Jellyfish Swarm to the Dance of Climate Change

Jellyfish copy

Compass jellyfish

Warmer and more acid seas, along with overfishing are promoting blooms of jellyfish, which now liberated from predation are swarming off many coastlines. “Like a karmic device come to punish our planetary transgressions, jellyfish thrive on the environmental chaos humans create. Is the age of the jellyfish upon us?” writes Karl Mathiesen in The Guardian (2015).


jellyfish bloomAdapting over Millennia

As a group, jellyfish have been around for more than 500 million years, making them one of the oldest and most successful species on Earth. Made of 98% water, without a brain or a heart, Jellyfish aren’t really fish; they are gelatinous zooplankton—which include Cnidaria, Scyphozoans (“true jellyfish”) and all similar animals such as comb jellies (ctenophores)—include 2,000 different types with about seventy that can sting.

Irukandji jellyfish

Irukandji jellyfish

The box jellyfish (Chironex fleckeri) is considered the most venomous—able to kill a human within minutes of one sting. The sting of the smallest box jellyfish—Irukandji jellyfish (including Carukia barnesi and Malo kingi)—is also the most deadly. The venom of this penny-sized jellyfish is 100 times more potent than a cobra’s. Turretopsis nutricula gets “younger” when threatened; it undergoes “cellular transdifferentiation” to become a polyp again then starts all over. Jellyfish can also wait for ideal conditions to occur, lying on the ocean floor as polyps for years then spawning only when conditions are favorable.

barrel jellyfish-Kynance Cove Cornwall UK

Barrel jellyfish off the coast of Cornwall, UK



Black sea nettle

Adapting to Change

The elegant and efficient structure of the jellyfish may be responsible for the group’s diversity and its long success. Being mostly water; the jellyfish’s remaining 5% contains the barest essentials needed to capture, consume, and use energy. They move using an elastic ring that contracts to rapidly expel water and propel it forward. Providing one of the most efficient modes of locomotion. Their trailing tentacles brush against prey, immobilizing them and conveying them to the jellyfish’s mouth — after which the jelly digests its prey in its body cavity then expels the remaining parts of the prey through the same opening.

The lack of complex physical features also makes jellies highly adaptable. Factors that restrict other marine animals—such as temperature, acidity, salinity, light, or darkness—don’t bother jellies. “They combine plantlike simplicity, animal-like mobility, and an almost bacterial ability to reproduce rapidly under favorable conditions,” writes Juliet Lamb of JSTOR Daily.

aurelia-aurita-moon jellyfish

Moon jellyfish

Along certain coastlines, where high concentrations of agricultural nutrients cause plankton to grow explosively, “dead zones” of depleted oxygen have been created.

“Most marine life can’t survive in an oxygen-deprived environment — except for jellyfish. Bloodless and brainless, jellyfish are able to exist with very little oxygen. Far from inhospitable, dead zones become competition-free plankton buffets. Once jellies take over, their tendency to consume fish larvae makes it difficult for other species to re-colonize the area even after oxygen levels return to normal,” writes Lamb.

Given their ancient beginnings, jellyfish have adapted to virtually all ocean ecosystems from the bottom of the sea to warm shallow coastal waters. They’re described in “swarms”, like insect infestations. And like insect pests, they tolerate harsh conditions and find ways to exploit catastrophic occurrence.

Sea wasp alatina alata

Sea Wasp (Chironex Fleckeri) Box jellyfish

Adapting to Climate Change

“When a stressor like climate change or overfishing opens up a niche for them they can really take advantage of that and rapidly proliferate,” said Lucas Brotz, a researcher at the University of British Columbia. “They can make millions and millions of copies of themselves and clone asexually. That’s when you get these massive blooms. I think that’s the secret to the success of jellyfish, the reason they’ve been around for hundreds of millions of years.”

Jellyfish appear ideally suited to change, particularly the stressful kind of change humanity is currently imposing on the environment. As overfishing wipes out their competitors and predators, jellies adapt. They have no problem living in warmer water. They proliferate in nutrient-enriched polluted waters, where other organisms fail due to low oxygen. Coastal developments provide convenient, safe habitat for their polyps to hide. In addition, “the great mixing of species transported across the world in the ballasts of ships opens up new, vulnerable ecosystems to these super-adaptors,” adds Mathiesen.

bluebottle-jellyfish shore

Blue Bottle Portugese Man of War jellyfish washes up on shore

Adapting to Humans

Many scientists agree that jellyfish populations are increasing worldwide. The rise in jellyfish throughout the world’s coastal areas has been reported from the UK to the Mediterranean Sea, the northeastern Atlantic, the Black Sea, and the Sea of Japan—to name just a few of the most prominent blooms.

The links between human activity and local jellyfish blooms are strong. In the Black Sea, invasive comb jellies dumped from the ballast of tankers have spawned greatly and destroyed the region’s fishing industryIn the Sea of Japan, fertiliser run-off has left an oxygen-depleted sea (“dead zones”) where little other than jellies can thrive. Early in the twenty-first century, blooms of massive Nomura’s jellyfish began occurring annually, when previously the blooms typically occurred once every forty years. Authorities blamed the sudden jellyfish abundance on agricultural runoff from neighboring China, which may have provided favorable conditions for jellies to spawn. The six-foot wide jellies clogged fishing nets, devastated fish populations both in fish farms and in the wild, and even caused a trawler to capsize.

Human Effect Jellyfish

Combined Effect of Humanity’s Activities on Jellyfish Populations

Despite these observations, many scientists remain reluctant to definitively conclude that jellyfish blooms are increasing with global warming. Despite the increased frequency and intensity of swarms in certain places, jellyfish remain elusive and are notoriously difficult to study—blooming unexpectedly in inaccessible places. Only a slight upward trend of blooms in recent years over the natural fluctuations previously observed could be acknowledged due to lack of sufficient data: not enough to go beyond the range of natural variability. Some coastal communities would laugh hysterically at this demure conclusion.

LionsMane Jellyfish

Lion’s mane jellyfish

Though the jellyfish-climate link was not refuted, the scientific community did suggest significant human-related causes such as:

Overfishing & Dismantling the Food Chain: Overfishing of predators of jellyfish has an obvious consequence of creating a trophic cascade in which jellyfish thrive in the absence of their predator. The greater consequence is that the jellyfish eat the eggs of smaller fish, as well as their food suppy—further exacerbating the cascade.

Increasing Polyp Habitat: Centuries ago, polyps clung to the hard surfaces available such as rocks and oyster shells; now they can cling to human-made structures such as piers, drilling platforms, plastic cigarette packets, offshore wind turbines, boats, etc.

Coastal Eutrophication: Fertilizers from farms and sewage from rivers first create organically enriched bays, leading to algal blooms, which, when they die off, create “dead zones” of oxygen-depletion. The unique physiology and structure of the jellyfish allows it to thrive in low oxygen when most marine life dies.  The best example comes from China, where pollution from the Yangtze River in western China has formed huge dead zones in the East China and Yellow Seas. Scientists believe that dead zones are behind the surge in Nomura jellyfish in Japan.


Nomura jellyfish off the coast of Japan

In the meantime, Japan is investigating ways to transform over-abundant jellies into an ecosystem service. One is to eat them.

jellyfish and shredded chicken

Jellyfish with shredded chicken




Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press (Vancouver) was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” will be released by Inanna Publications (Toronto) in 2020.

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