Freshwater Jellyfish in Europe: A Hidden Invader

Most people assume jellyfish belong in the sea: creatures of salt water, a hazard at the beach and something holidaymakers might encounter while wading. They are not usually imagined drifting through a lake.

New findings from a survey across 17 European countries suggest that assumption is far from reliable.

Since 1880, a jellyfish has been spreading through the continent’s lakes, reservoirs and ponds. Yet it appears to have attracted remarkably little public attention.

The most common freshwater jellyfish

The species is Craspedacusta sowerbii, the most widely distributed freshwater jellyfish in the world.

Probably native to China’s Yangtze River basin, it was first identified beyond that range in 1880, in an ornamental pond in London.

The species has since reached six continents. Its tiny polyp and dormant life stages can travel on aquatic plants, the legs of waterbirds and submerged objects transferred between bodies of water.

European records now extend from Spain and Italy to Finland and Russia, with populations confirmed in lakes, reservoirs and ornamental ponds.

For confirmed sightings of Craspedacusta sowerbii, Europe is second worldwide only to North America.

What the freshwater jellyfish survey revealed

A team led by Dr Guillaume Marchessaux at Aix-Marseille University (AMU) set out to establish just how little attention the animal received.

The researchers conducted a multilingual online survey over 22 months in 17 European countries.

Its 1,388 responses exposed a striking lack of awareness: more than 80% of participants could not name the species.

Just 33% recognised it as non-native, while nearly half – 49% – reported seeing it in the sea.

Marine jellyfish dominate the news

That final figure surprised the researchers. Craspedacusta sowerbii lives exclusively in freshwater, not in oceans, estuaries or brackish water. Even so, half of those surveyed confidently associated it with the beach.

The team puts the confusion down to media bias. Photographs in the news and nature documentaries are dominated by marine jellyfish, whereas freshwater species receive almost no coverage.

As a result, people tend to think of salt water whenever jellyfish are mentioned.

This misunderstanding lies at the heart of the paper. Marchessaux’s team calls C. sowerbii a cryptic invader.

Although established for over a century and now found in dozens of countries, it remains largely unnoticed by members of the public who share its waters.

Unsettling jellyfish blooms

Those who had encountered the jellyfish often admired it. Respondents described it as beautiful (21%), sublime (18%) and delicate (18%) – terms not usually associated with an invasive species.

Their response changed when large numbers appeared. During warm summer weeks, C. sowerbii can suddenly form blooms numbering in the thousands.

An entire pond can fill with translucent jellyfish the size of coins. More than half of respondents said seeing this made them nervous.

Mostly a harmless sting

C. sowerbii is not adapted to injure people. Among the 8% of respondents who said they had been stung, 56% experienced no sensation at all. Severe pain was effectively absent from the results.

Public reactions therefore occupy an unusual middle ground.

People find the jellyfish captivating to look at, although large numbers can cause unease. Even so, the species is largely harmless to humans.

“The species is perceived as an aesthetic curiosity rather than a direct physical threat,” the authors write.

Personal experience and public attitudes

The team built a statistical model to identify what shapes public attitudes. The researchers expected demographics – age, gender, education and scientific background – to be the main influences, but none dominated.

Instead, a simple factor predicted whether someone believed the species warranted official attention: whether they had seen one.

Seeing the animal directly, rather than reading a textbook chapter, determined how seriously a person regarded it.

“In the case of a little-publicized species, personal experience overrules scientific information in shaping public opinion,” the researchers noted.

That finding has practical implications for the reporting of aquatic invaders.

An open ecology question

It remains uncertain whether C. sowerbii harms ecosystems. Previous research found that jellyfish blooms can reduce zooplankton through predation; these tiny crustaceans underpin freshwater food webs.

Other effects may also exist but have not yet been fully explained.

Larger-scale experiments have indicated that even relatively low jellyfish densities can produce a sharp increase in algae as zooplankton numbers decline.

Such a reshaping of the food web can quietly reset a lake.

Broader implications of the study

The researchers argue that European invasive species policies currently overlook subtle invaders of this kind.

Charismatic species such as Asian hornets, signal crayfish and zebra mussels attract press releases and action plans. Less conspicuous species do not.

The survey is intended to help close that gap. The team wants management agencies and conservation groups to make greater use of citizen science platforms, where ordinary swimmers and anglers can record a sighting in seconds.

Warming waters are likely to make that need more urgent. An analysis of climate scenarios predicts that rising temperatures will lead to more frequent jellyfish blooms, and therefore more sightings in more places over the coming decades.

Before this paper, nobody had measured how little the European public understood the species. The answer was: very little.

The survey points to a way of narrowing that gap: direct sightings recorded in real time by ordinary observers.