In the 1980s, the Black Sea was overrun by non-native comb jellyfish in a storming that put even the most successful invasive species until that point to shame. This has been widely acknowledged as a Very Bad Thing™, though we can’t really blame the ctenophore itself for what happened.
Here’s how, thanks to human activity and an ocean away from its natural stomping grounds, one of the most ancient animal lineages on Earth got its second wind. Did we learn anything at all from its brief Eastern European reign?
Walnuts in the sea?
In the ‘58 sci-fi “The Blob”, the eponymous slime creature came to Earth from outer space. The protagonist of the sea walnut saga traveled over from a more familiar location: America.
Scientifically known as Mnemiopsis leidyi, the sea walnut is a member of the phylum Ctenophora, commonly known as the sea gooseberries or comb jellies. The “jelly” denomination refers to ctenophores’ texture, not the fact that they’re actual jellyfish — they’re not. They don’t sting, nor do they pulse like their spicy cousins, instead moving themselves along by waving rows of tiny cilia. These hair-like structures also happen to be responsible for their eye-catching iridescence.
On the east coast of the Americas, sea walnuts are a part of life. Areas like Chesapeake Bay regularly become overrun when blooms occur, turning the local waters into a gelatinous soup until the weather gets too cold. Things are a little different in Europe and the Near East. Ctenophores do exist here (they’re everywhere, really), but sea walnuts are absent. Excuse me, were absent.
The rise and reign of the sea walnut
Many highly invasive organisms have spread outside their natural range as a result of human activity. The ballast water taken in by oceanic cargo ships at their port of origin and dumped in their port of arrival is one of the most obvious migrant routes for non-native marine flora and fauna. Well-known invaders include the zebra mussel, European green crab, shipworm, and, indeed, the sea walnut.
The Black Sea, the inland sea marginally connected to the Mediterranean and bordering Turkey, Ukraine, and surrounding countries, experienced a sharp increase in shipping traffic in the 1980s. That’s a lot of ballast water being brought in, and unintended side effects were soon to follow. Sea walnuts were being observed in these waters as early as 1982.
By the late ‘80s, the sea walnut population had reached an extremely alarming density, with hundreds of walnuts per cubic meter of seawater in some areas. Pelagic fish stocks, especially anchovies, plummeted simultaneously on account of being this ctenophore’s favorite food. Panic ensued: blobs are eating the baby fish that keep the fisheries’ industry going!
To add insult to injury, the sea walnut has no concept of our imagined geographical boundaries, and as such quickly ventured beyond the Black Sea. It soon ended up in the nearby Sea of Azov, as well as the Caspian Sea (bordered by Georgia, Iran, Kazakhstan and surrounding countries). And… whoops! There it was in the northern Mediterranean, North Sea, and Baltic Sea as well.
Predator turned prey
No sooner was the sea walnut unleashed upon Europe and the Near East, and scientists were scrambling to figure out a way to get rid of it. Think of the anchovies! But before they could do much, the unlikely solution arrived in another load of ballast water: Beroe ovata, which appeared around 1997.
B. ovata is a comb jelly native to the Southern Atlantic and Mediterranean. It mainly feeds on other comb jellies, and it’s ravenous for sea walnuts. Noting that these walnut scourges need to consume about 20% of their own body weight daily, scientists observed that M. leidyi populations didn’t bloom nearly as catastrophically as before they appeared.
Although the penetration and establishment of two non-native species is probably not something we should be celebrating, it worked out in this case. The walnuts are still there, but the population no longer reaches extreme densities and the local zooplankton appears less affected. The ctenophores have been prevented from bringing the entire ecosystem to its knees… or have they?
From Iconographia Zoologica (Fauna und Flora des Golfes von Neapel und der angrenzenden Meeres-Abschnitte, 1880)
Guilty until proven innocent
As soon as the sea walnut began taking the Black Sea and the waters beyond by storm, scientists and the fisheries’ industry alike pointed the finger at this comb jelly as the cause of the collapse of the local zooplankton — and as such, fish, and as such, larger predators like dolphins… resulting in ecosystem collapse.
After the introduction of the heroic B. ovata calmed the situation somewhat, however, experts began to scrutinize these initial conclusions more closely. It’s true that the Black Sea proved to be an ideal habitat for sea walnuts (not too salty, few predators present at the time, and with mild winters), and scientists say it’s no surprise that it did so well. But could we be the real instigators of the mayhem we so conveniently blamed on the blobs?
The structure of the zooplankton and general ecosystems in the Black Sea had begun to change as far back as the 1960s. And that was definitely on us humans, who fished it relentlessly for anything that looked even remotely like a fish. In fact, according to a 2002 study, if these fish had still been present, they likely would have been able to keep the sea walnut population under control by competing with them for food (and by simply eating them).
Human activities like agriculture also resulted in the eutrophication of the Black Sea, a process whereby a surge in nutrients — such as from industrial fertilizer — cause a plankton explosion. Sea walnuts eat zooplankton, meaning they were able to gorge themselves unchecked. In short, while the walnuts were the bullet, we can state pretty confidently that humans pulled the trigger.
Current state of affairs
The sea walnut has firmly established itself in various seas far outside its natural range, but can’t be held responsible for directly causing damage to local ecosystems. The walnuts’ continued presence and spread are best viewed as symptoms of ongoing ecosystem-wide challenges associated with general human interference, not as direct causes of them.
This idea is strengthened by the fact that the sea walnut hasn’t appeared to be able to gain quite as much of a foothold in the few healthier parts of the Mediterranean. It’s a more urgent threat mostly in ecosystems already destabilized by human activity, such as the Adriatic Sea. A 2009 study also found a huge bloom in the Spanish Mar Menor—just one of many blobfests in the general area — which was surprising given that this lagoon boasts a salinity level well above what scientists previously thought to be the sea walnut’s limit.
Walnut, I forgive you!
We can expect this little ctenophore to keep expanding its range. Directing our efforts at getting rid of it directly, however, would more than likely be a waste of energy. Instead, the focus should be on restoring ecosystems as a whole: reducing overfishing and stopping industrial and agricultural waste from reaching the seas remain the top priorities.
Sources & further reading
Finenko, G. A., Romanova, Z. A., Abolmasova, G. I., Anninsky, B. E., Svetlichny, L. S., Hubareva, E. S., … & Kideys, A. E. (2003). Population dynamics, ingestion, growth and reproduction rates of the invader Beroe ovata and its impact on plankton community in Sevastopol Bay, the Black Sea. Journal of Plankton research, 25(5), 539-549.
Gucu, A. C. (2002). Can overfishing be responsible for the successful establishment of Mnemiopsis leidyi in the Black Sea?. Estuarine, Coastal and Shelf Science, 54(3), 439-451.
Malej, A., Tirelli, V., Lučić, D., Paliaga, P., Vodopivec, M., Goruppi, A., … & Shiganova, T. (2017). Mnemiopsis leidyi in the northern Adriatic: here to stay?. Journal of sea research, 124, 10-16.
Shiganova, T., Mirzoyan, Z., Studenikina, E., Volovik, S., Siokou-Frangou, I., Zervoudaki, S., … & Dumont, H. (2001). Population development of the invader ctenophore Mnemiopsis leidyi, in the Black Sea and in other seas of the Mediterranean basin. Marine biology, 139, 431-445.