Herring was one of the first fish species to colonize the newly formed Baltic Sea around 8,000 years ago. But the low salinity presented challenges for the fish that migrated from the Atlantic. Genetic analyzes now show how he adapted to it. The key therefore lay in the ability to reproduce in brackish water. Genetic changes that stabilized the eggs and sperm at low salt levels were therefore particularly important.
The Baltic Sea was formed around 8,000 years ago after the last ice age, when salt water from the North Sea flowed into the Baltic Sea basin, where there was previously a huge meltwater lake. One of the first fish species to migrate to the new brackish sea was the Atlantic herring. Today herrings in the Baltic Sea are among the most ecologically and economically important fish. “The herring is a link between the plankton on which it feeds and the fish, birds and marine mammals that in turn feed on the herring,” explains Leif Andersson from Uppsala University in Sweden. “The rich herring stocks have been an important part of the food supply for the population around the Baltic Sea for thousands of years.”
New home in brackish water
Together with a team led by first author Cheng Ma, Andersson has now investigated which genetic changes enabled the fish that migrated from the Atlantic to survive in the Baltic Sea. “In parts of the Baltic Sea, the salinity drops to 0.2 to 0.3 percent, compared to 3.4 to 3.5 percent in the North Atlantic,” explain the researchers. This difference is particularly important for the reproduction of herring. Since it reproduces through external fertilization, its sperm, eggs and embryos are directly exposed to seawater. To ensure that they do not swell or suffer other osmotically-related damage, they must be adapted to the salt content of the environment.
To find out what adaptations the Baltic Sea herring have developed, Ma and his colleagues sequenced the genomes of numerous herrings from different regions in the Atlantic, the Baltic Sea and from Ringkøbing Fjord on the Danish North Sea coast, which lies between the distribution areas of the Atlantic and Baltic Sea herrings. They discovered numerous genetic differences. “Changes in the genes that control reproduction and early development are particularly crucial,” reports the team.
Genetic modifications in the reproductive system
One of these adaptations involves an ion channel found only in sperm. In Baltic Sea herring, the diameter of this canal is increased due to a gene mutation. “The large diameter facilitates the transport of ions and osmolytes and probably prevents sperm swelling during spawning in waters with low salinity,” explain the researchers. The eggs are also protected from swelling through genetic modifications. Adapted proteins and enzymes ensure a more stable egg shell. This reinforced egg shell protects the embryo, but becomes a problem when it hatches. But natural selection has also solved this problem: Compared to Atlantic herring, Baltic herring have 20 additional copies of a gene for breaking down the egg shell.
“This study is a textbook example of how strong natural selection leads to genetic changes in multiple genes that together ensure successful reproduction in a new environment,” says Andersson. “What is remarkable is that we were able to identify specific genes and mutations and explain the biological mechanisms why these changes were crucial for adaptation to the brackish water environment of the Baltic Sea.”
To date, Baltic herring has been classified as a subspecies of Atlantic herring. “However, the striking genetic differences between Baltic and Atlantic herring that have now been uncovered provide an argument for classifying Baltic herring as a separate species,” says Andersson.
Source: Cheng Ma (Uppsala University, Sweden) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2601861123