How a Comb Jelly Is Reshaping the Baltic Sea
In the delicate balance of the Baltic Sea, a silent invasion is unfolding, led by a deceptively beautiful creature with a voracious appetite.
Imagine a creature that is 95% water, has no brain, and resembles a transparent, gelatinous walnut. This is Mnemiopsis leidyi, a comb jelly that has traveled thousands of miles from its native waters along the Atlantic coast of North America to become one of the Baltic Sea's most successful and concerning invaders. Its arrival in Northern Europe in 2005 sparked fears of a complete ecosystem collapse, mirroring the damage it had previously inflicted in the Black Sea 1 3 .
This article delves into the fascinating ecology of this gelatinous plankton, exploring how it feeds, reproduces, and spreads, and examines the crucial scientific experiments that are helping us understand its impact on the fragile Baltic environment.
The gelatinous composition of Mnemiopsis leidyi
Despite its simplicity, it's an efficient predator
Traveled thousands of miles from native waters
Mnemiopsis leidyi is not a jellyfish, but a ctenophore, distinguished by the eight rows of beating, comb-like cilia it uses for movement. These cilia scatter light into shimmering rainbows, creating a beautiful display that belies the animal's efficiency as a predator.
Originating from the temperate and subtropical coasts of North and South America, this species is remarkably ecologically plastic, capable of thriving in a wide range of temperatures and salinities 1 . This inherent flexibility prepared it for success in the varied conditions of the Eurasian seas.
Historical Precedent: Its initial invasion into the Black Sea in the 1980s, likely via ballast water from ships, resulted in a catastrophic ecosystem shift 1 . Population explosions of M. leidyi led to the collapse of commercial fisheries, as the comb jelly consumed vast quantities of zooplankton, fish eggs, and larvae 4 .
Research has revealed that M. leidyi's distribution in the Baltic is anything but uniform. The population is heavily constrained by a key environmental factor: salinity 3 5 .
The Baltic Sea's salinity decreases from south to north, creating a powerful barrier to the comb jelly's expansion. The highest abundances are consistently found in the higher saline waters of the Kattegat and Skagerrak areas, where populations can be 60 times denser than in the central Baltic 3 .
A 2012 Ph.D. thesis confirmed that adults and larvae are consistently absent from the low-salinity northern Baltic, suggesting the central Baltic population is largely dependent on advection, or transport by currents, from the south 3 5 .
Southern populations can be 60x denser than central Baltic 3
| Factor | Impact on Mnemiopsis leidyi |
|---|---|
| Salinity | The primary limiting factor; higher abundances in southern, higher-salinity regions (Kattegat/Skagerrak) 3 |
| Temperature | Population blooms are associated with warmer temperatures, with highest abundances typically observed in October 1 3 |
| Food Availability | Can reproduce efficiently even at low food concentrations, allowing for population booms even when prey is limited 3 |
| Currents (Advection) | Essential for maintaining populations in the central Baltic, which may rely on larvae and adults transported from southern source regions 3 |
This distribution has led scientists to propose the existence of two distinct eco-types in Eurasia: a "southern" type (adapted to the Black, Caspian, and Mediterranean seas) and a "northern" type (found in the Baltic and North seas), each with ecology specific to their new environment 1 .
One of the key secrets to M. leidyi's invasive success is its extraordinary reproductive strategy.
Most remarkably, it can produce thousands of eggs daily, even when food is scarce 3 .
This ability to achieve maximum reproduction rates at low food concentrations helps explain how the species can reach such high densities in localized areas of the Baltic, despite limited resources 3 .
This fecundity is critically regulated by salinity; reproduction rates drop drastically in the low-salinity conditions of the central Baltic, which acts as a natural check on its population growth 3 5 .
To assess the true threat M. leidyi poses to Baltic fisheries, scientists have conducted meticulous laboratory experiments. One crucial line of investigation focuses on its predation on the early life stages of commercially important fish, like herring.
A 2024 study designed feeding experiments to investigate several key questions about the predator-prey relationship between M. leidyi and herring larvae 4 .
M. leidyi individuals of various sizes were collected from Kiel Fjord.
Yolk-sac herring larvae were hatched from fertilized eggs of wild-caught herring. The larvae were grouped by age (0-1 days old and 3-4 days old).
Individual M. leidyi were placed in bottles with seawater at a Baltic-relevant salinity of 16. They were exposed to different prey scenarios.
The experiments ran for 22-24 hours in the dark. Afterward, the water was analyzed to count the number of remaining prey.
The findings provided clear, and somewhat alarming, answers:
All tested sizes of M. leidyi were able to capture and digest herring yolk-sac larvae. In 88% of the trials, at least one larva was ingested 4 .
Younger herring larvae (0-1 days old) were significantly more vulnerable than older larvae (3-4 days old) 4 .
The presence of copepods significantly reduced the predation on herring larvae. When alternative prey was available, the clearance rate for herring larvae dropped by nearly half 4 .
| Experimental Variable | Core Finding | Ecological Implication |
|---|---|---|
| Predator Size | All sizes of M. leidyi consumed herring larvae. | Even young comb jellies can pose a threat to the earliest life stages of fish. |
| Larval Age | 0-1 day old larvae were consumed at higher rates than 3-4 day old larvae. | Newly hatched larvae are most vulnerable, likely due to poorer escape abilities. |
| Alternative Prey | Clearance rates on herring dropped by ~47% when copepods were present. | M. leidyi is an opportunistic feeder; high zooplankton abundance may protect fish larvae. |
This experiment demonstrated that while herring larvae are indeed part of M. leidyi's diet, the overall impact is complex and depends on the context of the wider ecosystem, including the availability of other food sources 4 .
The story of Mnemiopsis leidyi in the Baltic Sea is one of both invasion success and ecological limitation. Unlike in the Black Sea, the unique low-salinity environment of the central and northern Baltic acts as a powerful brake on its expansion 3 5 . While the comb jelly is a capable predator of fish larvae, its impact appears to be context-dependent, influenced by the availability of alternative prey and the specific conditions of its habitat 3 4 .
The ongoing scientific research, combining field monitoring with careful experiments, is vital. It allows us to move from fear to understanding. By unraveling the complex interactions of feeding, distribution, and reproduction, scientists can provide policymakers with the evidence needed to manage this invasive species and protect the fragile, economically vital ecosystem of the Baltic Sea.
References would be listed here in the final version of the article.