December 11, 2023


North Sea: Where did the gaseous comb jelly come from?

Jelly comb Mnemiopsis Leidyi, native to North America, is one of the world’s most successful invasive species. This comb jelly has also proven to be present in the North Sea and the Baltic Sea. Using DNA analyzes, scientists have now been able to reconstruct its source and how this species spread in the seas of Europe. This reveals that comb jellyfish come from two different indigenous groups and have been introduced to Europe several times.

Biological invasions are one of the major threats to biodiversity today. Because if non-native species settle into new habitats, it could disrupt existing ecosystems and displace native species. Many of these types of invasive animals are brought in by humans; In the ocean, for example, they reach their new home on the hull of ships or in ships’ ballast water.

On the trail of jellyfish comb Mnemiopsis Leidyi

One of the most famous marine invaders is the comb jelly Mnemiopsis Leidyi. Animals, up to ten centimeters in size, are very strong and adaptable. They live in water temperatures from about freezing point to a good 30 degrees and tolerate even brackish and nearly salt-free water. Invertebrates also benefit from enormous reproductive potential: they can produce up to 3000 eggs per day. So it’s no wonder that this species, which was native to the east coast of America, was able to establish itself very quickly elsewhere.

Comb jelly was first seen in the Black Sea in the 1980s, where it drastically altered the marine ecosystem. Predatory species destroyed 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. Cornelia Jaspers of the Technical University of Denmark explains: “No matter how significant it is, the specific invasion dynamics and spread during The introduction is not yet known – as is the case with most non-native marine species.”

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DNA comparisons reveal the origin of the invaders

Together with her colleagues, she conducted DNA analyzes of 72 comb jellyfish samples from two indigenous regions of North America, as well as the Black Sea, Mediterranean Sea, North Sea and Baltic Sea. By comparing the genomes, the team was able to reconstruct where the newly created populations in Europe came from. “Because this species is now found in the North Sea and Baltic Sea, we have to understand its exact history of invasion, including a possible decrease in genetic diversity,” explains co-author Thorsten Reusch of GEOMAR Helmholtz Center for Ocean Research Kiel. “By re-sequencing the entire genomes of individuals from five different indigenous and invasive groups, we were able to reconstruct invasion routes and the demographic history of at least two invasion events.”

DNA analyzes revealed that the European population of Mnemiopsis Leidyi had different origins: the comb jellyfish introduced into the Black Sea in the 1980s came from a southern population native to the Gulf of Mexico and off the coast of Florida. The animals may have been transported across the Atlantic with the ballast waters of large ships. “This is consistent with the fact that there was heavy freight traffic between Cuba and the ports of the Soviet Union on the Black Sea during this period,” the team explains. Genetic comparisons also indicate that large numbers of comb jellyfish reached the Black Sea at once. And from the Black Sea, this invasive population spread further into the Mediterranean.

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The North Sea has been “occupied” several times

However, contrary to what was previously assumed, the comb jellyfish created in the North and Baltic Seas does not come from these southern inhabitants. Instead, they return to a different population from the northeastern coast of the United States, the researchers found. So they must have arrived in Europe independently of the earlier advances. “This northern invasion appears to be more recent than the southern invasion,” Jabsers and colleagues say. The relatively high genetic diversity of Mnemiopsis Leidyi in the North Sea, exceeding even local starting stocks, indicates that ships repeatedly brought smaller quantities of this comb jellyfish into the North and Baltic Seas.

“Our data indicate that the North Sea frequently suffers from mnemiopsis, despite international agreements to limit the introduction of the species,” Jaspers says. “Revealing this temporal relationship is important for understanding the current risks of invading territories and stopping 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