Hordes of cannibalistic jellyfish are spreading through Venice’s famed lagoon and wreaking havoc on local fisheries. Usually found in the western Atlantic, the warty comb jellyfish, also known as the sea walnut, is now thriving in the Adriatic Sea.
Despite its name and appearance, Mnemiopsis leidyi is not a true jellyfish but a comb jelly, which is in a different phylum than true jellyfish. Among other differences, the gelatinous, translucent creatures do not have stingers. The name “comb jellies” comes from their method of moving through the water using rows of hair-like structures called ciliary combs.
Scientists believe the species came to the Adriatic accidentally in the ballast water of cargo ships. A sometime cannibal, it will eat its own young when food is scarce. The bizarre creature has a temporary anus that appears only when it needs to expel waste, then vanishes again. Unfortunately, it is also one of the world’s top invasive species and is causing chaos for the Italian city’s fragile lagoon ecosystem.
The Sea Walnut. Photo: Shutterstock
Warm lagoon, happy jellyfish
The lagoon is almost perfectly suited to its needs, allowing the sea walnuts population to explode at an alarming rate. A recent two-year study found that it can thrive in a wide range of temperatures and salinity levels. The species blooms from early spring through to early autumn, when warmer waters and stable salt levels allow it to reproduce rapidly. Researchers note that climate change is fueling the jellyfish’s success in the lagoon and that the situation is likely to worsen.
The sea walnut may sound harmless, or even cute, but that is far from the truth. The species devours plankton, fish eggs, and fish larvae, stripping the lagoon of the foundations of its food web. For local fishers, the invasion is already having serious consequences. The gelatinous creatures are clogging up fishing nets, making them heavier, harder to haul in, and less effective. At the same time, fish stocks are crashing because the comb jellies are demolishing most of their fish eggs and larvae.
