The comb jelly Mnemiopsis Leidyi, originally from North America, is one of the most successful invasive species in the world. This comb jelly has also established itself in the North and Baltic Seas. Using DNA analyzes, scientists have now been able to reconstruct where it came from and how this species spread in Europe’s seas. These reveal that the comb jellyfish come from two different original populations and have been introduced to Europe several times.
Biological invasions are considered to be one of the major threats to biodiversity today. Because if non-native species establish themselves in new habitats, this can disrupt existing ecosystems and displace native species. Many such invasive animal species are brought in by humans; in the ocean, for example, they get to their new home on the hull or in the ballast water of ships.
On the trail of the comb jellyfish Mnemiopsis Leidyi
One of the most notorious marine invaders is the comb jelly Mnemiopsis Leidyi. The animals, up to ten centimeters in size, are extremely robust and adaptable. They survive in water temperatures from almost freezing point to a good 30 degrees and tolerate even brackish and almost salt-free water. The invertebrate invaders also benefit from an enormous reproductive potential: They can produce up to 3000 eggs per day. It is therefore no wonder that this species, originally native to the east coast of America, was able to establish itself very quickly elsewhere.
The comb jelly was first spotted in the Black Sea in the 1980s, where it led to a drastic change in the marine ecosystem. The predatory species decimated the stocks of many fish and small marine animals. Since then, Mnemiopsis Leidyi has spread across large parts of Europe and western Eurasia and has also been found in the North and Baltic Seas since 2006. “Regardless of their great importance, the specific invasion dynamics and the spread during the introduction are so far unknown – as is the case with most non-native marine species,” explains Cornelia Jaspers from the Technical University of Denmark.
DNA comparisons reveal the origin of the invaders
Together with colleagues, she carried out DNA analyzes of 72 specimens of these comb jellyfish from two regions of origin in North America, as well as the Black Sea, the Mediterranean Sea, and the North and Baltic Seas. By comparing genomes, the team was able to reconstruct where the populations newly established in Europe once came from. “Since this species is now found in the North and Baltic Seas, we have to understand its exact history of invasion, including the possible reduction in genetic diversity,” explains co-author Thorsten Reusch from GEOMAR Helmholtz Center for Ocean Research Kiel. “By whole genome re-sequencing of individuals from five different indigenous and invasive populations, we were able to reconstruct the invasion routes and the demographic history of at least two invasion events.”
The DNA analyzes revealed that the European populations of Mnemiopsis Leidyi have different origins: the comb jellyfish introduced into the Black Sea in the 1980s come from a southern population that is native to the Gulf of Mexico and off the coast of Florida. The animals were probably transported across the Atlantic with the ballast water of large ships. “It fits in with the fact that there was heavy shipping traffic between Cuba and the large Black Sea ports of the Soviet Union during this period,” explains the team. The genetic comparisons also suggest that large numbers of the comb jellyfish made it into the Black Sea at once. From the Black Sea, this invasive population then spread further into the Mediterranean.
North Sea has been “conquered” several times
However, contrary to what was previously assumed, the comb jellyfish established in the North and Baltic Seas do not come from these southern populations. Instead, they go back to a different population native to the northeast coast of the USA, as the researchers found. They must therefore have reached Europe independently of previous introductions. “This northern invasion seems to be more recent than the southern one,” report Japsers and her colleagues. The comparatively high genetic diversity of Mnemiopsis Leidyi in the North Sea, which even exceeds that of the local starting stocks, also suggests that ships repeatedly brought and bring smaller amounts of these comb jellyfish into the North and Baltic Seas.
“Our data indicate that the North Sea is repeatedly plagued by mnemiopsis, despite international conventions to curb the introduction of species,” says Jaspers. “Uncovering this temporal relationship is important in order to understand the current risk of invasion of areas and to stop the introduction of species in the long term.”
Source: GEOMAR Helmholtz Center for Ocean Research Kiel; Technical article: Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2116211118