Giant jellyfish like this one are taking over parts of the world's oceans as overfishing and other human activities open windows of opportunity for them to prosper, say researchers.
In this photo, a diver is attaching a sensor to track a monster Echizen jellyfish, which has a body almost 5 feet across, off the coast of northern Japan.
Jellyfish are normally kept in check by fish, which eat small jellyfish and compete for jellyfish food such as zooplankton, researchers said. But, with overfishing, jellyfish numbers are increasing.
These huge creatures can burst through fishing nets, as well as destroy local fisheries with their taste for fish eggs and larvae.
Anthony Richardson of CSIRO Marine & Atmospheric Research and colleagues reported their findings in the journal Trends in Ecology and Evolution to coincide with World Oceans Day.
They say climate change could also cause jellyfish populations to grow. The team believes that for the first time, water conditions could lead to what they call a "jellyfish stable state," in which jellyfish rule the oceans.
The combination of overfishing and high levels of nutrients in the water has been linked to jellyfish blooms. Nitrogen and phosphorous in run-off cause red phytoplankton blooms, which create low-oxygen dead zones where jellyfish survive, but fish can't, researchers said.
"(There is) a jellyfish called Nomura, which is the biggest jellyfish in the world. It can weigh 200 kilograms (440 pounds), as big as a sumo wrestler and is 2 meters (6.5 feet) in diameter," Richardson said.
Richardson said jellyfish numbers are increasing in Southeast Asia, the Black Sea, the Gulf of Mexico and the North Sea.
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