Pacific oysters are adapting to less saline waters and may soon spread into the Baltic Sea, aided by warming waters and genetic traits. Pacific oysters are an invasiveThe species, despite being less than a decade old, has successfully adapted to and reproduces in the low-salinity waters off the coast of Skåne. University of GothenburgThe discovery, reported by researchers, suggests that the species has the potential to colonize the western Baltic Sea.

The Pacific oyster has colonized the Bohus province at record speed and is now found even in the depths of the channel. The question is whether these oysters will continue their journey to the Baltic Sea. Image source: YoukGreeve

Pacific oysters were first introduced to European oyster farms in the 1970s to replace dwindling native oyster populations, and began spreading outside aquaculture facilities soon thereafter. They arrived in the Bohus province in northern Sweden in 2006 and have since migrated south along the west coast of Sweden, now appearing south of the Oresund Strait.

Until recently, scientists thought the low salinity of the Baltic Sea would act as a natural barrier to further spread of the species. However, this new study shows that Pacific oysters can quickly adapt to saltwater environments, raising the possibility that they will continue to expand into the Baltic Sea.

"We know today that oysters in the Sund Strait can reproduce and that their larvae can be carried by ocean currents to the Baltic Sea, but we don't know if they can also adapt to the saltwater living conditions there," said Pierre de Wit, a marine biology researcher at the University of Gothenburg.


Pierre DeWit is a senior lecturer in conservation biology at the University of Gothenburg. Photo credit: Susanne Liljenström

As early as the 1970s, Sweden tried to cultivate Pacific oysters, but failed to reproduce, possibly because the water temperature was too low. Pacific oysters require water temperatures above 20 degrees Celsius for a period of time to mature, and it was not until the 21st century that this became the norm in Swedish waters.

Today, oysters breed in Sweden every summer. A female oyster can lay hundreds of millions of eggs, and the larvae drift with the ocean currents for 2-3 weeks, so they can reproduce quickly.

The study's authors crossed female and male oysters in waters with varying salinities, ranging from Baltic Sea salinity (8 parts per thousand) to full-ocean salinity (33 parts per thousand).

Oysters collected from the waters of Hallands Väderö in Skåne reproduce well at salinities as low as 13‰, while oysters from Bohuslän cannot adapt to salinities below 18‰. Research shows that an oyster's ability to reproduce depends on its breeding habitat and genetics. At the lowest salinity, genetics is the most important factor.

"We don't know how low salinity Pacific oysters will need to reproduce in the future. But many studies suggest that they have genes that allow them to tolerate saltier water than they do today," said Alexandra Kinnby, a marine biology researcher at the University of Gothenburg and lead author of the study.

Pacific oysters use in vitro fertilization - adult oysters release eggs and sperm into open water, and the sperm must then find the eggs and fertilize them. But sperm is sensitive to factors such as temperature and salinity. In the new study, researchers show that genetic diversity in sperm on certain key genes determines whether oysters can reproduce in low salinity.

"It is not yet clear whether oysters can colonize the Baltic Sea, or what happens when oyster larvae drift into the Baltic Sea. Do they drift back to land again with ocean currents, or do they remain in the distant sea? We don't have the answer yet," says Pierre de Wit.

Compiled from /ScitechDaily