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Why Jellyfish are taking over the world - Lisa-ann Gershwin’s “Stung! On jellyfish blooms and the future of the ocean”

Move aside Steven King, jellyfish are worse than any of your demons, worse than any Grade-B monster that’s graced the silver screen. Unlike The Blob, which can be stopped by freezing, you can’t kill them. Not with chemical repellents or biocides or nets or electric shocks or introducing species that eat jellyfish like the striped sea slug. If you shoot, stab, slash, or chop off part of a jellyfish, it can regenerate lost body parts within two days. Not even the past 5 major extinction events which killed up to 90% of all life on earth, killed off the jellyfish.

Meanwhile they’re on a rampage, doing millions of dollars in damage clogging intakes of nuclear, coal, and desalination plants, killing millions of farmed fish, and destroying fishing nets with their sticky icky bodies.

The more we overfish, pollute, acidify and warm the ocean, create vast dead zones, and trawl ocean bottoms, the better the jellyfish do.

The oceans make the earth habitable for us. They generate most of the oxygen we breathe, stabilize temperatures, drive climate and weather, and absorb a third of the CO2 we’re emitting. Over 3 billion people depend on the oceans for their livelihoods; 2.6 billion depend on seafood as their main source of protein.

Most alarming of all, 40% of phytoplankton has died off globally since the 1950s – they’re not only at the base of the food chain, but they generate most of the oxygen we breathe, as well as absorb half of the carbon dioxide, and their increasing death rate will make the ocean get warmer even faster.

Why Jellyfish are taking over the world

Prolific, hard to kill, breed fast, and more – no wonder they’re so successful:

They’ve everywhere, spread around the world in ship ballast or sea currents.
Ubiquitous – from top to bottom of the ocean, from pole to pole, year-round.
Grow faster than other species to quickly take advantage of any food, and they’ll eat almost anything — copepods, fish eggs, larvae, flagellates. They eat past when they can keep consuming, spit food out, waste a great deal other creatures could have eaten. Even when they’re full, their tentacles keep capturing prey.
If there’s no food, jellyfish can consume their own body mass and get smaller and smaller until they find food again, and rapidly return to normal. Even when they grow smaller they can still reproduce.
Consume many times their body weight in high-value food but are of low-value themselves because they provide little energy, ounce for ounce, compared to the food they ate. So they have few predators.
When 2 weeks old they can lay 10,000 eggs a day that hatch 12-20 hours later
They reproduce many ways: massive orgies, fission, fusion, cloning, hermaphroditism, external fertilization, self-fertilization, copulation.
If they lose a body part, they can regenerate it within 2 days.
They are the “Last Man Standing” in eutrophication zones because they need less oxygen
Many species can tolerate any salinity level, from fresh water to salt water
They’ve survived ice ages, hothouse climates, all five mass extinctions, predators, competitors, and us.
Jellyfish in the oceans have been known to live over 10 years
Many of them avoid predators by long vertical migrations from the deep sea to the surface at night and back down again by daylight

They can wait a long time for the right conditions to bloom

Just as plants have seeds which can endure many years waiting for optimum conditions to grow, jellyfish have a seed-like state called a polyp that waits for good conditions, and can clone themselves to create armies of ‘seeds’ waiting to burst into jellyfish blooms seemingly overnight. Polyps don’t “grow up” to become jellyfish. They spawn what we think of as jellyfish – the medusa — which then mate sexually to produce polyps, which stick to rocks, shells, man-made structures, plastic, etc. Both the polyps and the medusa could be considered “immortal” – when a polyp dies it’s clones live on, and there is one species of jellyfish, where after it dies, its pieces turn back into polyps (“Logically, it would seem that other species probably do it too, but we have yet to identify others,” according to Gershwin in a reply to this book review).

Jellyfish are at the top of the food chain

That seems so wrong– a primitive brainless blob? But jellyfish eat much larger clams, crabs, starfish, snails, and fast, smarter fish and squid.

They’re also at the top because not much wants to eat them.

Worse yet, they outcompete other sea life by devouring the eggs and larvae of species that would have grown up to eat jellyfish larvae. It’s a double whammy since these larvae never grow up, leaving a lot more food for jellyfish to consume. A jellyfish bloom can clear the water of all eggs, larvae, copepods, and small plankton in less than a day. This makes it almost impossible for some overfished species to make a comeback.

We’re helping the jellyfish take over by overfishing

Many of the small fish that compete with jellyfish for the same food, such as anchovies and sardines, are being overfished and turned into farmed fish food, pet food, and fertilizer. We harvest a whopping 44% of these small fish at the base of the food chain, which are also what cod, snapper, tuna, and halibut feed on, which prevents the recovery of fish we’d much rather eat.

We’ve already fished out 90% of all large fish in the ocean. And it’s only a matter of time before we find the other 10% with sonar, radar, LORAN, GPS, and spotter aircraft.

The United Nations has predicted all commercial fish species will be extinct by 2048. In 2002 we were fishing 72% of fish stocks faster than they could reproduce. 90 fish stocks around the world have had no recovery in population even 15 years after they collapsed.

Few small fish left, few big fish left – that opens up a lot of space for jellyfish to move in and take over. We’re creating a feedback loop that favors jellyfish.

Worse yet, overfishing can create trophic cascades when we remove keystone predators. We’ve nearly driven 11 species of large sharks along the Atlantic coast into extinction. They kept the ray population in check, but now that they’re gone, the ray population has exploded, and they’re devouring almost a million tons of scallops, clams, and oysters a year. Fishermen only harvested 330 tons. The Chesapeake used to famous for shellfish, now it’s best known for its jellyfish (p261-263).

You’ve probably heard of bycatch – all the unwanted and unintended dolphins, turtles, fish and so on that are discarded, most so mangle they don’t survive when thrown back. I was unaware that tropical shrimp are the worst of the worst because they’re obtained by bottom trawling and have a bycatch of 125 to 830% more than the shrimp captured. In the Gulf of Mexico shrimp fishery 12,000,000 juvenile snappers and 6,000,000 pounds of sharks are discarded every year. Since most bycatch is unreported, these figures are probably too low. Further destroying the fish are the thousands of miles of “ghost nets” – the nets lost from boats that drift aimlessly still catching fish.

Jellyfish even eat other jellyfish, so when we’ve caught most of the fish, or otherwise destroyed them by dredging, ocean warming and acidification, pollution, dead zones, etc., jellyfish will still survive.

Trawling and Sewage favor jellyfish

Sewage provides nourishment for jellyfish since they can get 10 to 40% of what they need by absorbing nutrients through their skin. And there’s plenty of sewage for them. In just 7 days a 3,000 passenger cruise ship generates 210,000 gallons of sewage, a million gallons of gray water, 37,000 gallons of oil bilge water, 8 tons of solid waste. In the USA, animal feedlots produce 500 million tons of manure a year, 3 times as much as humans.

Bottom-trawls weigh thousands of tons and rake the seafloor for sole, halibut, cod, haddock, plaice, rockfish, rays, skates, prawns and son on, destroying corals and sponges as trawls rake across miles of seafloor, crushing what isn’t scooped up. The raking creates a fog of tiny particles. Fish can’t find their food in this dense fog of raked up particles or murky sewage, but guess who can….jellyfish, who just dangle their tentacles and it capture any food that drifts or swims into them.

Trawling dredges up toxic DDT, PCBs, hydrocarbons, mercury, radioactive particles, heavy metals, and plastics that add to eutrophication, destroy clams, scallops, bryozoans, tunicates, and other creatures. These substances, which had been buried in the sediment and removed from the food chain are released back again, and incorporated into the muscle, bone, blood, and fat of sea organisms.

Jellyfish don’t have these tissues, so they’re not much affected. Nor do they live long enough to store a high concentration of harmful toxins, or develop mutations or cancer.

Dredging creates many more areas for jellyfish polyps to attach to as pieces of plastic and other flotsam are dredged up, increasing the size of jellyfish blooms.

Jellyfish can take the heat

As climate change raises temperatures, the metabolic rate of all creatures rise, and they have to catch more food to stay alive.

The ocean has risen 1.8 F the past century, most of that the past 30 years, and may increase another 3.6 F over the next 100 years. In the ocean heat is even harder on organisms because warmer water has less oxygen. This means increased respiration which uses more energy and finding more food to eat. A creature that can’t respire fast enough will suffocate.

Warmer oceans are a dream come true for jellyfish – they can grow fast very quickly while other species are struggling. Phytoplankton blooms make even more food available. Jellyfish rates of reproduction increases and they can reproduce longer too.

Climate change also means far more unpredictable weather, another advantage for jellyfish, since they respond quickly to change and bloom explosively to cope. They’re the first to arrive and the last to leave. And jellyfish can tolerate a wide range of temperatures.

Jellyfish can even increase CO2 levels because

Their goo and poo are preferred by bacteria that emit high amounts of CO2.
Jellyfish displace fish, whose fecal pellets would have sunk to the bottom and sequestered CO2

We’re tipping the ecosystem in their favor

The more we damage and stress the ocean, the more likely the sedentary polyps will feel compelled to produce the next generation, the getaway medusa jellyfish who can escape the eutrophication, warming temperatures, changes in salinity, pollution, acidification, oil spills, or whatever else we’ve thrown at them. The medusa disperse to safer areas, and new areas, live to see another bloom, and eat and outcompete fish.

Dead zones, eutrophication, hypoxia favor jellyfish

Jellies can survive low oxygen conditions because they store oxygen in their tissues and breathe through their skin. They can swim in the top layer of water above and form a wall of slime that keeps fish out.

They can cause eutrophication by eating so many copepods that phytoplankton blooms erupt, die, and tilt the balance towards flagellate-based organisms, which jellyfish eat but fish don’t. And also their goo and poo favors microbes that respire a lot which generate CO2 and increase ocean acidification. Jellyfish can survive low oxygen levels better than most creatures.

The more jellyfish, the more jellyfish

As we create conditions that favor larger jellyfish blooms, their concentrations grow more dense, so when they release sperm and eggs the odds of contact and fertilization are greater.

And the greater the density of jellyfish, the more likely prey will be unable to escape. Nor will the small predators of jellyfish larvae be able to do so – the dense numbers of parent medusae will eat the small predators before their own larva can be consumed.

Larger jellyfish blooms makes even larger jellyfish blooms more likely, ratcheting up their ascent to dominance in the oceans.

Farmed fish won’t keep fish around – jellyfish kill them too

Jellyfish harm salmon farms through their mucous, bacteria, and stinging. The salmon waste and uneaten food also probably change the ocean to favor jellyfish and algal blooms.

Other jellyfish facts

- There are 1,500 known species of jellyfish, but probably quite a few more we haven’t identified yet
- They have no heart, brains, ears, heads, feet, gills, or bones
-They range from the size of a pea to 8 feet in diameter with tentacles that can be 200 feet long
- Kinds of jellies: moon, comb, pink meanies, rainbow, box, fire, sea wasps, sea nettles, sea gooseberries,Venus’s girdles, lion’s manes, purple people eaters, blubbers, snotties, agua vivas, blue bottles, the long stingy stringy thingy, etc.
-They’ve been here at least 565 million years practically unchanged, long before predators with shells or teeth evolved
-The Box Jellyfish is the world’s most venomous animal that can kill within 2 minutes. There are other lethal jellyfish as well.

Conclusion – We’ve turned the tide in favor of Jellyfish

This is one of the best books you can read about the myriad ways we’re destroying the ocean, which Gershwin has to explain so that she can then explain how that relates to how those factors affect jellyfish. Gershwin’s writing is witty and funny, making this grim topic easier to take. The natural history of jellyfish is amazing and bizarre. And despite this long book review, I’ve left out quite a bit, the story is far too complex to summarize — I hope you’ll read this book to learn more.

Even if we stopped overfishing, polluting, and so on, once we tip the ecosystem into one controlled by jellyfish, they will become the “new normal” and that will quite likely be impossible to change.

What a dismal future — an ocean of slimy, repulsive, stinging, sticky, lethal, spooky, scary, alien jellyfish. Bye-bye fish, oysters, shrimp, scallops, lobsters, Beluga caviar, abalone, sharks, whales, seals, sea lions, penguins, dolphins, sea otters, polar bears. Hello jelly-O.

The time when jellyfish rule is not far away, it could be in your lifetime, or your children’s lifetime. The climate and chemistry of the ocean is becoming like the Ediacaran ocean 565 million years ago, when jellyfish ruled the oceans for over 100 million years as the top predators.

In the last chapter, Gershwin writes that in the end, jellyfish are “also outcompeting the human race, because we depend on the oceans’ fish for our own food.”

Gershwin wrote this book assuming she’d have advice at the end of actions you could take to bring back the fisheries and keep jellyfish from dominating the oceans, but she ends the book saying it’s too late to do anything. Hold the presses — perhaps not, Lisa replied to this book review and said “I welcome thoughts that you or your readers may have toward saving the oceans and fixing the damage… the subject of my next book!”.

I like Gershwin’s honesty, and the willingness of the University of Chicago Press to publish her book, since most publishers won’t print a book that doesn’t have a happy ending (and also why our political and economic leaders deny or don’t talk about peak oil, climate change, and other insoluble problems.)

When will the fish, whales, dolphins, etc., return?

People have asked me when the fish would come back, since after all, they’re here now, they must have defeated the jellyfish in the past. That’s why you need to read this 344 page book. the ocean ecosystem is complex and Gershwin spends most of the book explaining how it works in order to then say how this relates to jellyfish. I’ve only reported on jellyfish part of what she wrote.

One important concept I didn’t cover was on low versus high-energy food chains, since that’s a big part of why the ocean is tipping in favor of the jellyfish, who do better in a low-energy system like the Ediacaran oceans hundreds of millions of years ago (read pages 288-344).

We’re returning the oceans to an Ediacaran state — warm oceans favor jellyfish, low energy food chains favor jellyfish, low oxygen favors jellyfish, ocean acidification favors jellyfish, billions of jellyfish consuming most fish eggs, larva, and juveniles favors jellyfish, ability to catch food in murky water favors jellyfish, their ability to bloom and grow faster than any other creature, humans removing most of the jellyfish predators and competitors from overfishing, the amazing adaptability of jellyfish, their being at the top of the food chain, and the synergy of all of these and the dozens of other factors above.

When this becomes a stable state, how do you get back?

“The Earth without us” gave me great hope. Because we’re at peak fossil fuels the climate change scenarios won’t be as bad as the worst forecasts (perhaps), without oil there will be only a billion people or less, who can’t do nearly as much harm without oil-powered vehicles and combustion engines.

A day will come when the earth cools, oceanic oxygen and pH levels go up, and fish and sea mammals will return. If they’ve survived, that is. The problem with an extinction like this 6th one we’re causing is that the hangover can last for millions of years before evolution refills the lost niches of extinct creatures, sigh.

Alice Friedemann


It's the stuff of a B-Grade horror movie! The oceans being overwhelmed by these jelly-slimy monsters. It's the stuff of a computer or arcade game -spot and destroy the enemy!
Prolific breeders, high volume of consumption, hard to control their numbers, threatening other species, taking over resources, unbalancing ecosystems and "managing" the oceans for their own interests! It's the marine version of what humans are doing to the rest of the planet.