Swimmers and small-scale fishers in Venice are confronting an unsettling new resident in the city’s famed lagoon: a translucent, walnut-sized invader that eats its own young.

Scientists say populations of the cannibalistic comb jelly Mnemiopsis leidyi have surged in the Venice Lagoon in recent years, raising alarms about the future of local fisheries and adding a fresh ecological worry for one of the world’s most visited coastal destinations.

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Cannibal Comb Jelly Takes Hold in Venice Lagoon

Italian and international researchers have confirmed that Mnemiopsis leidyi, sometimes called the warty comb jelly, has become firmly established in the Venice Lagoon after first being detected there more than a decade ago. Native to the western Atlantic, the species is believed to have reached European waters via ballast water discharged from commercial ships and has since colonized semi-enclosed seas from the Black Sea to the Adriatic.

New studies by the National Institute of Oceanography and Applied Geophysics and collaborating universities report seasonal population explosions of the comb jelly in the lagoon, especially in late spring and again from late summer into early autumn. Warmer water temperatures and stable salinity levels are providing ideal conditions for reproduction, allowing the animal’s numbers to climb rapidly during these windows.

Mnemiopsis leidyi is already infamous among marine scientists. In the late 1980s it triggered a regime shift in the Black Sea when an explosive bloom devastated European anchovy stocks and contributed to the collapse of local fisheries. Researchers warn that similar mechanisms could be at work in the Venice Lagoon as the species continues to expand.

A Predator of Fish Eggs, Larvae and Its Own Offspring

Unlike stinging jellyfish that belong to the cnidarian group, Mnemiopsis leidyi is a ctenophore, or comb jelly, a separate branch of gelatinous zooplankton. It lacks stinging cells but is nonetheless a highly efficient predator, feeding voraciously on zooplankton, fish eggs and larvae, and other small gelatinous organisms in the water column.

Laboratory and field observations have shown that the comb jelly will also consume its own eggs and larvae when food is scarce, a behavior scientists describe as cannibalism. This strategy may help stabilize the population when prey is limited, but it does little to reduce the pressure on the broader plankton community and early life stages of fish.

Because many of the Venice Lagoon’s key commercial species, including clams and crabs, have planktonic eggs or larvae, researchers say they are directly exposed to predation by Mnemiopsis leidyi. A recent interdisciplinary study on the lagoon’s artisanal fisheries concluded that the ctenophore has likely exerted a significant negative impact on fishery resources over the past decade, though it interacts with a suite of other environmental changes in the system.

Artisanal Fisheries Report Empty Nets and Rising Costs

For small-scale fishers working out of Chioggia, Pellestrina and other lagoon communities, the ecological story is manifesting as an economic crisis. Many describe hauling in nets heavy with gelatinous biomass but light on sellable fish and shellfish. Comb jellies not only compete with fish for food and eat their eggs, they also clog nets and gear, forcing crews to spend valuable time cleaning equipment instead of fishing.

Industry representatives say landings of some traditional lagoon species have trended downward for years, squeezed by a combination of warming waters, habitat changes, pollution and now invasive species. Fishers interviewed by Italian media describe Mnemiopsis leidyi as “everywhere” in certain seasons, adding that fuel and labor costs are rising as they travel farther or make more sets to achieve the same catch.

Scientists point out that isolating the exact share of blame carried by the comb jelly is challenging, because the Venice Lagoon has undergone extensive engineering works to protect the city from flooding, alongside broader climate and nutrient shifts in the northern Adriatic. Nonetheless, recent peer-reviewed research concludes that the invasive ctenophore is now a major stressor on an already fragile fishery system and should be explicitly considered in management plans.

Tourism and Public Perception in a Jellyfish Era

While Mnemiopsis leidyi does not sting humans, the words “cannibal jellyfish” have captured global headlines and could influence traveler perceptions of Venice as a seaside destination. The lagoon and adjacent beaches along the Lido and Cavallino-Treporti coast are key attractions for the millions of tourists who pour into the region every year, particularly during the peak summer season.

City authorities and tourism operators are keen to emphasize that the comb jelly poses no direct physical threat to swimmers. Its tiny, transparent body is delicate, and contact is harmless. Nonetheless, dense swarms of jelly-like organisms can be disconcerting for bathers, especially when photos and social media posts frame them as exotic or dangerous invaders.

Officials face a delicate communication task: acknowledging the seriousness of the ecological impact while avoiding alarmist messages that might deter visitors. At the same time, tourists are increasingly sensitive to visible signs of environmental stress in destinations they visit, from coral bleaching to plastic pollution. The presence of invasive gelatinous species is becoming another marker of change in Mediterranean holiday hotspots.

Part of a Wider Mediterranean Jellyfish Surge

The situation in Venice is unfolding against a broader Mediterranean backdrop in which jellyfish and comb jelly blooms are becoming more common and more intense. Coastal regions in Spain, France, Italy and Greece have all reported surges of Pelagia noctiluca, the mauve stinger jellyfish known for its painful sting and capacity to close beaches when it washes ashore in large numbers.

Recent summers have seen purple jellyfish outbreaks off Catalonia’s Costa Brava, the Gulf of Corinth and the Northern Sporades, prompting advisories and beach closures. Local biodiversity observatories and citizen science networks have documented waves of Pelagia noctiluca along the French Riviera and Corsican beaches, with concentrations that authorities describe as “invasions.”

Although Mnemiopsis leidyi and Pelagia noctiluca belong to different groups and behave differently, both benefit from similar environmental drivers. Warmer waters, fewer predators due to overfishing and altered nutrient patterns appear to be giving gelatinous organisms a competitive edge in many semi-enclosed basins and coastal zones.

Climate Change, Overfishing and a Shifting Lagoon

Marine ecologists stress that the success of Mnemiopsis leidyi in Venice is tightly linked to ongoing climate change. The northern Adriatic has warmed significantly in recent decades, with heatwaves now pushing summer sea surface temperatures close to or above 30 degrees Celsius in some years. These conditions not only favor comb jellies, they also stress traditional cold- and cool-water fish species that once dominated regional catches.

Overfishing further tilts the balance. Large predatory fish that would once have consumed substantial numbers of jellyfish and comb jellies have declined in many parts of the Mediterranean. With fewer natural checks on their populations, gelatinous organisms can capitalize on food resources that would otherwise support fish communities. International fisheries agencies have warned that such feedback loops may prevent depleted fish stocks from recovering and help lock ecosystems into jellyfish-dominated states.

In the Venice Lagoon, additional pressures come from human engineering and pollution. Major infrastructure projects aimed at protecting the city from extreme high tides have altered hydrodynamics, while historical nutrient reductions and sediment changes have reshaped habitats. Researchers say Mnemiopsis leidyi is thriving in this new configuration, taking advantage of calm, warm, relatively enclosed waters that suit its life cycle.

Monitoring, Mitigation and Possible Responses

Authorities and scientists are now grappling with what can realistically be done to curb the comb jelly’s impact. Eradication of Mnemiopsis leidyi from such a large, open coastal system is considered virtually impossible. Instead, efforts focus on close monitoring, better forecasting of bloom events and management measures designed to support the resilience of fish stocks and the broader lagoon ecosystem.

Research groups working in the Venice Lagoon have begun mapping seasonal patterns of the ctenophore, identifying temperature and salinity thresholds that seem to prefigure population spikes. This information can help fishers anticipate when and where they are most likely to encounter heavy comb jelly concentrations, potentially allowing them to adjust gear or timing to reduce economic losses.

Longer term, experts argue that addressing root causes will be key. That includes reducing greenhouse gas emissions to slow ocean warming, rebuilding fish stocks through sustainable harvest strategies, and designing coastal protection and dredging works with ecological impacts in mind. Some scientists suggest that supporting populations of natural gelatinous predators, where appropriate, might also help restore balance, though such interventions must be approached cautiously to avoid introducing new invasive risks.

What Visitors and Locals Should Know

For residents and tourists in Venice, the rise of Mnemiopsis leidyi is a reminder that even the most iconic destinations are enmeshed in global environmental change. Beachgoers who encounter small, shimmering comb jellies in the shallows should understand that these animals do not sting and are part of a complex web of ecological interactions rather than a direct hazard to human health.

Public information campaigns in the wider Mediterranean already explain how to recognize different jellyfish species, what to do in case of stings from species like the mauve stinger, and why blooms are occurring more often. In the case of the Venice Lagoon, outreach is beginning to focus more on the invisible economic and ecological effects of gelatinous invaders and the ways local communities are adapting.

Marine scientists emphasize that citizen reports can be valuable for tracking the spread and intensity of jellyfish and comb jelly blooms. Apps and online platforms allow users to log sightings, contributing to real-time maps that help both researchers and coastal managers respond more effectively to sudden changes in the water.

FAQ

Q1. What exactly is the “cannibal jellyfish” invading the Venice Lagoon?
It is Mnemiopsis leidyi, a comb jelly native to the western Atlantic that has become invasive in several enclosed seas, including the Black Sea and now the northern Adriatic. Despite the nickname, it is not a true jellyfish but a ctenophore, a different group of gelatinous marine animals.

Q2. Why is Mnemiopsis leidyi described as cannibalistic?
Scientists have observed that Mnemiopsis leidyi will consume its own eggs and larvae, especially when other food sources are scarce. This cannibalism appears to be a way of recycling energy within the population, but it does not prevent the species from reaching very high numbers during bloom periods.

Q3. Does the invasive comb jelly sting or pose a danger to swimmers?
No. Mnemiopsis leidyi does not have stinging cells and cannot sting humans. Contact with the animal is generally harmless, although its gelatinous texture can be unpleasant for some bathers. The primary concerns relate to its impact on marine ecosystems and fisheries, not to direct risks for people in the water.

Q4. How does this species threaten fish and shellfish in the Venice Lagoon?
The comb jelly feeds heavily on zooplankton, fish eggs and larvae, and the planktonic early stages of shellfish such as clams and crabs. By consuming these life stages and competing for the same food as young fish, Mnemiopsis leidyi can reduce recruitment to fish stocks and undermine the productivity of local fisheries.

Q5. What role do climate change and overfishing play in the invasion?
Warming sea temperatures in the northern Adriatic create more favorable conditions for Mnemiopsis leidyi to reproduce and survive, while overfishing has reduced populations of large fish that might help control gelatinous organisms. Together with local habitat alterations, these factors give the comb jelly a competitive advantage and make blooms more likely.

Q6. Are there any successful examples of controlling Mnemiopsis leidyi elsewhere?
In the Black Sea, the arrival of another comb jelly that preys on Mnemiopsis leidyi helped reduce its numbers and allowed some fish stocks to recover. However, deliberately introducing new species as biocontrol agents carries significant ecological risks, so most experts do not see this as a model to replicate in the Venice Lagoon.

Q7. How are Venetian fishers being affected on a day-to-day basis?
Fishers report catching large volumes of gelatinous biomass in their nets, which can damage gear and require long hours of cleaning. At the same time, catches of marketable species are often lower, forcing crews to work harder and spend more on fuel to maintain income. For many small-scale operators, these combined pressures are eroding already thin profit margins.

Q8. Could the jellyfish situation discourage tourism to Venice and nearby beaches?
Tourism officials are watching perceptions closely, but so far there is no sign of a broad decline in visitor numbers linked specifically to Mnemiopsis leidyi. Clear communication that the species does not sting, together with accurate information about water quality and beach conditions, is considered essential to prevent misunderstandings that might influence travel decisions.

Q9. What monitoring or early warning systems are in place?
Research institutes and local authorities are conducting regular surveys of the lagoon’s plankton community and measuring environmental parameters like temperature and salinity. Combined with citizen science reports and satellite data, these efforts aim to detect bloom conditions early so that fishers and coastal managers can prepare and, where possible, adapt their activities.

Q10. Is there anything ordinary visitors or residents can do about the problem?
Individuals cannot directly control jellyfish populations, but they can contribute by reporting unusual aggregations to local observatories, supporting sustainable seafood choices that help rebuild fish stocks, and backing climate and conservation policies that address the underlying drivers of change in the lagoon. Staying informed and sharing accurate information also helps prevent unwarranted fear and supports evidence-based management.