The Sea Walnut (Mnemiopsis leidyi) as an Invasive Species

Another example of a species that has been transported via ship ballast water is the Mnemiopsis leidyi. This small jellyfish is native to the Atlantic coast of North and South America and have been observed to grow up to 160-180 mm in the Black Sea and 20-30 mm in the Azov and Caspian Seas. This creature can withstand a wide range of different environments so it can establish itself in many different locations in the world.

The Sea Walnut (Mnemiopsis leydyi) was first introduced to the Black Sea from the American Atlantic coast in the 1980s. It feeds on zooplankton and eggs and larvae of fish that are native to the environment. Like a lot of species that become invasive, it has no natural predators in this new home which makes it a very easy place for this jelly to thrive. During the 1990s it was reported that this species made up roughly 90% of the biomass in the Black Sea which is more than the world’s annual fish catch. It spread south the Sea of Marmara and the eastern Mediterranean. Within just a few years of its introduction it was already causing high stress to local fisheries and in the neighboring Azov Sea, they have caused anchovy fisheries to completely collapse. By 2005 it was found in the Adriatic sea and first recorded in 2006 in the Baltic and the North Sea. Because the fish that live here are being outcompeted the numbers have declined which directly contributes to the lower numbers of larger organisms such as dolphins. There are so many that they have even lowered the overall oxygen in the Black Sea and over time they will most likely destroy the entire ecosystem. They are spreading rapidly and will most likely wreak the same havoc wherever they go.

Marine invasive species have been recognized as a major national concern by the United Nations Conference on Environment and Development (UNCED).  It takes time to effectively deal with the issue of invasive species and requires collaboration between governments and non-government organizations, economic sectors, and international treaty organizations. Most invasions are irreversible but the more the world is aware and the more everyone collaborates to attack the issue the better chances there are of preventing it in the future.



Ballast Water Management. (2017). [online] Available at: [Accessed 22 Feb. 2017].

Mnemiopsis leidyi (sea walnut). (2017). [online] Available at: [Accessed 22 Feb. 2017].

Shipping problems: Alien invaders. (2017). [online] Available at: [Accessed 22 Feb. 2017].





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