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

Many species have been transported or introduced into ecosystems in which they’re not native, however not all are considered invasive. An invasive species is an organism that causes environmental harm, harm to human health or economic harm when in an ecosystem to which it’s not indigenous. These species often find their way into new environments through the direct activities of humans, either accidentally or deliberately. The comb jelly (Mnemiopsis leidyi), is native to the Atlantic waters off the coast of America, however, in the 1980’s it was found in the Black sea, where it quickly established a breeding population. How did it arrive in the Black Sea and how did it become so successful?

Comb jellies are 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 microscopic organisms that are suspended within the water column called plankton and more specifically upon zooplankton and ichthyoplankton.

Arrival in the Black Sea?

It’s been surmised that the comb jellies found their way into the Black Sea by being accidentally transported within the ballast water of vessels travelling from the comb jellies native waters in the Atlantic to the Mediterranean. This is a common way non-native species arrive into new environments, although in many cases most of these species die before being able to reproduce and form a breeding population.

Why was the comb jelly so successful?

There are a number of reasons why the comb jelly has been so successful within this new ecosystem. The first of these reasons is the way it reproduces. The comb jelly is a hermaphrodite, which means it’s capable of self fertilisation.  Another reproductive adaptation that has led to the success of the comb jelly, is its ability to regenerate from a third of its original body; meaning if fishermen  attempt to kill this species by chopping them up and throwing them back into the water it can have the detrimental effect of actually increasing the number of individuals.

A further 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 to 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 for most of the year, apart from winter when their activity is limited.

Another contributing factor is the lack of natural predators. As they are an invasive species, they had no natural predators within this new ecosystem. With no predators, the population could increase drastically. The last, but possibly the main reason for the success of this species, was the high abundance of prey. The Black Sea has many large fisheries and the years before the introduction of M. leidyi they had need exploited by overfishing, causing this abundance in food.  A combination of all these factors allowed the population to grow out of control.

The collapse of the Black Sea ecosystem!

Comb jellies are voracious feeders, capable of eating up to 10 times their own body weight every single day . And with an uncontrolled population, this caused the diversity and abundance of ichthyoplankton and mesozooplankton to decrease drastically.  In an study 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 abundance put great pressure on native planktonic fish, as well as fish essential to the Black Sea fisheries. The invasive species wasn’t only limiting the amount of prey, but also affecting 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 populations of mega-fauna such as dolphins and seals also decreased. Two of the biggest Black Sea fisheries anchovies and horse mackerels, both saw a dramatic fall in landing. In 1988 the landing for anchovies  was 295,000tonnes, but by 1990 this had fallen to only 66,000tonnes. The mackerel showed a similar fall, but wasn’t to the extent of the anchovies, dropping from 101,000tonnes in 1986 to 90,000tonnes in 1989.

Recovery

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

Fortunately, the ecosystems that were affected by this alien species have shown 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 it. This fortuitous introduction has helped keep the population of M. leidyi under control and has given  the native fish species the opportunity to recover and stabilise the ecosystem. It’s interesting to see how the introduction of just one non-native species could have such a negative effect on an ecosystem, and could turn it into such an extreme environment and habitat.

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