Many species have been transported or introduced into ecosystems in which there not native, however not all are considered invasive. An invasive species is an organism that causes environmental harm, harm to

Figure 1-
photo of Mnemiopsis leidyi, the Comb Jelly.
Photo taken by Vidar A from Gozo, Malt

human health or economic harm when in an ecosystem that it’s not indigenous. This species often find their way into new environment through the direct activity of human, either accidentally or deliberately. The comb jelly (Mnemiopsis leidyi), which is originally found in Atlantic waters of the coast of America. However, in the 1980’s it was found in the Black sea, where it quickly established a population. But how did it arrive in the Back sea and how did it become so successful?

Comb jellies is a jellyfish like organism within the phylum Ctenophore, commonly identified for their combs which are specialised groups of cilia used for feeding and swimming. They typically feed upon microscoping organisms that are suspended within the water column called plankton. But more specifically they feed upon zooplankton and ichthyoplankton.

Arrival in Black sea-

It’s thought that the comb jellies found their way into the black sea by being accidentally transported within the ballast water of vessels travelling from their native water to the Mediterranean. This is a common why non-native species arrive into new environments, although most of these species dead before doing able to reproduce and form a breeding population.

What made the comb jelly so successful?-

There are many reasons the comb jelly were so successful within this new ecosystem. One of the reasons is due to the way it reproduces. The comb jelly is a hermaphrodite, which means its capable of self fertilisation.  Another reproductive adaptation that lead to the success of the comb jelly, is its ability to regenerate from a third of its original body; meaning if fisherman haul in a net full of this species and decided to chop them up and throw them back into the water it could greatly increase the number of individuals.

Another key factor to the comb jellies success is its large tolerance range to biological factors such as temperature and salinity. Comb jellies are known to be able survive temperatures ranging from 1.3 to 32 oC and salinities ranging from 3.4 to 34 PPT. This wide range of tolerances allows these comb jellies to feed and breed all year round, apart from winter where they are limited.

Another contributing factor is the lack of natural predators. As they are an invasive species, it’s unlikely to have any predators within this new ecosystem. With no predators to control the abundance the population can increase drastically. The last but thought to be the main reason for the success of this species, is the high abundance of pray. A combination of all these factors allowed the population of to grow out of control.

The black sea ecosystems fall-

Comb jellies are voracious feeders, capable of eating upto 10 times its own body weight every single day . And with an uncontrolled population, this caused the diversity and abundance of ichthyoplankton and mesozooplankton to decrease drastically. And within an examination conducted by Shiganona , it was confirmed that there was a relationship between decreasing abundance of fish eggs, larva and zooplankton and the abundance of M. leidyi. This caused large amounts of pressure on native planktonic fish as well as fish essential to the black sea fisheries, as the invasive species wasn’t only limiting the amount of pray but also effecting the life cycle of the fish; as the comb jelly was predating upon the fish eggs and larvae of the native fish. With this increased pressure, the population of native fish began to decrease, so much so that the effects could be seen at the top of the trophic structure where population of mega-fauna such as dolphins and seals also decrease.


Figure 2- Photo of Beroe ovata
Photo taken by Eric Heupel

Fortunately, the ecosystems that were affected by this alien species show signs of recovery. These improvements are thought to be due to the introduction of another non-native species Beroe ovata.  Beroe ovata is also a comb jelly and is a natural predator of M. leidyi and it feeds almost exclusively on the invasive species. This introduce has help keep the population of the invasive species under control and has given chance for the native fish species and stabilise the ecosystem. It’s interesting to see how the introduction of just one non-native species that could have a negative effect on an ecosystem, could turn it into an extreme environment and habitat.


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