Mastigias jellyfish flood Jellyfish Lake, a marine lake in Palau, an island nation in the Pacific Ocean. Here, researchers found that pulsating jellyfish stir up the oceans with as much vigor as tides and winds, making them major players in ocean mixing. CREDIT: K.Katija/J.Dabiri.

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Though some reports suggest jellyfish are taking over the world’s

oceans, long-term records of these gelatinous animals fail to show a

global increase in jellyfish blooms likely caused by pollution, warming,

coastal development and other human influences.

While the analysis of a team of researchers who have pulled together

records of jellyfish presence going back to the 19th century don’t

support a rising gelatinous menace, the team did find a surprise: roughly 20-year cycles in the abundance of jellies.

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Part of a recent rise-and-fall cycle may have prompted the perception

of a global swell in jellyfish, according to the international team,

whose researchers are part of the Global Jellyfish Group. They point

specifically to the rising phase that began in 1993 and peaked in 2004.

Blamed for stinging swimmers, clogging fishing nets, overrunning ecosystems

and wreaking other havoc, jellyfish blooms — when these animals appear

in massive numbers — have caught the attention of the media and

scientists alike. A number of research papers have suggested that not

only are blooms increasing on a global scale, but humans are likely

responsible, because humans alter the oceans in ways that favor

jellyfish. (See Stunning Photos of Jellyfish Blooms)

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However, others have maintained information on jellyfish populations just isn’t sufficient to draw such conclusions.

This most recent study drew upon 37 data sets, each of which included at

least 10 years of records of jellyfish presence in an area.

Even though the records don’t evenly represent the ocean — the majority

came from the Northern Hemisphere, particularly in the Atlantic Ocean

and the Mediterranean Sea — they include all available annual

measurements, including datasets used to support work indicating

increases in jellyfish, the authors write in a study published online today (Dec. 31) in the journal Proceedings of the National Academy of Sciences.

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Giant jellyfish (Nemopilema nomurai) clogging fishing nets in Japan. Also called Nomura’s jellyfish, these gelatinous creatures can grow up to 6.7 feet (2 meters) in diameter. CREDIT: Dr. Shin-ichi Uye

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Language has added to the challenge. In common use, the term jellyfish

lumps together organisms that can be quite different from one another.

For the purposes of this study, the researchers included records for

true jellies, the type most familiar to beachgoers; their relatives the

hydrozoans; comb jellies, which use tiny hairs, called cilia, to swim;

and another group of free-swimming invertebrates called salps.

From around 1940 to present, the records show the 20-year rising and

falling cycles. Prior to that, researchers saw signs of oscillations in

regions where data were available; however, this isn’t enough

information to draw conclusions about global patterns, said lead

researcher Rob Condon, a marine scientist at Dauphin Island Sea

Laboratory in Alabama. 

Multidecadal cycles are not uncommon in nature, whether in organisms’

growth patterns and populations, or physical phenomena, such as the

oxygen concentration of the oceans.

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