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The dramatic proliferation of jellyfish in oceans around the world, driven by overfishing and climate change, is a sure sign of ecosystems out of kilter, warn experts.

"Jellyfish are an excellent bellwether for the environment," explains Jacqueline Goy, of the Oceanographic Institute of Paris. "The more jellyfish, the stronger the signal that something has changed."
Brainless creatures composed almost entirely of water, the primitive animals have quietly filled a vacuum created by the voracious human appetite for fish.