Our species has enjoyed mussels for thousands of years. This modest mollusc has in fact been consumed since prehistoric times, and its shells have been found in large numbers on the sites occupied by our ancestors. The latter certainly did not imagine that their food remains would become tools for their descendants to explore past biodiversity…
The increase in ocean temperature due to climate change and the modification of their composition (in particular due to their acidification and pollution) threaten the survival of many marine and coastal species, as well as that of their ecosystems.
This is particularly the case in the Arctic, which is warming three times faster than the planet’s average and which is recognized as a hotspot for climate change. To monitor the consequences of these environmental upheavals over time, our team found an unexpected ally: Greenland mussels.
Mussels, privileged witnesses of environmental changes
The oceans cover 70% of the earth’s surface and contain the greatest biodiversity on our planet. These ecosystems are highly sensitive to the global environmental changes underway since the start of the Industrial Revolution in the 18th century.e century. To better understand the impacts that these modifications due to human activities can have, over time, on marine species, mussels are ideal models.
They are in fact abundant along the coasts of very diverse environments and play an important role in coastal ecosystems. In addition, they have been part of the diet of human groups since prehistoric times, making them one of the molluscs whose shells are most frequently found in archaeological sites.
Based on these observations, we sifted through archaeological sites and museum collections to constitute a sampling of ancient mussel shells from the west coast of Greenland. The age of these shells is known by radiocarbon dating of the sites from which they originated, or, for the most recent ones, by museum documentation recording the dates on which they were collected. To be able to compare with the current situation, we selected mussels representative of the populations that live in this region today.
Today, there are two species whose respective distributions are explained by their differences in tolerance to the conditions of their environment. Lower temperatures in northern Greenland would benefit the species Mytilus trossulus which would be more suitable than Mytilus eduliswhich is found more abundantly to the south. Between environmental extremes, the two species reproduce with each other to give rise to hybrid mussels Mytilus edulis x Mytilus trossulus.
According to different hypotheses, only one of the species is endemic to the west coast of Greenland. The other species would have recently returned to Greenland under the effect of climate change or human activities, such as long-distance maritime transport for example.
Comparing ancient and current mussel shells at different sites on the west coast of Greenland could reveal whether population structure has indeed changed over time. If changes exist, their dynamics could be linked to variations in environmental conditions reconstructed from the composition of Greenland ice cores and climate models, and from the scientific literature.
Problem: the two species, and even more so the hybrids, are difficult to differentiate if we only base ourselves on the observation of their shell. This is why, to determine the distribution of species and the structure of ancient and contemporary populations, we implemented analyzes of complete genomes, in other words of the entire DNA of a mussel.
DNA in the shells
If extracting DNA from the tissues of mussels that have just been dislodged from their rock does not present any particular difficulty, how can we analyze that of old mussels, when their soft tissues have disappeared? By recovering the DNA trapped in… their shells.
We have indeed discovered that the shells of mussels and other molluscs also contain DNA. If we do not yet know precisely how the latter is found in the shell (made of calcium carbonate), our work suggest that it is integrated during its formation, in other words throughout the life of the mussels.
Clio Der Sarkissian, Provided by the author
Ancient DNA to the rescue
Our work also revealed that the DNA present in mussel shells can be preserved over the very long term. Thus, the oldest shell from which we were able to extract DNA has been dated to over 100,000 years ago. This exceptional preservation was made possible by the optimal conditions offered by the continually frozen soils of the Siberian permafrost in which the said shell was found.
This property is valuable for being able to trace possible changes in the structure of populations over long periods of time. However, it must be emphasized that although DNA can persist for a long time in the shells, it is nonetheless subject to the ravages of time. Over the years, these molecules fragment and are chemically modified, which complicates analyses.
Studying ancient DNA is no easy feat. This must in particular be done in special “clean room” type laboratories, in other words extremely controlled environments, in order to limit contamination by modern DNA. State-of-the-art sequencing and analysis tools are also required.
Finally, you should know that the shells do not only contain the DNA of molluscs, but also other DNA, coming from microbes of the marine environment trapped when molluscs filter the water to feed (a mussel pumps on average 25 L of water per day). The DNA of infectious agents responsible for diseases in molluscs could also be identified in shells.
The shells can therefore be “read” like archives by researchers from various disciplines, who will unearth not only the DNA of the molluscs, but also that of their pathogens and the marine microbial communities that lived at the same period.
What has ancient DNA taught us?
Against all expectations, DNA analysis of 125-year-old mussel shells showed that M. trossulus and hybrids were already found at mid-latitudes along the west coast of Greenland. Our analyzes also confirmed the presence of M. edulis in southern Greenland for at least 600 years.
This preliminary work therefore does not detect any difference in the distribution of mussel species in Greenland between the past and the present, despite significant environmental changes and intensified pressure due to human activities.
The question now is whether within each species, it is really the same populations that have persisted over time. To find out, more detailed analyzes are currently underway. This could confirm the resilience of mussel populations in the face of environmental changes as well as the stability of hybridization zones. These hypotheses had already been formulated based on the study of current populations, but had never been tested over the long term. It will then remain to understand the factors behind such resilience, if it exists.
Either way, the next time you’re walking on a beach, when your eyes land on empty shells, remember that these are not just remains of molluscs, but real archives biodiversity!
