Jellyfish: On the Rise or Not?

Jelly Fish by Mike was used under a Creative Commons license and is available at :


Is it possible that jellyfish are slowly taking over the world’s oceans? The idea seems like the plot of an old B-list science fiction movie, but reports of population explosions around the world for these creatures have made headlines for much of the last decade. Studies throughout the last decade have supported these claims, many of which have suggested that human-related activities, such as over-fishing, eutrophication, and climate change, have contributed to these increases.

“In recent years, jellyfish blooms have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters,” said Anthony Richardson of the University of Queensland, whose 2009 study explored what he termed as the ‘jellyfish joyride.’ “The most dramatic have been the outbreaks in the Sea of Japan involving the gargantuan Nomura jellyfish, which can grow up to 2 meters in diameter and weigh 200 kilograms.”

However, the scientific community still remains divided about the reality of such population changes.

“Clearly, there are areas where jellyfish have increased, and the situation with the Giant Jellyfish in Japan is a classic example,” said Rob Condon of the Dauphin Island Sea Lab in Alabama. “But there are also areas where jellyfish have decreased or fluctuate over the decadal periods.”  

Background on Jellyfish

Jellyfish, members of the Cnidaria Ctenophora phyla, are sometimes referred to as medusae. They inhabit every ocean in the world at all kinds of depths. Made of 95 percent water, most lack specialized organ systems. Lifespans typically range from a few hours to several months, but one species is reported to live as long as 30 years. They are carnivorous predators of mostly plankton, but are known to feed on some crustaceans, small fish and fish eggs, and other jellyfish, using tiny stinging structures called nematocysts (or cnidocytes) located on tentacles to inject venom and kill prey. Only 70 of the 2,000 different species catalogued so far are dangerous to humans; the most dangerous is the box jellyfish (Chironex fleckeri), which could kill within three minutes. It’s speculated that there could be as many as 300,000 different species of jellyfish yet to be discovered.

Jellyfish reproduce rapidly and exhibit fast growth rates. Their ability to feed via touch rather than by sight allows for feeding to occur at night and in turbid waters, which lets them out-compete fish for the same prey. The transparency of skin and muscles are the perfect camouflage.

“Jellyfish are voracious predators,” says Condon. “They impact food webs by capturing plankton that would otherwise be eaten by fish and converting that food energy into gelatinous biomass. This restricts the transfer of energy up the food chain, because jellyfish are not readily consumed by other predators.”

Reproduction occurs daily if there is enough food, and happens in sexual and asexual stages, potentially yielding blooms of 100,000 individuals in most species. Jellyfish can also thrive in environments that are stressful for most other sea creatures, such as salty waters with poor oxygen levels.  They are aggressively invasive; eight years after comb jellyfish were introduced to the Black Sea in 1982, they totaled about 900 million tons and accounted for more than ten times the weight of the total annual fish catch from around the world. There are over 400 marine Dead Zones -- areas polluted for almost all life except jellyfish.

Despite this, jellyfish are still poorly understood creatures, especially with regard to population dynamics. There is little quantitative data of historic populations to compare with current data. The biology and reproductive habits of jellyfish, like other underwater animals, are difficult to study.

“Unfortunately,” said Richardson, “jellyfish are fragile and difficult to sample, hampering the collection of time series.”

The sheer number of different species leaves scientists cautious of generalizing findings of one species for the rest.  It’s easy to see why the notion of population rises in jellyfish is still disputed.

What does all this have to do with humans?  

There is much speculation that human activities are, in fact, primarily to blame for these recent population rises. Blooms are dependent on currents, nutrient availability, sunshine, temperature, prey availability and oxygen concentrations. Overfishing gives jellyfish an opportunity to fill niches once held by fish.  Richardson is quick to remind that jellyfish are both predators of fish and competitors of smaller prey; thus, they remove eggs and larvae of fish/shrimp/crabs and exacerbate the problem.

Human-induced eutrophication (the addition of excessive nutrients) caused by fertilizer runoff and sewage into coastal waters can alter the environment and lead to algal and plankton blooms, which in turn can augment jellyfish food sources.  Low oxygen levels that result from algal blooms are also not a big problem to jellyfish, since they are able to survive low O2 environments. Most pressing, however, is the problem of climate change. Jellyfish, like many ocean fauna, thrive best in warmer temperatures. But as winters are cut shorter, ice begins to melt sooner, paving the way for easier access to warmer waters and more sunlight for plankton.

What is observed is a positive feedback in which existing circumstances lead to larger jellyfish blooms, which are uninhibited to continue reproducing and invading other environments.

Already there has been $350 million in losses to the Black Sea’s fishing and tourism industries resulting from the invasion of the comb jelly into the Black Sea, and losses from the ongoing comb jelly invasion of the Caspian Sea are expected to exceed that. In November 2007, jellyfish blooms wiped out Northern Ireland’s salmon farm, killing more than 100,000 fish. Population outbreaks have also been linked to power outages in the blockage of cooling intakes at coastal power plants and the blocking of alluvial sediment suction in diamond mining operations. It’s thought that jellyfish could also act as immediate vectors for various aquatic parasites.  

Fears of being injured or killed as a result of jellyfish stings can economically hurt beach communities dependent on tourism. About 150 million people annually are exposed to jellyfish around the world. Currently about 500,000 people are annually stung by jellyfish in the Chesapeake Bay and about 200,000 people are annually stung by jellyfish in Florida.

“Mounting evidence suggests that open-ocean ecosystems can flip from being dominated by fish, to being dominated by jellyfish,” Richardson said. “This would have lasting ecological, economic and social consequences.”

The debate goes on…

But are jellyfish populations really increasing so suddenly? Condon and others don’t think it’s wise to assume so just yet. A recent paper authored by him in collaboration with approximately 30 other scientists and published in BioScience argues that much of the current paradigm about jellyfish populations is built much more on perception rather than actual data. There is not a large enough human frame of reference to make sound conclusions that blooms aren’t simply in a state of regular fluctuation. To Condon and colleagues, it’s more prudent to understand the long-term data rather than try to draw conclusions from short-term observations.

“People seemed to be extrapolating from these regional increases to claim that jellyfish populations were increasing worldwide,” said co-author Mary Beth Decker from Yale University. “In some of these areas, like the Bering Sea, subsequent population decreases were not acknowledged in the literature, and the paradigm continued to be perpetuated.”

Cathy Lucas of the University of Southampton echoed these sentiments. “There has been a tendency to ‘add up’ all the conclusions from various local and regional studies to come up with a ‘global conclusion’ that jellyfish blooms are a relatively new phenomenon and are occurring all around the world in response to climate change or man-induced perturbations.” Obsessive media reporting can only serve to skew reports to the public. “These reports only highlight the population outbreaks and the negative effects of jellyfish,” explained Lucas. “It doesn't make exciting news to report that there have been no jellyfish!”

Condon, Lucas, Decker, and many others worked together to develop a strategy for testing the validity of these reports. Their paper highlights the result:  the creation of the Jellyfish Database Initiative (JEDI), which seeks to collate as much of the world’s raw data on jellyfish populations as possible in order to test the paradigm.

“This is the first time an undertaking of this size on the global scale has been attempted, but it is important to know whether jellyfish blooms are human-induced or arise from natural circumstances,” said Condon.

Lucas elaborated on the sheer variety and range of data compiled, which includes “oceanographic databases, unpublished data from scientists, published data from journals and reports, fisheries bycatch data, long-term plankton monitoring programs, public sightings, etc.”

Decker acknowledges that developing JEDI was not easy. “For the most part, scientists who had published their data were happy to contribute data sets to JEDI, but some people who had data but had not yet published it were, understandably, sometimes reluctant to provide it, so in some cases it took careful negotiation to be granted access to the data. Overall, however, we feel confident that we have gained access to more than 90 percent of the time series data available for jellyfish.”

“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,” said Carlos Duarte of the University of Western Australia’s Oceans Institute. “The important aspect about our synthesis is that we will be able to support the current paradigm with hard scientific data rather than speculation.”

It will take some time before a consensus can be reached on the state of jellyfish populations, and what the implications will be for oceanic ecosystems and human activities. Regardless, the debate underscores the necessity for caution in scientific research.  Interest in jellyfish will probably not disappear soon, but how we perceive them is in a constant state of flux, dependent on how that research is conducted.

This science feature article was written under the guidance of JYI Science Writing Mentor Christie Wilcox.

JYI staff members in the Research Department help student authors every step of the writing, editing, and peer-review process.
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