Marinko Babić
By Nenad Jarić Dauenhauer
A voracious warty comb jelly that infamously devastated Black Sea fisheries is now thriving in the coastal areas of the northern Adriatic.
The arrival of these invasive animals in the Adriatic was first noted in 2005. But this year huge swarms have been spotted and photographed along the coast of northern Croatia, and the coasts of Slovenia and Italy, as far south as Pesaro. Since July the jellies have literally filled lagoons in northern Italy.
“This is the first time that this species has existed in such masses in the Adriatic,” says Valentina Tirelli at the National Institute of Oceanography and Geophysics in Trieste, Italy.
“At certain points, population densities were estimated to be up to 500 specimens per square metre,” says Davor Lučić at the Institute for Marine and Coastal Research in Dubrovnik, Croatia. “Estimates were made for the adults only, but we assume that there were significantly higher numbers of juveniles.”
Though not dangerous to humans, scientists are alarmed by the new boom in Mnemiopsis leidyi because it has already devastated fish stocks in the Black Sea. It is one of the best-documented alien invaders, arriving in oil tanker ballast water from the American Atlantic in 1982.
Rapid spread
With no natural enemies, the comb jelly spreads at an alarming rate, feeding on zooplankton, the food choice of commercially important fish, as well as their eggs and larvae.
By the mid-1990s it had cost the seafood industry billions of euros and spread to the neighbouring Sea of Azov. Now it has arrived in the Caspian and Baltic Seas, as well as the Adriatic.
Scientists say that Mnemiopsis probably reached the Adriatic via ballast waters – something a new international treaty that comes into force next year should hopefully prevent in the future. The ship traffic in the Adriatic is growing, and some of the largest ports, such as Trieste, Koper, and Rijeka, are all in the north.
“The 2005 introduction presumably originated from the Black Sea; there were direct connections between North Adriatic and the Black Sea ports,” says Alenka Malej at the National Institute of Biology in Piran, Slovenia. “This year a different lineage, better adapted to the Adriatic, could have been introduced.”
Though the situation is serious, scientists believe that the Adriatic invasion may not be as devastating as the Black Sea one.
The northern Adriatic is more open with a faster exchange of waters, and is less polluted and more diverse, making it more resilient to alien species.
But the comb jellies must be carefully monitored. Mnemiopsis is a voracious predator, it cuts the food supply for other predators, triggering changes in abundance, species diversity and ecosystem functioning all the way down to bacteria and viruses. Most importantly it appeared in large masses during the spawning season of anchovies, one of the most important commercial fish in the region.
The main natural enemy of Mnemiopsis is another comb jelly Beroe ctenophore, also an alien in the Adriatic, and there is some hope that one alien may now feed off another, more devastating one.
Read more: Jellymageddon: Can we stop the rise of the jellyfish?
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