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Despite the name, a jellyfish is not actually a fish, but a cnidarian, a spineless, soft-bodied animal. Close to 2000 different species of jellyfish are found in every ocean from the surface to the deep sea. Perhaps best known for their sting—which can be toxic to humans—jellyfish are now getting attention for a far more disturbing reason. "Jellyfish populations have exploded dramatically in the last few years, thanks to overfishing and climate change, leading experts to caution that it could signal irreversible changes to marine ecosystems," writes Kimberley D. Mok. "Jellyfish are an excellent bellwether for the environment," adds Jacqueline Goy, of the Oceanographic Institute of Paris. "The more jellyfish, the stronger the signal that something has changed."

These jellyfish near shore are a message the sea is sending us saying, 'Look how badly you are treating me,'" says Dr. Josep-María Gili, a leading jellyfish expert, from the Institute of Marine Sciences of the Spanish National Research Council in Barcelona.

Idea: Stop treating the ocean badly.

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