Mussels as biodiversity archives

Through their filtering diet, mussels accumulate genetic traces of the creatures in their environment. © Damocean/iStock

How has biodiversity in marine ecosystems developed over the last decades? Researchers report that the analysis of environmental DNA from archived mussel tissue can provide insights into this question: By evaluating the genetic traces of other organisms collected by the filter feeders, the change in a living environment can be reconstructed. This may also reflect the history of the spread of invasive species. The scientists illustrate this using the example of an Australian barnacle species on the German North Sea coast.

Tracking down living creatures with “forensics” – a new process has been introduced into biodiversity research in recent years: traces of genetic material can be detected in environmental samples that can be assigned to specific original organisms. This so-called environmental DNA comes from detached cellular material that lands on surfaces or gets into the air or water. Modern genetic methods can now capture these traces more and more effectively. If there is reference data, the identified sequence sections can then be assigned to known species. This procedure can be used to explore biodiversity or to detect a specific species in a habitat.

Sample material from 40 years

By studying environmental DNA in water samples, researchers can already gain insight into the biodiversity of aquatic ecosystems. But only the current state of settlement by the various living creatures is apparent. A team of researchers at the University of Trier now came up with the idea that special archive material could perhaps enable longer-term observations: At the university, samples of mussels that have been collected for 40 years to monitor environmental pollution on German coasts are stored frozen.

The team investigated the extent to which environmental DNA contained in the archive material can provide insights into the organisms of the mussels' habitat at the time of sampling. The chances seemed promising. Mussels filter more than a liter of seawater per hour and in the process accumulate organic matter in their digestive tract - probably including environmental DNA. This is exactly what the studies confirm: The scientists were able to show that the mussels are actually full of genetic traces of the organisms in their environment. “By sequencing the DNA obtained, we were finally able to characterize the different mussel-associated habitats,” writes the team.

How climate change affects biodiversity

Using archive material from the past 40 years, the researchers were able to show long-term changes in biodiversity. “With our study, we were able to point out an important ecological consequence of climate change for biodiversity: the gradually warmer winters pave the way for heat-loving bioinvasors to multiply and displace native populations,” says lead author Isabelle Junk from the university Trier. The team was able to demonstrate this specifically in the case of the Australian barnacle. This crustacean species, which comes from the global south, was introduced to Europe during the Second World War, settled on European coasts and then displaced native species there.

The long-term data from the mussels were now able to show strong fluctuations in the population of the invasive species on the German North Sea coast, which correlated with the temperature trends. In cold winters, Australian barnacle populations declined massively. In some cases, the population only managed to recover after several years. In the warm periods, however, the invaders were able to almost completely displace the native barnacles, which was reflected in the mussel data. As the team emphasizes, this case is one example among many: the Australian barnacle is one of many other invasive marine animals that can spread to new regions as a result of human activities, threatening ecosystems.

The spread of these invaders often goes unnoticed and the consequences and influencing factors remain unclear. “Time series available to date are often short or incomplete and do not allow reliable statements to be made about whether changes in biological diversity are short-term population fluctuations or long-term trends,” says senior author Henrik Krehenwinkel from the University of Trier. “This evidence could now be provided by analyzing environmental DNA from long-term archived environmental samples,” said the scientist.

Source: University of Trier, specialist article: Current Biology, doi: 10.1016/j.cub.2023.07.035

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