Meet the Sea Walnut — A Cannibalistic Jelly Currently Invading Venice’s Lagoon
Don’t be fooled by this harmless and cute jelly floating in the waters of the Venice Lagoon. Comb jellies exhibit strange behaviors and characteristics, such as cannibalism and lack of a permanent anus.
Recently, this small, slimy sea creature, native to the east coast of North and South America, has moved into a new habitat, spreading at a worrying rate and wreaking havoc on Italy’s local marine ecosystem.
A new study published in Science of estuaries, coasts and continental shelf Italian researchers explored the causes of the comb jelly’s expansion in the Mediterranean lagoon and found that it was perfectly adapted to the temperature and salinity of the lagoon water. Researchers say climate change will only worsen the population boom, calling for immediate action to mitigate the spread of comb jelly.
Comb frosts can threaten ecosystems
Comb jelly (Mnemiopsis leidyi), also known as warty jelly or sea walnut, has a transparent body measuring up to 3 to 5 inches and belongs to a group of stingless gelatinous animals called ctenophores, according to the University of Chicago Marine Biological Laboratory. Instead of stinging, they use colloblasts, microscopic sticky cells on their tentacles, to capture their prey.
Mr. Leidyi is famous for its stunning bioluminescence and glides gracefully through water using eight rows of cilia, microscopic hair-like paddles that beat rhythmically to cause movement. In fact, seanuts are the largest animals known to use their cilia for movement.
Interestingly, Mr. Leidyi is the only animal observed without a permanent anus. Its backend only germinates when it needs to defecate.
Seanuts have a varied diet of crustaceans, fish eggs and larvae, which can make them a major destabilizer when introduced to new ecosystems.
Learn more: Brainless jellyfish need rest like humans and reveal the evolutionary purpose of sleep
Why the Venice Lagoon is ideal for encroaching jelly
The impact of sea walnut on marine environments has already been studied in several regions, but its behavior in Mediterranean lagoons remains underexplored. The Italian research team observed Mr. Leidyi and studied it in the laboratory for two years, discovering that the invader follows a seasonal pattern in which populations flower in late spring and again in late summer.
They demonstrated that jellies can withstand wide temperature changes, ranging from 50 to 90 degrees Fahrenheit, and salt concentrations between 10 and 34 parts per thousand, making the warm and changing salinity levels of the Venice Lagoon an optimal habitat.
Overall, the results suggest that ongoing climate change will further shift lagoon conditions in favor of sea walnut, leading to larger populations and even greater impacts on local marine species that prey on them. In their study, the researchers encouraged specialized monitoring for invasive species in the lagoon, hoping that this would reduce problems seen in the local fishing industry as well as the broader ecosystem.
Seanuts may resort to cannibalism
According to the Smithsonian, the sea walnut’s invasive history began in 1982, when Mr. Leidyi was accidentally introduced into the Black Sea by ship ballast water and quickly spread to the Caspian, Mediterranean, Baltic and Northern Seas. There, sea walnut has contributed to serious fishing accidents as populations have grown exponentially without local predators to control them.
In its natural habitat along the US East Coast, other ctenophores, jellyfish and fish feed on Mr. Leidyithereby helping to better control population sizes, as described in the Global Invasive Species Database.
Another survival strategy, common among jellyfish and other gelatinous zooplankton, may also play a role in the sea walnut’s survival toolbox. According to a 2020 study in Nature Communications Biology, Mr. Leidyi in the western Baltic Sea appear to maintain energy reserves in nutrient-poor habitats by cannibalizing their own larvae.
The sea walnut proves that nothing in nature is black or white: species can fascinate us with their beauty and original way of life, while their invasive behavior can still threaten valuable ecosystems.
Learn more: Rare giant ghost jellyfish and well-preserved whale skeleton spotted off the coast of Argentina
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