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Russian scientists reconstruct pearl mussel’s settlement after glaciation period

Pearl mussels are facing the threat of extinction, and the climate changes are among negative factors

ARKHANGELSK, August 8. /TASS/. Russian scientists reconstructed how pearl mussels (Margaritifera margaritifera) settled after the glaciation in Europe and North America and analyzed the species’ genetic diversity. The studies featured experts from the Arctic Complex Studies Center at the Russian Academy of Sciences’ Urals Branch, from the Academy’s Karelia scientific center, and from the Fishery and Ocean Studies Institute, the Arctic Complex Studies Center’s Director Ivan Bolotov told TASS.

Pearl mussels are facing the threat of extinction, and the climate changes are among negative factors, he added.

"Scientists have methods to use genetic codes to track, to reconstruct how this or that organism has settled. Such a method was used in the case of pearl mussels. The species has a very interesting distribution history: it lives in Europe, and its second habitat is in North America - from Delaware in the US to Newfoundland in Canada. Experts have argued: what came first - Europe or America, from where does pearl mussel originate? Results of our studies show that it has come from Europe to America," the scientist said.

Europe’s eastern boundary is the Solza River in the Arkhangelsk Region. The mussels live in the reservoirs of Karelia, Novgorod and Leningrad regions, in the Baltic States, Germany, and some separate populations are found in France and on the Iberian Peninsula.

The life cycle is connected with salmon and trout. Larvae parasitize on salmon species, they attach to the gills, and this process contributes to their further settlement.

Scientists from Arkhangelsk and Karelia have gone to expeditions to collect material for this research, and additionally they have taken data from open international databases - thus they have covered the entire area. For tests the researchers used a section of the mitochondrial gene that gradually accumulates mutations. By analyzing similarity of fragments in different individuals, scientists can assess their relationship and determine settlement processes. During the glaciation, the mussels preserved in so-called refugiums - fauna shelters free from ice. The mussels managed to survive the glaciation on the Iberian Peninsula, in the south of France and on the British Isles together with salmon fish, which contribute to the mussels’ settlement. From there, 15-12 thousand years ago, the habitat began to spread northbound.

Overseas relatives

At the same time, studies have pointed to the biggest genetic similarity between the mussels in Europe’s north-east and those in North America. This proves the European pearl mussels have repeatedly crossed the North Atlantic traveling on salmon, and this consequently has caused the exchange of genes.

According to Ilya Vikhrev of the Arctic Center, biologists have identified the greatest genetic diversity in pearl mussels in Karelia. "North Karelia is an important area in terms of genetic diversity and preservation of the species diversity. This is the place where two migration flows meet: the flow from the South, which apparently comes from the Baltic Sea, the Gulf of Finland, and the flow from the North, which comes from the North Atlantic, North America," he explained. "Populations from neighboring tributaries may be quite different genetically. Such regions are important for the species because there preserves the genetic diversity, and - the higher the diversity, the more stable is the system."

Scientists have found a reservoir where no diversity was detected in the studied genome section. It is a small river, the Peypia, near St. Petersburg. There are quite a lot of pearl mussels there, but the habitat is quite close to a megalopolis. "The population is under a constant anthropogenic pressure. Thus, we can see a zero genetic diversity, that is, one haplotype," the scientist said. "If the conditions there change somehow, it will be very complicated for this population to get adapted at the genetic level. The risk of extinction is extremely high."

Warming’s impact

European pearl mussel is listed on the Red Data Book of the Russian Federation and the Red Data Books of all the countries where it lives. The number of its populations has been decreasing. There are several reasons for this. One of them is a decrease in the numbers of salmon and trout. Noteworthy, on pink salmon, which has been introduced into the Russian northern rivers from the Far East, the larvae cannot settle.

These mussels filter the water and are an indicator of its conditions. In some reservoirs, the populations are affected by contamination coming from industrial and domestic wastewaters. Another factor is changed riverbeds, to which quite often mussels are unable get adapted.

The European pearl mussel belongs to long-living species. Specialists have registered a life span of 210 years, and an average life term is about 150 years. The mussels from northern regions are larger and live longer, those from southern areas usually live less and are smaller in size. The rising average annual temperatures are reasons of shorter life terms. Scientists explain this supposing most likely the metabolic processes are accelerated, and the life cycle gets shorter. The warming changes the hydrological regimes in reservoirs. "In the worst-case scenario of carbon dioxide emissions into the atmosphere, the population will survive only in Scandinavia and in Russia’s north-west," the expert added.

Currently, Center’s specialists together with colleagues from Germany and Karelia are analyzing specific fragments of the mussels’ nuclear genome, which point to the changes that have occurred in the genes over the recent few hundred years. This study will assess how humans and climate factors can affect the mussels’ genetic diversity.