Invasive species part 2:The comb jelly Mnemiopsis leydyi

The small but spectacularly beautiful comb jelly Mnemiopsis leydyi, can reproduce at a dismaying speed, and copes with warm and cold water and even different amounts of salinity. And when it spreads to areas where it isn’t naturally found, it can cause devastating damage. As in the Black Sea in the 1980s.

Today, species mainly spread by stowing away in the ballast water on cargo ships. Huge amounts of ballast water are involved, up to four billion tonnes a year, transported between the ports of the world. 

Ships that aren’t fully loaded need stabilising. They used to be stabilised with stones or sand but these days the tanks are filled with water. This ballast water is pumped in after unloading and then emptied out again in the next port when new goods are loaded. 

But the water comes with stowaways in the form of animals, plants, eggs, larvae and micro-organisms. Most of them die in the dark, often toxic space, while others die as soon as they reach the new environment. Some, however, survive and some of these become what are termed invasive. In other words, they cause a huge amount of damage in the ecosystems where they spread.

There is no really good system for stopping invasive species. This means that the problems are growing at an alarming speed. Currently, a new species establishes itself somewhere in the world every two months. 

So what happened in the Black Sea?

Mnemiopsis arrived there in the early 1980s, presumably carried in the ballast water of a ship.

Once established in the Black Sea, it started eating vast numbers of fish eggs, larvae and plankton, rapidly out-competing other fish. It got so bad that sprat and anchovy fishing fell by 90 percent and was on the verge of total collapse.

Today the fishing has recovered, mainly due to the appearance of another alien comb jelly. Mnemiopsis maneterna featured in its diet and eventually the ecosystem attained a new equilibrium. 

However, introducing different species to get rid of invasive species isn’t generally a simple equation, basically because the consequences are too difficult to control. It can work well, as it did in the Black Sea, but it can also go very badly wrong. 

Reportage: Lena Scherman
Photo: Tobias Dahlin

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