Invasive comb jelly expands in the Baltic Sea

Mnemiopsis leidyi, a gelatinous invertebrate that resembles a jellyfish but lacks stinging cells, first reached European waters in the 1980s after being transported by ships calling at Black Sea ports from the Atlantic coasts of North and South America. W.M. Graham and K.M. Bayha note in *Biological Invasions by Marine Jellyfish* that the species quickly became dominant in its new habitats. At the peak of the invasion in 1988, densities reached up to 400 individuals per cubic metre. Although Baltic numbers have not yet matched that peak, a study published in January 2026 in the *Journal of Plankton Research* reports that the comb jelly’s spread is now accelerating.

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Mnemiopsis leidyi feeds on zooplankton, sprat and sardine eggs, and larvae, thereby reducing future fish stocks. It also exhibits cannibalism, consuming its own larvae when food is scarce, which helps it survive unfavorable conditions. In the Baltic, the three-spined stickleback (*Gasterosteus aculeatus*) can prey on comb-jelly larvae, providing a natural control. However, increasing hypoxia—low-oxygen “dead zones”—limits stickleback activity and reduces larval predation, according to the same 2026 study.

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Rising water temperatures linked to climate change create shallow, warm, nutrient-rich coastal zones that favor Mnemiopsis leidyi. Simultaneously, progressive hypoxia expands oxygen-desert areas, further weakening stickleback populations. Climate change also lowers Baltic salinity, a condition that is not wholly advantageous for the comb jelly, which prefers slightly higher salinity. Researchers emphasize that these interacting factors make forecasting the species’ future distribution challenging.

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The comb jelly’s arrival is one of several signals that the Baltic Sea ecosystem is under pressure. Scientists have documented expanding oxygen deserts, record water-level fluctuations, and other climate-related phenomena that alter habitats for native species. While some alien organisms, such as the American crab and the sand-burrowing clam *malgiew piaskolaz*, have integrated into local food webs, Mnemiopsis leidyi’s rapid expansion and predation on early-life stages of commercially important fish raise concerns for fisheries and biodiversity.

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Continued monitoring of Mnemiopsis leidyi densities, hypoxia trends, and stickleback populations is essential for assessing long-term impacts. The complexity of interacting climate, chemical, and biological drivers means that predicting the Baltic Sea’s future composition remains a major scientific challenge.

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