Okay, maybe they haven't quite taken over the world but the do have quite an impact. Here is an awesome special report by the National Science Foundation.
http://www.nsf.gov/news/special_reports/jellyfish/
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1 day ago
I first became aware of jelly fish when I was 13 years old. I would swim in Sunny Cove outside of Seward Alaska. The water was cold and so was the air. There was a 4' by 6' piece of driftwood we pushed out into the cove so we could jump off. Before I jumped I always checked for jellyfish because in the summer the Lion's Mane jellys were thick and huge. The bell was the size of a dingy and the tentacles stretched 30 feet.