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 was widely acknowledged as a Very Bad Thing™, though in hindsight, we can’t really blame the ctenophore itself for what happened.
Looking back at its introduction and what happened after—fisheries collapse, general panic—it becomes clear that the saga of the sea walnut and the Black Sea is little more than a particularly pressing symptom of a much larger ailment.
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 from outer space. In the saga of the sea walnut, it traveled over from a more familiar location: America. Scientifically known as Mnemiopsis leidyi, the sea walnut (or warty comb jelly) is a member of the phylum Ctenophora, which doesn’t really roll off the tongue that pleasantly, so we usually refer to them as 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 do: they move themselves along by waving rows of tiny, hair-like cilia, which are also 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. Things are a little different in Europe and the Near East, where ctenophores do exist (they’re everywhere, basically), but sea walnuts are absent. Excuse me, were absent.
The rise and reign of the sea walnut
These ctenophores are one of the many intensely invasive species that have been 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 flora and fauna. By the 1980s, one area was experiencing a sharp increase in shipping traffic; it would soon be confronted with an unintended side effect.
Zebra mussel, European green crab, shipworm—and sea walnut. It all started back in 1982, when the species was first noticed in the Black Sea, the inland sea marginally connected to the Mediterranean and bordering Turkey, Ukraine, and surrounding countries.
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 places. Basically jelly soup. At the same time, pelagic fish stocks, especially anchovies, were plummeting spectacularly on account of being this ctenophore’s favorite food. Panic ensued: these blobs are eating all the baby fish, causing the fishing industry to collapse!
To add insult to injury, the sea walnut has no concept of our imagined geographical boundaries, and as such did not remain in the Black Sea. It quickly ended up in the nearby Sea of Azov, as well as the Caspian Sea (bordered by Georgia, Iran, Kazakhstan and surrounding countries). And… oops! 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 the 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 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 does have a taste for sea walnuts. Observing that these walnut scourges need to consume about 20% of their own body weight daily, scientists noted that M. leidyi populations didn’t bloom nearly as catastrophically as before they popped up.
Although the penetration and establishment of two non-native species in the Black Sea (and beyond) is probably not something we should be celebrating, it worked out in this case. Although the walnuts are still there, the population doesn’t reach the extreme densities and the local zooplankton appears less strongly affected.
From Iconographia Zoologica (Fauna und Flora des Golfes von Neapel und der angrenzenden Meeres-Abschnitte, 1880)
Guilty until proven innocent
When the sea walnut started taking the Black Sea and waters beyond by storm, scientists and the fisheries industries alike quickly started pointing 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… ecosystem collapse!
After things calmed down a little with the introduction of the heroic B. ovata, 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, the humans, who fished it relentlessly for both large and small fish species. 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, even more simply, by just eating them).
And that wasn’t all. Human activities like agriculture also led to eutrophication of the Black Sea, a process whereby increased nutrients in the water—such as from industrial fertilizers—cause an explosion in plankton. Sea walnuts eat zooplankton, meaning they could gorge themselves unchecked. In short, while the walnuts were the bullet, we can say pretty confidently that humans pulled the trigger.
Current state of affairs
As of right now, the sea walnut has firmly established itself in a bunch of places outside its natural range. However, as we concluded above, it’s not necessarily true that they’re causing massive damage to local ecosystems—their continued presence and spread are best viewed as symptoms of ongoing ecosystem-wide issues caused by human interference, rather than 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 healthier parts of the Mediterranean (of which there aren’t many, admittedly) and beyond. It’s a more urgent threat 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 this lagoon boasts a salinity level well above what scientists previously thought to be the sea walnut’s limit.
Conclusion: Walnut, I forgive you!
We can expect this little ctenophore to keep expanding its range, but it’s probably not the best idea to direct our efforts at getting rid of it directly. Instead, the focus should be on restoring ecosystems as a whole, such as by reducing overfishing and stopping industrial and agricultural waste from reaching the seas.
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., LuÄŤić, 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.