Beautiful, but Deadly

Jellyfish Blooms Appear to be on the Rise. Is Global Warming Causing an Ocean Swarming?


photo of a moon jelly, from below
photo of a jelly
photo of a jellyUnless noted, all jellyfish from

On the morning of November 21, 2007, John Russell awoke to his worst nightmare. Billions of small jellyfish, covering a patch of water 10 miles wide, had infiltrated his fish farm off the coast of Northern Ireland. The sea was so dense with jellyfish that the boats used to try to rescue the fish couldn’t cut through the gooey mush. “The sea was red with these jellyfish and there was nothing we could do about it, absolutely nothing,” Russell told the BBC. By the time the swarm had moved on, the damage was done: Hundreds of thousands of Russell’s salmon were dead, and his company had lost more than $2 million of inventory.

The swarm of jellyfish that overwhelmed Russell’s fish farm would be little more than a science fiction-esque curiosity if it didn’t have such alarming consequences for ocean ecosystems worldwide. Reports of voracious jellyfish swarms are on the rise – they’re being seen in greater numbers, tighter concentrations, and more areas than ever before. To make matters worse, many of the most disruptive swarms are occurring in seas that were, until recently, too cold for jellyfish. As global ocean temperatures steadily rise, many fishermen, scientists, and beach-goers may be watching a real-life environmental horror movie unfold before their very eyes.

Scientists worldwide are moving quickly to understand the dimensions of the problem and its root causes. Already, millions of people have been affected, and nearly every ocean-based industry is getting stung.

Fishermen haul up their nets to find gelatinous bycatch. Beachside resorts struggle to ensure swimmer comfort and safety. Ocean freighters, desalination facilities, seabed mining dredgers, and nuclear power plants have all been forced to curtail operations while jellyfish ooze is excavated from clogged pipes. In Japan, some coastlines have been inundated with hundreds of millions of refrigerator-sized jellyfish, so many and so large that they have forced the temporary closure of electricity plants that provide power to Tokyo.

In order to understand the harmful effects of jellyfish blooms, it’s important to appreciating the animals’ evolutionary panache. The term “jellyfish” is a taxonomist’s torment. Strictly defined as stinging animals of the phylum Cnidaria, the title now colloquially encompasses most gelatinous creatures of the sea, a wide range of “jellyfish-like organisms.” (Because both groups have similar ecological roles, jellyfish and jellyfish-like organisms will be collectively referred to as “jellyfish” in this article.) To add to the confusion, jellyfish – boneless and gill-less – are not actually fish.

photo of a jelly Anastasia Shesterinina

Although broadly defined and clumsily named, jellyfish have certainly found their ecological niche. “Evolutionarily, they’re extremely successful animals,” says Dr. Monty Graham, a senior marine scientist at Dauphin Island Sea Lab. Graham cites the fact that jellyfish morphology has remained largely unchanged for more than 600 million years as “tremendous evidence that they’re successful animals.” The key lies in their prolific replication strategy, an unusual combination of sexual and asexual reproduction. Adults release eggs and sperm in the water at impressive rates, with some species dispersing up to 40,000 eggs each day. When fertilization occurs, a larva is formed, which affixes itself to a hard surface as it enters the polyp stage. This is where jellyfish tend to slip off the radar. Polyps are rarely found in the wild, though researchers believe they cover extensive tracts of the ocean floor, biding their time for up to several years until conditions are right. When salinity, temperature, and food levels are favorable, each polyp elongates and buds off, producing many young jellyfish. Because the conditions promoting budding are regional in scale, fields of polyps release their young at roughly the same time, leading to vast flotillas of jellyfish. A bloom is born.

“Jellyfish are going to
become the top predator
because they’ll just replace everything. That’s happening in many places all over the world. What’s going to
displace them? Nothing.”

Once a bloom becomes active, little can stop it. Dr. Anthony Moss, an associate professor at Auburn University, describes jellies as “very voracious predators” that seize control of ecosystems from the bottom up. Jellyfish eat all the fish eggs, larvae, and zooplankton they can get their tentacles on, an approach that both eliminates potential predators and limits the food available to the competition. In the process, “They’re going to become the top predator because they’ll just replace everything,” Moss notes. “And that’s what’s happening in many places all over the world. What’s going to displace them? Nothing.”

As frightening as these developments are, most of the evidence of increased jellyfish populations is anecdotal, and it is dangerous to read too much into what may be a few freak events. So are jellyfish numbers really rising?

“I approach this thing with a big degree of skepticism,” says Dr. David Agnew, a fisheries scientist at Imperial College London. Agnew notes that jellyfish blooms have been a natural part of oceanic ecosystems for millions of years. The animals grow and reproduce quickly in order to take advantage of ideal, and often fleeting, conditions. But as people continue to move toward the coasts and we become increasingly dependent on the sea for food and resources, even normal blooms have a larger impact on human activities.

In a field where scientists are lucky if a data set extends just a few decades into the past, it’s impossible to evaluate how modern jellyfish levels compare to a reliable historic baseline. That said, there is a growing consensus among marine scientists that jellyfish blooms, if not absolute numbers, are rising.

photo of a jelly

“There isn’t enough information available at this point to give a definitive answer,” Moss says, “but many of us in the field are pretty sure that’s what’s happening.” Dr. Mary Beth Decker of Yale University says, “There’s some speculation that we may be seeing more frequent, larger blooms.” No matter what the exact number, the blooms, which have stunned ecosystems from the Black Sea to the Bering Sea, are worrisome, especially given that large swaths of ocean appear ripe for equally disruptive jellyfish explosions.

What is causing the apparent surge of jellyfish blooms around the world? Graham compares the situation to another familiar case of fluctuation: “Like following the ticker on the stock market, natural ups and downs are going on.” But, he notes, it’s the longer-term trends, the true departures from historic norms, that reveal fundamental changes in how ecosystems are balanced.

Dr. Jack Costello, a scientist at Providence College, has been closely monitoring one such shift. Costello studies Mnemiopsis leiydi, a small gelatinous organism also known as the “sea walnut” because of its domed shape. Off the coast of New England, Mnemiopsis populations follow seasonal patterns, expanding during the summers and contracting over the winters. Starting in the late 1990s, however, Costello noticed something peculiar: The jellies were blooming up to two months earlier than expected. He and his team ultimately discovered that the organisms were retreating to smaller “refuge sites” that warm up earlier in the year than the open water. “These refuge areas were being most affected by climate alterations,” he explains, “and that’s what was driving the larger system.” Earlier blooms allowed mature Mnemiopsis to consume predators and competitors before they became a real threat. Warming waters have also allowed Mnemiopsis to move farther north than ever before, into regions once deemed too cold for the temperate species. “We’re seeing range expansion, but,” Costello cautions, “we don’t know how permanent that is.” Moss has also noticed other jellyfish species colonizing new waters, thanks to global warming. “I’ve seen the effects and I’m astonished,” he says. “I can read the data like anyone else, but I never expected it would slap me in the face.”

A change to any ecosystem invariably creates winners and losers – and jellyfish, sometimes referred to as “the cockroach of the sea,” have almost always been on the winning side of the equation.

Few studies have attempted to systematically evaluate the effect of global warming on jellyfish populations, but one of the best examinations was conducted by Dr. Jennifer Purcell of Western Washington University. In a research paper published by the Marine Biological Association of the UK, Purcell reviewed jellyfish levels and climate records from around the world, concluding 18 out of 24 species she looked at grew in response to warmer water conditions. She warns that “global warming could result in expanded temporal and spatial distributions and larger populations of jellyfish” – as shrill a statement as you’re likely to find in academic literature.

Rising water temperatures are playing a role in the jellyfication of the seas, but the full story is much more complicated. In addition to global warming, “We think there may be other things that we’re doing that make conditions right for jellyfish,” Decker says.

One such activity with unexpected consequences is global shipping, which inadvertently precipitated ecological and economic disaster in and around the Black Sea. Before a ship leaves port, it often takes water on board to provide stability. This “ballast water” often contains many of the smaller flora and fauna native to the departure port’s ecosystem. Upon arrival, the ship dumps the ballast water, unwittingly inoculating the destination port with any surviving organisms.

During the early 1980s, one ocean freighter, likely departing from a more temperate port in the US, introduced Mnemiopsis into the Black Sea. The first reports of the invasive species appeared in 1982, and the population ballooned soon thereafter as the jellyfish feasted on zooplankton, fish eggs, and larvae. By 1989, at the peak of the explosion, there were some 400 jellies per cubic meter of water, and native fish populations were decimated. Finfish and anchovy stocks were hit particularly hard, and fishing fleets in bordering nations were hammered. Turkey alone lost an estimated $43 million in the early 1990s as a result of the collapsed fishery.

photo of a jelly

Mnemiopsis has since spread to the Caspian Sea, the Sea of Azov, the North Sea, and the Baltic Sea, often with similarly destructive ecological and economic consequences. The introduction of invasive species via ballast water is particularly dangerous because even the smallest organism can forever upset an elegantly balanced ecosystem. In its native waters, Mnemiopsis has developed a dynamic relationship with its predators and prey. Elsewhere, however, other fish have not evolved specific strategies to counter the habits of the jellyfish, leaving them high and dry. As Moss puts it: “A pretty jellyfish in the Gulf of Mexico” can become “a raging monster in the Caspian Sea.”

Some jellyfish species have thrived thanks to another human alteration of the seascape – ocean “dead zones.” At some 150 offshore locations around the world, huge quantities of agricultural runoff – nitrogen-heavy chemical fertilizers, pesticides, and animal wastes – have led to explosions of algae in the sea. The algae suck oxygen out of the water and block sunlight from reaching other plankton lower in the water column. With less oxygen and fewer types of food on offer, many larger fish move elsewhere or die. The Gulf of Mexico contains the largest dead zone in the Western Hemisphere: In 2008, it was measured at 8,000 square miles, and it is expected to surpass 10,000 square miles by year’s end.

While most other species have declined in the Gulf of Mexico dead zone, jellyfish have proliferated, emphatically bucking the trend. During the 1980s and 1990s, the sea nettle and moon jellyfish steadily increased their ranges, and now invasive species like the Australian jellyfish have joined the party, swarming over thousands of square miles. Many fish species struggle to find food when the water column becomes more opaque, but jellyfish, which are not visual hunters, are unbothered. Also, “Jellies can do okay in low-oxygen environments,” Decker says, “because they don’t have complicated bodies, don’t have huge oxygen requirements.” In the murky waters of the dead zone, simplicity pays off, and jellyfish are reaping the rewards.

Overfishing is another man-made factor that may be bolstering jellyfish populations. As human consumption removes large fish from ocean habitats, jellyfish benefit. With fewer predators and competitors, jellies can gorge on more plankton in safety, tightening their grip on the ecosystem. In the Bering Sea, a skyrocketing jellyfish population in the late 1990s was blamed partially on depleted stocks of pollock, a local predator. Graham points out that “the vast majority of our fisheries are depleted,” and the situation isn’t going to improve if the status quo holds. “At our current rate of using oceans, the problems will undoubtedly mount, and jellyfish will likely play a greater role,” he says.

It isn’t only what we’re taking out of oceans that affects jellyfish populations; it’s also what we’re putting in. As ingenious as the jellyfish’s reproductive strategy is, the polyp stage requires a hard surface, or substrate, on which to wait for favorable conditions. According to Decker, “Some people hypothesize that as we’ve added lots of structures to coastal waters – oil rigs, docks, marinas – maybe we’re providing additional substrates for polyps.” More polyps translate into larger blooms, which in turn out-graze competitors and generate a positive feedback loop of gelatinous domination.

photo of a jelly

The rise of jellyfish cannot be blamed exclusively on global warming or any single factor; rather, a perfect storm of man-made changes in our ocean environments has conspired to provide opportunities that jellies have been quick to exploit. A change to any ecosystem invariably creates winners and losers – and jellyfish, sometimes referred to as “the cockroach of the sea,” have almost always been on the winning side of the equation.

“It doesn’t take a whole lot in terms of perturbing an ecosystem to get it to respond with increases in jellyfish,” Graham says. Jellyfish take advantage of warming waters to expand their range and accelerate their rates of reproduction. They hitchhike in ballast water to seas that are evolutionarily unprepared to deal with their presence. Their simple ways of life pay dividends in the primordial conditions of dead zones. They fill in the gaps as commercial fishermen harvest their predators and competitors, and they co-opt humans’ ocean infrastructure as their very own nursery. In all of these cases, jellyfish are relentless, quick to turn a toehold into a chokehold. “These things can respond,” Moss says. “They’re plastic. They can change. They can reproduce rapidly. They are astonishing animals.”

Despite growing awareness of the economic and environmental threats posed by increasing swarms, the jellyfish challenge isn’t likely to be resolved anytime soon. “We can see what’s happening, but is there anything to do?” Moss asks. “We need to think in terms of much bigger things” that could restore ecosystems to their previous levels of diversity.

Graham agrees: “Don’t treat the jelly blooms as the problem,” he advises. “Dig deeper and then try to cure the patient, not the symptoms.”

Curing ocean ills, however, will take time, money, and sustained political will. Dramatically reducing greenhouse gas emissions, restoring dead zone habitats, and managing fisheries in sustainable ways are all enormous challenges. “None of those is a very easy problem to tackle on short time scales,” Graham says, “so it will take real long-term commitment on a societal level.” Although such problems’ contribution to rising jellyfish populations has only recently become apparent, these issues are already well fixed on the environmental agenda. A number of groups – including governments, scientists, and activists – are working on each of these challenges, and hopefully oceanic ecosystems can be restored to balance in the process.

With no end to the increasing presence of jellyfish in sight, what can we expect of our future oceans? Will jellies soon dominate oceans worldwide? Graham believes that “at our current rate of using oceans, the problems will undoubtedly mount, and jellyfish will likely play a greater role in regulating the flow of nutrients and energy through tomorrow’s oceans just as they did in yesterday’s oceans.”

Chefs have found a silver lining: The slimy bells, appreciated for their rubbery and crunchy texture, are ingredients in a number of trendy dishes.

In this sense, the jellyfish resurgence runs against the evolutionary grain, placing a decidedly less-evolved organism in a dominant ecological position. As a result, other species lose out. “If diversity of the ecosystem is what we define as good,” Costello says, “then we’re most probably seeing decreased diversity.” But, Graham points out, “Life on this planet is extremely diverse, and this overall diversity we have, over deep evolutionary time, is not going to go away.”

Despite the gloomy predictions, there may be a silver lining. Even as some scientists, environmentalists, and politicians seek ways to address the underlying cause of the slime, others are finding ways to adapt to the new age of jellyfish.

Japanese researcher Kiminori Ushida is one person who believes he has found a benefit to increased jelly blooms, a way of transforming a terror into a treasure. Ushida and his colleagues are perfecting a technique for harvesting jellyfish’s mucin protein, a substance that displays antibacterial properties and is used in drug delivery, cosmetics, and food additives. Ushida’s team has also extracted from jellyfish a previously unknown type of mucin, the therapeutic effects of which are currently under investigation. These findings open the door to a new branch of pharmacology, possibly revealing these “pest” organisms as an untapped medicine cabinet.

Diners may also experience the growing role of jellyfish in the food chain. The slimy bells, appreciated for their oddly satisfying rubbery and crunchy texture, are trendy ingredients in a number of dishes, particularly in Chinese cuisine. Joe Lai, head chef at London’s renowned Dragon Castle restaurant, mixes strips of boiled jellyfish with cucumbers and sesame seeds into a popular appetizer. Nutritionally, Lai sees many advantages: “It’s rich in protein, but it doesn’t have much fat – it’s a healthy snack.” If consumers can be convinced that jellyfish are a palatable alternative to other protein sources, the sea creatures may soon be taking over the dinner table as well.

photo of jellies being sorted for consumptionHo New / ReutersGiant jellyfish, like these caught off the Japanese coast, are so
many and so large that they have forced the closure of electricity
plants. Some chefs believe they have a solution –
eating the pests.

The rise of jellyfish is among the latest unintended effects of wide-scale environmental degradation, and the problems are likely to get worse before they get better. “It will become what it will become,” reflects Moss philosophically. “That’s how biological systems work, but it won’t be what we like or what we expect.” Ships’ pipes will be clogged, fish farms will be damaged, ecosystems will become increasingly homogeneous, and swimmers will be stung. Having set in motion the conditions that have allowed jellies to dominate, the only recourse is to redouble our efforts to combat climate change, overfishing, and dead zones. In the meantime, there’s little to do but manage the consequences – and develop a taste for boiled jellyfish.

Jeff Marlow is a Marshall Scholar working on the search for life on Mars at Imperial College London. He blogs about science and exploration for the Nature Network.

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