Are the jellyfish coming?

For the last decade there have been regular reports of jellyfish invading beaches across the world. These "jellyfish blooms" have also caused serious problems for power stations.

What's more, many scientists have claimed that jellyfish populations are increasing worldwide. Their studies have triggered alarming headlines like "Jellyfish taking over ocean, experts warn" and "What you need to know about the coming jellyfish apocalypse".

Anyone who has seen a jellyfish bloom for themselves might find this plausible. There are certainly local and regional explosions in jellyfish populations.

However, the idea that jellyfish are taking over the oceans seems to be built on wobbly foundations. A recent study has found evidence that poor scholarship has led to the evidence behind such a claim being hugely inflated.

Jellyfish populations are believed to rise and fall over several decades. In certain areas, jellyfish populations do go through dramatic increases, leading to periods where there seem to be more jellyfish than usual in a given spot.

Jellyfish also often move in large groups known as "fluthers" or "smacks". These can certainly seem overwhelming.

But are the oceans as a whole becoming steadily jellified? Marina Sanz-Martín at the Mediterranean Institute for Advanced Studies in Spain and her colleagues investigated how scientists have gone about answering this question.

Sanz-Martín started investigating the literature on jellyfish blooms in 2012. She picked the issue because she had heard about the widely-reported problem of the jellification of the oceans.

But when Sanz-Martín looked more closely at the sources that jellyfish researchers were using to back up these claims, she found that many of them were not well-founded.

There was a tendency for nuanced language in one paper to be reported as firmer and more concrete in the next paper. Some findings were exaggerated, and sometimes even distorted, from one paper to the next.

One study stood out as a particular source of confusion

Of the literature Sanz-Martín surveyed, she found that just under half of publications cited papers incorrectly. Some papers misinterpreted their sources, while others cited irrelevant papers or referred to papers selectively in order to fit the author's argument.

Sanz-Martín had to sift through hundreds of papers in minute detail. It was also a challenge on a personal level, she says, because "criticising the others' work is very difficult". For completeness, she and her colleagues also analysed papers that they themselves had written on jellyfish. They found exactly the same citation mistakes.

The researchers have published their findings in the journal Global Ecology and Biogeography.

One study stood out as a particular source of confusion.

Claudia Mills is an independent scientist affiliated with the University of Washington in Friday Harbor, US. In 2001 she published a paper that has had a huge impact in the field of jellyfish biology.

Mills's paper was a review, in which she questioned whether there was a global trend in jellification. Her answer was not a definitive "yes" or "no", so she framed her title as a question: "Are populations increasing globally in response to changing ocean conditions?"

I seemed to have started this rumour that went completely out of control

Many of the scientists who went on to cite her work seem to have assumed that her answer to that question was a firm "yes". "It didn't occur to me that by posing it as a question, and inviting people to make their own conclusions, that they wouldn't read it carefully," says Mills.

About 10 years later, at a meeting on jellyfish blooms at the National Center for Ecological Analysis and Synthesis, Mills saw the impact of her work directly. "They had a hand show of who thinks jellyfish are increasing, and more than half of the people thought so," she says. Mills did not raise her hand.

"I seemed to have started this rumour that went completely out of control," says Mills. "I am really kind of horrified that it probably took me most of a year to write that paper, and it was pretty nuanced. To see that no one probably even bothered to read it is really amazing." She calls these sloppy citation practices "irresponsible".

There are many possible reasons why scientists might be doing such a bad job of citing each other.

The sheer scale of the literature that scientists have to get to grips with could be one cause, Sanz-Martín says. "It is very difficult to handle all this information and also to be balanced," she says.

It is also hard to accurately sum up someone's complex, nuanced piece of work when you can only dedicate a single sentence to it.

Much of the existing evidence for the increase cannot be taken at face value

Furthermore, scientists need to compete for research funding, and this puts pressure on them to be bolder in their claims than they might otherwise be. "You need to have a very good reason to do research and to gain funding," Sanz-Martín says. "You need to get funding and you need to publish a certain amount of papers every year."

Mills adds that, in many parts of the world, it is hard to get access to scientific papers. This is especially true for jellyfish research, because it attracts people who love the animals and study them keenly, but do not necessarily have a full university affiliation. Naturalists and citizen scientists are not likely to have fully paid-up access to lots of journals through university libraries.

For this reason, Mills says many of her more established colleagues will only publish in "open access" journals that anyone can read online for free. But scientists do not always get to be so picky about where their work ends up.

Without doing a follow-up study, Sanz-Martín cannot say for sure which – if any – of these factors played a role. Without more detail, it might be hard to come up with ways stop similar mistakes happening in the future.

One possible solution would be for scientists to check all papers that cite their work, to make sure they have not been misinterpreted. "But it would be a huge amount of work," says Sanz-Martín.

Besides, the problem is not confined to jellyfish biology. A 2010 study found that on average a quarter of the citations in marine biology papers were flawed. Other research fields, such as business studies, seem to be no better.

Finally, what does all this mean for the jellification of the ocean?

Sanz-Martín's findings do not mean that jellyfish populations are not on the rise. Instead, it means that much of the existing evidence for the increase cannot be taken at face value. In other words, we are back to square one.

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