American Comb Jellies Comb Jellies by benbenzhou


American Comb Jellies Comb Jellies

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A feared and invasive species.
Possibly a threat to marine life on the West Coast.
An intruder with no enemies.
In Swedish waters.
A lot has been said about the American comb jelly, Mnemiopsis, and this is what it looks like.
But, what is actually so special about this type of comb jelly?

"The special thing about it is that it can multiply and increase in size extremely quickly. In a
very short time they can be in exceedingly large numbers. They eat the same food as other
jellies and comb jellies, namely zoöplankton, fish eggs and very young fish. But, when there
are so many of them, and they grow in number so quickly, they have a huge effect on our
ecosystem. And this is a species which shouldn't be here."
"Why is it here, then?"
"Usually they are to be found on the east coast of North and South America. But, there, they
have a large number of natural predators pursuing them." They can now also be found in the
Black Sea and the Caspian Sea. Presumably, they ended up there in ballast water during the
1980's. They didn't have any predators there, either, when they arrived, and could therefore
grow in number very quickly, and, in turn, had a huge effect on the system. Fishing collapsed
in the Black Sea, as young fish had competition for food. And the comb jelly has now spread
out to large parts of northern Europe.

Lene Friis Möller at the University of Gothenburg, Sweden, is conducting research on comb
jellies at a research station in the Gullmarsfjord in Bohuslän County, where she is studying
the Common Comb Jelly. But, when it was discovered, during the Summer, that the American
comb jelly had more or less invaded the Bohuslän coast, she chose to specialise in this and, in
August, she started her experiments.

"I can show you a feeding test with comb jellies. We have comb jellies in the aquarium and
I've measured the volume of water. And, with the help of a microscope, I know how much
zoöplankton I have in the filter. Then, I give them to the comb jellies.

Lena travels out in a boat once a week to collect sea water containing jellies and plankton.
When she returns to the lab, she counts the jellies to see how much they have grown. She has
also checked how many phytoplankton and zoöplankton there are per cubic metre of water.
And, with these measurements, she can roughly work out how much plankton would be eaten
by the jellies if they were to continue invading Swedish waters.

"Have you noticed large differences during the time you've been here?"
"Yes, I've noticed, both, that there are more and more of them, and that they are growing in
size every week. As soon as they arrive they are of a size where they really affect the system.
Even if we can see all sizes, from 2 millimetres to ten centimetres, the average size is just 3
centimetres. So, they can grow a lot more. What we fear is that they are eating all the
zoöplankton, meaning that there is no food for the young fish."
"Can this be solved?"
"We can't solve it directly. There's nothing we can do. In the Black Sea, another jelly arrived
which ate Mnemiopsis, and it was therefore held in check. But, it didn't disappear."

But is the American comb jelly dangerous to us humans?

"No, it isn't dangerous to us. It's just that, when there are so many of them, they become a
danger to the system."

So, what will happen in the future? Will we see more of the American Comb Jelly up here in
northern Europe?

"I believe the comb jelly is here to stay. And, we must follow its development extremely
carefully, to see what effect it is having on the system."

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