In 1982, an unwelcome visitor arrived in the Black Sea. An oil tanker traveling from the American Atlantic dumped its ballast water into the sea, releasing a flood of warty comb jelly stowaways along with it. The alien invaders have since ravaged the area’s native fish populations, and now New Scientist reports that they’ve become a threat along the northern Adriatic coast.

Warty comb jellies (Mnemiopsis leidyi) have been seen in the Adriatic sea since 2005, but this summer marked the first time they’ve been present in such great numbers. Davor Lučić of the Institute for Marine and Coastal Research in Dubrovnik, Croatia told New Scientist that the clusters get as dense as 500 jellies per square meter in some spots. That estimate is based on fully-matured specimens—the number of juveniles is likely even higher.

The swarms have been documented along the Adriatic coast from Slovenia to Pesaro, Italy. Lagoons in northern Italy have been clogged with the creatures since July. The animals pose no direct threat to people, but their appetite has already proven disastrous to whatever ecosystem they invade.

A few of the warty comb jelly's meals of choice include fish eggs, fish larvae, and zooplankton. Zooplankton also happens to be the main food source for many commercial fish in the area. Less than a decade after the introduction of the comb jelly into the Black Sea, local anchovy and sardine fisheries were devastated. The seafood industry had lost billions by the mid-'90s.

Now, there’s threat of a repeat catastrophe in the Adriatic Sea. The mass emergence of the species coincides with anchovy spawning season, a crucial time for one of the sea’s most commercially significant fish. Some scientists are looking on the bright side: the Adriatic is more open and less polluted than the Black Sea, and its local fauna is more diverse. This makes the native populations better equipped to survive the invasion. Additionally, Mnemiopsis leidyi isn’t the sea’s only uninvited guest. Another comb jelly, Beroe ctenophore, has also invaded the waters, and scientists hope the aliens might contribute to one another’s demise.

Mnemiopsis likely entered the Adriatic through a ship’s ballast, the same Trojan horse it rode into the Black Sea. This problem isn’t limited to invasive jellies: A list of destructive species from ants to mussels have been introduced to new environments this way. A global treaty that aims to put an end to the issue will go into effect next year.

[h/t New Scientist]

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