes known as “chromosomal inversions.”
In less than 30 years, the shell of the original crab snail has evolved to resemble that of a wave snail.
In the initial generations, researchers witnessed an intriguing phenomenon known as “phenotypic plasticity”: right after “recolonization,” the snails changed their shape to adapt to the new environment. But the population also quickly began to change genetically. Researchers could predict the extent and direction of these genetic changes, particularly concerning the chromosomal inversions.
They demonstrated that the rapid and powerful transformation of the snail species could be attributed to two complementary processes: One is the rapid selection of characteristics that were present but less expressed in the newly migrated crab snail population, and the other is gene flow from neighboring wave snails (just a few hundred meters away) potentially drifting into mating.
Evolution Under Environmental Pressure
Theoretically, scientists understand that a species with sufficiently high genetic variation can adapt more quickly to change. However, few studies aim to experimentally test the evolutionary process over time in nature. Garcia Castillo noted: “This work allows us to examine the repeatable evolutionary process more closely and predict how a population could develop characteristics that evolved separately in the past under similar conditions.”
The research team now aims to understand how species can adapt to modern environmental challenges such as pollution and climate change. Westram stated: “Not all species have access to large gene pools, and developing new traits from scratch is a slow and tedious process. The adaptation process is very complex, and our planet is also facing unpredictable changes due to severe weather events, rapid climate change, as well as pollution and new parasites.” She hopes this work will further promote research on maintaining species with large and diverse genetic structures. Westram concluded: “Perhaps this study will help convince people to protect more natural habitats so that species do not lose their genetic diversity.”
