A species that resembles a jellyfish but, unlike a jellyfish, has no stinging cells reached Europe in the 1980s. As W.M. Graham and K.M. Bayha point out in the bookBiological Invasions by Marine Jellyfish (source in Polish), it was carried there by ships calling at Black Sea ports from the Atlantic coasts of North and South America. It soon became one of the dominant organisms in those waters.
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In 1988, at the height of the invasion, the density of Mnemiopsis leidyi reached as many as 400 individuals per cubic metre. Shortly afterwards, however, as it consumed huge quantities of zooplankton, its main source of food, the comb jelly’s population began to decline.
In the Baltic Sea this species has not yet reached such high numbers, but the latest research indicates that its spread is accelerating. One of the factors that may be aiding its expansion is the growth of oxygen-depleted zones, also referred to as oxygen deserts.
“Sea cannibal”
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 serious threat to fisheries. Species such as sprats and sardines are particularly at risk, because the comb jelly feeds on their eggs and larvae, thereby reducing the size of future fish stocks.
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.
What is more, this species has developed a unique survival strategy. When food resources begin to run out, the comb jelly is able to eat its own larvae, and adult individuals produce large numbers of eggs which, in periods of food shortage, can serve as an additional source of energy. This specific form of cannibalism allows it to survive even under unfavourable conditions.
It thrives best, however, in shallow, warm, nutrient-rich coastal waters.
In new ecosystems the comb jelly often encounters no natural enemies that could effectively keep its numbers in check. In the Baltic the situation is somewhat different: there is a species that can prey on the comb jelly’s larvae. The problem is that ongoing environmental change is weakening this natural control mechanism.
Natural enemies
In January 2026 the journal Journal of Plankton Research (source in Polish)published the resuts of a study indicating that the three-spined stickleback (Gasterosteus aculeatus), native to the Baltic Sea, may act as a natural enemy of the comb jelly because it feeds on its larvae.
The latest observations, however, show that increasing hypoxia – a drop in the oxygen content of the water that leads to the formation of so-called dead zones – is limiting the activity of sticklebacks and reducing the extent of their feeding on comb jellylarvae.
This means that the invasive invertebrate’s success in expanding its range depends not only on rising water temperatures linked to climate change, but also on the progressive loss of habitats suitable for its natural enemies.
Paradoxically, climate change is also contributing to a fall in the salinity of the Baltic Sea. This is not unequivocally beneficial for Mnemiopsis leidyi, because the species thrives best in waters with slightly higher salinity. The complexity of the processes taking place in the Baltic means that predicting future changes remains a major challenge for scientists.
An ecosystem under pressure: the Baltic in transition
The appearance of the “sea cannibal” is just one of many signals indicating that the Baltic Sea ecosystem is under severe pressure. In recent years scientists have observed, among other things, the growth of oxygen deserts, record fluctuations in water levels and increasingly frequent phenomena associated with climate change.
These processes are affecting living conditions for native species and may facilitate the expansion of alien organisms that are better adapted to the new environmental conditions.
However, not every alien species necessarily poses a threat to the Baltic ecosystem. Some of them gradually become part of the local web of interactions, and some may even increase biological diversity.
One example is the American crab, which has become an important source of food for cod, flounder and waterbirds, and the sand-burrowing clam małgiew piaskołaz – a species that buries itself deep in bottom sediments, where it helps to loosen them and improve their oxygenation.
The Baltic Sea therefore remains a dynamic and sensitive ecosystem in which natural processes are increasingly intertwined with the effects of human activity and global climate change.
