They can stop the world's biggest warships, cripple power stations and kill humans with a single sting but jellyfish are not about to take over the world and turn our oceans to slime, as some reports have claimed.
In a paper published this month in BioScience, a team of international marine scientists - including Winthrop Professor Carlos Duarte, Director of the Oceans Institute at The University of Western Australia - said they found no hard evidence to support claims that jellyfish numbers are increasing worldwide at an alarming rate.
Despite widespread media reports of such claims, the lack of a comprehensive database of jellyfish numbers had prevented a reliable scientific assessment of global trends, Professor Duarte said.
However, researchers had now assembled more than 500,000 records dating back to 1750 to form a community-based Jellyfish Database Initiative (JEDI), against which to measure future changes in jellyfish blooms.
Professor Duarte said jellyfish clogging seawater intakes of the world's biggest nuclear-powered warship, the USS Ronald Reagan, had prompted the captain to withdraw his 20 storey-tall aircraft carrier from Brisbane Harbour for safety reasons during a 2006 visit.
Similarly, some coastal power stations and water desalination plants had been forced to shut down or install slicers to stop jellyfish blooms choking seawater intakes, as occurred in Palma Beach, Florida last December.
The team of 30 experts from the international Global Jellyfish Group found that jellyfish numbers had risen in some places such as Japan but decreased or fluctuated elsewhere.
"The study was prompted by increased media speculation and discrepancies in climate and science reports about current and future jellyfish blooms," Professor Duarte said.
"After examining the evidence, we believe that claims of a big rise in jellyfish numbers is very much a case of misreporting of qualified data.
"There are major consequences for getting the answer correct for tourism, fisheries and management decisions as they relate to climate change and changing ocean environments."
The study group will use JEDI to assess past trends in jellyfish blooms in a more rigorous manner than was possible previously.
It will assess whether current jellyfish blooms are caused by human-made actions or whether there is simply more awareness of them due to their impact on human activities, such as over-harvesting of fish and increased tourism.
"The importance of this study is that we will be able to support current knowledge with hard scientific data rather than speculation," Professor Duarte said.