An invasive marine organism that glows in the dark and practices cannibalism is spreading more aggressively through the Baltic Sea, raising concerns among fisheries scientists and ecologists. The comb jelly Mnemiopsis leidyi, often described as a sea cannibal, first reached European waters in the 1980s via ship ballast water discharged into the Black Sea from the Atlantic coasts of North and South America, as documented by researchers W.M. Graham and K.M. Bayha.
By 1988, the species had reached densities of up to 400 individuals per cubic metre in the Black Sea, overwhelming the local ecosystem. Its population later declined after it consumed vast quantities of zooplankton, its primary food source. In the Baltic, numbers have not yet reached such extremes, but recent studies indicate that the invasion is accelerating, driven in part by the growth of oxygen-depleted zones, also known as oxygen deserts.
A threat to commercial fish stocks
The comb jelly poses a direct threat to fisheries because it feeds on the eggs and larvae of species such as sprats and sardines, reducing future fish stocks. It also competes with native organisms for zooplankton, disrupting the natural balance of the food web. The species has developed a remarkable survival strategy: when food becomes scarce, adult jellies consume their own larvae, and they produce large numbers of eggs that can serve as an emergency energy source. This cannibalistic behaviour allows the population to persist even under unfavourable conditions.
Mnemiopsis leidyi thrives best in shallow, warm, nutrient-rich coastal waters. In new ecosystems it often encounters no effective natural predators. In the Baltic, however, there is one native species that can prey on its larvae: the three-spined stickleback (Gasterosteus aculeatus). A study published in January 2026 in the Journal of Plankton Research identified the stickleback as a potential natural enemy of the comb jelly.
Yet the same research shows that increasing hypoxia — a drop in oxygen content that creates dead zones — is limiting stickleback activity and reducing their consumption of comb jelly larvae. This means the invasive species’ success 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 reducing the salinity of the Baltic Sea, which is not unequivocally beneficial for Mnemiopsis leidyi, as it prefers slightly saltier waters. The complexity of these interacting processes makes it difficult for scientists to predict future changes with certainty.
An ecosystem under pressure
The spread of the sea cannibal is just one symptom of a broader environmental crisis in the Baltic. Scientists have observed expanding oxygen deserts, record fluctuations in water levels, and increasingly frequent extreme weather events linked to climate change. These changes are altering living conditions for native species and may favour alien organisms better adapted to the new environment.
Not every non-native species is harmful. The American crab, for instance, has become an important food source for cod, flounder, and waterbirds. The sand-burrowing clam Mya arenaria helps loosen and oxygenate bottom sediments. But the comb jelly’s rapid expansion, combined with the weakening of natural controls, represents a serious challenge for the Baltic’s already stressed ecosystem.
As oxygen deserts continue to expand across the region, the balance between invasive success and native resilience will depend on how quickly European policymakers and scientists can address the underlying causes of eutrophication and climate change. For now, the sea cannibal is tightening its grip on one of Europe’s most vulnerable seas.
