"Blue mussels are an iconic species along the shore. Anyone living by the sea has probably collected mussels at some point. However, there have been fewer of them over the past couple of decades, and we're not entirely sure why."

This is according to Professor Ane Timenes Laugen from the University of Agder (UiA). Together with colleagues at UiA and the Institute of Marine Research, she has conducted a genetic study of blue mussels along the coast from Kristiansand to Gothenburg.

In an article from this study, the researchers show that there is significant geographical variation in the genes of blue mussels in this sea area. This indicates that different groups of mussels do not mix easily with others, despite being a species that spreads readily.

There are many theories about what happened to the mussels, but no one is quite sure what happened. Laugen hopes their findings might provide a piece of the puzzle.

Drifting with the current

Like many other marine organisms, blue mussels do not reproduce like humans, who produce one offspring at a time. Instead, they release thousands or millions of eggs and sperm that meet in the water.

Fertilised eggs develop into larvae, which are carried by ocean currents before turning into mussels after about three weeks.

There are many obstacles along the way. Many blue mussel larvae are swept westward by currents along the Skagerrak coast and onward to the North Sea. But in the Skagerrak, there is a large, circulating ocean current that occasionally pulls the larvae out into the open sea, reducing their chances of finding a suitable place to settle and mature.

Ocean currents can also benefit mussel larvae. One such current is the Baltic surface current, which flows from the Baltic Sea, along the Swedish west coast and down the Norwegian Skagerrak coast.

"This surface current moves quickly, and the larvae can travel quite far in three weeks. When we nevertheless discover such great genetic variation in the Skagerrak, it is therefore somewhat unexpected," says Laugen.

"Our challenge as researchers is the lack of systematic data on mussel occurrences. We get reports from the public and a few studies here and there, but we need long-term data series to really understand the causes of the decline," says Laugen.

The researchers have found some studies on mussel populations along the Swedish coast between Gothenburg and Strömstad from the '70s, '80s and '90s. However, they are not sure if similar data exists in Norway.

"Locally on the Swedish west coast, there are significant genetic differences between mussel populations within the archipelago and those further out. The same pattern might exist in Norway, but we have not yet investigated this," Laugen adds.

The fact that some mussel populations are genetically very distinct from others means that few larvae come from different places.

"If something happens to these isolated mussel populations, natural recruitment of new individuals is difficult. We may have lost some mussel habitats because they only reproduced locally," the researcher says.

"In the summer of 2024, we've seen large numbers of new blue mussels along the coasts of Skagerrak and Kattegat, but we will only see over the next couple of years whether these mussels survive to reproductive age and can contribute to future mussel generations," she continues.

In the article, the researchers caution against relocating blue mussels.

One reason is that, just as different species have intrinsic value, so do genetically distinct populations. More importantly, it's uncertain if the mussels can cope with relocation.

"If all the blue mussels in Skagerrak disappear, we should be cautious about relocating mussels from Trøndelag. When we move individuals with a specific genetic composition to a new environment, they might not thrive," Laugen warns.

She also notes that shells can carry many diseases, and careless relocation could inadvertently spread these diseases to new areas.