Snails on a small rocky islet developed new traits right in front of scientists. Marine snails were brought back after a harmful algal bloom decimated their population on the skerry. Interestingly, even though researchers introduced a different group of the same snail species, these snails evolved to closely resemble the original population that had vanished over 30 years ago.
Back in 1988, the Koster archipelago, located near the Swedish-Norwegian border, suffered a severe algal bloom, which led to the extinction of marine snail populations. You might wonder why the extinction of a few snails on a tiny rock in the vast ocean matters. This event, however, provided a rare chance to witness and predict evolution right before our eyes.
Prior to the algal bloom, the islands and their small intertidal skerries—rocky islets—boasted rich and diverse populations of the marine snail species Littorina saxatilis. While the snail populations on larger islands—with some being reduced to under 1%—were able to bounce back in two to four years, many of the skerries struggled to recover from this setback.
Kerstin Johannesson, a marine ecologist at the University of Gothenburg, Sweden, recognized a unique opportunity. In 1992, she reintroduced L. saxatilis snails to their lost habitat on a skerry, initiating an experiment that would yield significant insights more than thirty years later. This led to a collaborative effort among researchers from the Institute of Science and Technology Austria (ISTA), Nord University in Norway, the University of Gothenburg in Sweden, and The University of Sheffield in the UK, allowing them to observe evolution as it happened.
Wave snails and Crab snails
L. saxatilis is a prevalent marine snail species found along North Atlantic shores, where different populations have developed traits suited to their environments. These adaptations can include body size, shell shape, color, and behavior. The differences are especially noticeable between the Crab and Wave ecotypes. These snails have independently evolved in various locales, either in areas vulnerable to crab predators or on rocky shores that face wave action without crabs. Wave snails are generally smaller, with thin shells that exhibit specific colors and patterns, a large rounded opening, and bold behaviors. Contrarily, Crab snails are larger, have thicker, unpatterned shells, and feature a smaller, elongated opening. They also exhibit cautious behavior due to the presence of predators.
The Koster archipelago hosts both types of L. saxatilis snails, often located close to one another on the same island or merely a few hundred meters apart across the sea. This proximity became crucial as the events unfolded.
Rediscovering old traits
Recognizing that the Wave snail population on the skerries had been completely eliminated due to the toxic algae, Johannesson decided in 1992 to reintroduce Crab-ecotype snails to the skerry. Anticipating that these Crab snails would adapt to their new habitat within one or two generations, she observed a transformation unfolding before scientists’ eyes. “Our collaborators noted signs of the snails adapting within the first ten years of the study,” shares Diego Garcia Castillo, a graduate student in the Barton Group at ISTA and co-author of the study. “Over the course of 30 years, we effectively predicted how the snails would evolve and which genetic regions would be involved. The changes were both rapid and remarkable,” he adds.
However, the snails did not develop their new traits entirely from scratch. Co-corresponding author Anja Marie Westram, formerly a postdoc at ISTA and now at Nord University, explains, “Some genetic diversity was already present in the initial Crab population, albeit at low levels. This is a reflection of the species having faced similar conditions in their recent history. The abundance of genetic variation contributed to this speedy evolutionary process.”
Diversity is key to adaptation
The research team monitored three elements throughout the experiment’s duration: the snails’ physical characteristics, individual gene variability, and larger genetic alterations involving regions of chromosomes known as “chromosomal inversions.”
In the initial generations, the researchers observed an intriguing phenomenon called “phenotypic plasticity.” The snails quickly adapted their shapes to fit their new surroundings. However, genetic transformations also began shortly thereafter. The scientists could predict the nature and trajectory of these genetic changes, particularly regarding the chromosomal inversions. The rapid transformation of snails seemed to arise from two complementary mechanisms: rapid selection for traits that were already present at low frequencies in the reintroduced Crab snail population and potential gene exchange from neighboring Wave snails that may have drifted across the 160-meter distance to the skerry.
Evolution in the face of pollution and climate change
Scientists understand that species with substantial genetic variation can adjust more swiftly to changes in their environment. Nonetheless, few studies have examined evolution over a prolonged period in natural settings. “This research provides us with a unique glimpse into repeated evolutionary processes and allows us to predict how a population might develop traits that have previously evolved under similar conditions,” observes Garcia Castillo.
The team aims to explore how species can adapt to contemporary environmental pressures, such as pollution and climate change. “Not all species have access to extensive gene pools, and developing new traits from scratch takes an inordinate amount of time. Adaptation is intricate, especially in our rapidly changing world that faces severe weather events, fast-accelerating climate change, pollution, and emerging parasites,” Westram notes. She hopes this study will encourage further exploration into preserving species with diverse genetic resources. “Perhaps this research will motivate people to safeguard a variety of natural habitats to maintain genetic variation among species,” Westram concludes.
Currently, the snails Johannesson reintroduced to the skerry in 1992 have flourished into a population of approximately 1,000 individuals.
