An invasive invertebrate known as Mnemiopsis leidyi, which resembles a jellyfish but lacks stinging cells, has become a significant concern for the Baltic Sea ecosystem. First introduced to European waters in the 1980s via ships traveling from the Atlantic coasts of North and South America to Black Sea ports, the species quickly established itself as a dominant organism. At the height of its initial invasion in 1988, population density reached 400 individuals per cubic metre in some areas, though numbers later declined after the species exhausted its primary food source, zooplankton.
This remarkable invertebrate, which – as described, among other places, in an article published in Tutoring Gedanensis (source in Polish) – is capable of bioluminescence, poses a direct threat to regional fisheries. By consuming the eggs and larvae of species such as sprats and sardines, it reduces potential future fish stocks. Furthermore, studies have also shown that Mnemiopsis leidyi consumes vast amounts of zooplankton, competing for food with many other marine organisms and disrupting the ecosystem's natural balance. The species thrives particularly well in shallow, warm, and nutrient-rich coastal waters.
Scientists have noted a unique survival strategy that has earned the organism the moniker of "sea cannibal." When food becomes scarce, the comb jelly is capable of consuming its own larvae. Additionally, adults can produce large quantities of eggs that serve as an alternative energy source during periods of nutritional stress, allowing the species to persist under unfavorable conditions. In new environments, the comb jelly often faces an absence of natural predators to regulate its population.
The situation in the Baltic Sea is distinct because the native three-spined stickleback (Gasterosteus aculeatus) has been identified as a predator that consumes comb jelly larvae, according to a January 2026 study published in the Journal of Plankton Research. However, this natural control mechanism is currently being compromised by ongoing environmental shifts. The expansion of the invasive species is driven by a combination of rising water temperatures associated with climate change and the loss of habitats essential for its natural enemies.
Climate change is also influencing the salinity of the Baltic Sea, which is currently falling. Because Mnemiopsis leidyi prefers environments with higher salinity, the impact of these changes remains complex and difficult for researchers to predict. The presence of this "sea cannibal" is one of several indicators that the Baltic ecosystem is under severe pressure, alongside phenomena such as the growth of oxygen deserts and erratic fluctuations in water levels.
While these environmental changes may facilitate the spread of alien organisms better adapted to new conditions, not all non-native species are detrimental. Some, like the American crab, have integrated into the local food web as a food source for cod, flounder, and waterbirds. Similarly, the sand-burrowing clam has contributed to the ecosystem by loosening bottom sediments and improving oxygenation. Ultimately, the Baltic Sea remains a dynamic and sensitive environment where natural processes are increasingly shaped by the intersection of human activity and global climate change.
