The invasive Pacific oyster have adapted to life in less salty seas and are reproducing off the coast of Skåne, although having been there for less than ten years. This discovery by researchers from the University of Gothenburg suggests that the oysters could colonise the western Baltic Sea in the future.
Pacific oysters were imported to oyster farms in Europe in the 1970s to replace native oysters whose stocks had collapsed. But they quickly began to spread from the farms, reaching northern Bohuslän in Sweden in 2006. Since then, they have spread southwards along the Swedish west coast and are now found as far south as the Sound.
Scientists have assumed that a low salinity would act as a barrier to a further spread into the Baltic Sea, but a new study shows that the oysters can adapt quickly and possibly spread into brackish water.
"We know today that the oysters in the Sound are able to reproduce and that their larvae can be transported into the Baltic Sea by ocean currents, but we don't know if they also can adapt to the brackish living conditions there," says Pierre De Wit, researcher in marine biology at the University of Gothenburg.
Spreading helped by warmer seas
As early as the 1970s, attempts were made to cultivate Pacific oysters in Sweden, but they failed to reproduce, probably due to the water being too cold. Pacific oysters need water temperatures above 20 degrees Celsius during a period of time to reach maturity, and it is only in the 2000s that this has become regular in Swedish waters.
Now, oysters reproduce every summer in Sweden. A single female can release hundreds of millions of eggs, and the larvae drift with ocean currents for 2-3 weeks, allowing them to spread very quickly.
A combination of environment and genetics
The authors of the study crossbred 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' breeding habitat and their genetics. In the lowest salinities, genetics was the most important factor.
"We don't know how low salinity a Pacific oyster will need to reproduce in the future. But much suggests that they have genes that 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.
Sperms are important
Pacific oysters have external fertilisation – eggs and sperm are released in open water by the adults, and the sperm must then find the eggs and fertilise them. But the sperms 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 determine whether oysters can reproduce in low salinity.
"It's still unclear whether the oysters will be able to establish in the Baltic Sea, or what will happen to the larvae when they drift in that direction. Will they drift back towards land again with the currents or will they remain far at sea? We don't have those answers yet," says Pierre De Wit.
