The invasive Pacific oyster have adapted to life in less salty seas and are reproducing off the coast of Skåne, despite having been there for less than ten years. The discovery by researchers from the University of Gothenburg shows that the oysters may be able to colonise the western Baltic Sea in the future.
Pacific oysters were imported to farms in Europe in the 1970s to replace our native oysters, whose stocks had collapsed by then. But they quickly spread from the farms along the coast, reaching Bohuslän in 2006. Since then, they have spread southwards along the west coast of Sweden and are now found as far as the Sound.
Scientists have assumed that the low salinity would act as a barrier to further spread into the Baltic Sea, but a new study shows that the oysters can quickly adapt and possibly spread into brackish water.
"We now know that the oysters in the Sound can reproduce there and that their larvae can be transported into the Baltic Sea by ocean currents, but we don't know if they can adapt to a life there as well," says Pierre De Wit, researcher in marine biology at the University of Gothenburg.
Warmer seas help spread
Attempts were made to cultivate Pacific oysters in Sweden as early as the 1970s, but they failed to reproduce, probably because the water was too cold. Pacific oysters need water temperatures above 20 degrees for some time to become sexually mature, and it is only in the 2000s that this has become common in Swedish waters.
Now oysters reproduce every summer in Sweden. A single female can release hundreds of millions of eggs, and their larvae drift with ocean currents for 2-3 weeks, allowing them to spread very quickly.
Environment and genetics
The authors of the study crossed female and male oysters in waters with different salinities, from Baltic Sea salinity (8 parts per thousand) to full marine salinity (33 parts per thousand).
Oysters collected from the waters of Hallands Väderö in Skåne were able to reproduce well down to 13 parts per thousand, while oysters from Bohuslän could not cope with salinities lower than 18 parts per thousand. The study showed that the oysters' ability to reproduce depends on a combination of the oysters' growing environment and their genetics. In the lowest salinities, genetics was the most important factor.
"We don't know how low in salinity a Pacific oyster will be able to reproduce in the future. But there are many indications that their genes allow them to tolerate even more brackish water than today," says Alexandra Kinnby, researcher in marine biology at the University of Gothenburg and first author of the study.
Sperm are important
Pacific oysters have external fertilisation - eggs and sperm are released into the water by the adults, and the sperm must then find the eggs and fertilise them in the open water. But the sperm are sensitive to factors such as temperature and salinity. In the new study, the researchers show that the genetic diversity of sperm in certain key genes determines whether oysters can reproduce in low salinity.
"It is still unclear whether the oysters will be able to establish themselves in the Baltic Sea, what happens to the larvae when they drift into it. Do they drift with the currents back towards land again or do they stay far out at sea? We don't have those answers yet," says Pierre De Wit.
Scientific article in Molecular Ecology: The roles of plasticity and selection in rapid phenotypic changes at the Pacific oyster invasion front in Europe
