12 Terrifying Facts About Jellyfish and Why They’re Taking Over

Editor’s note: This post is a Care2 favorite. It was originally published on September 18, 2013. Enjoy.

Some say that cockroaches, those survivalists par excellence, could inherit the earth. If they do, it’s likely they will be joined by jellyfish populating the oceans or whatever might remain of them. As scientist Lisa-ann Gershwin details in her book, “Stung! On Jellyfish Blooms and the Future of the Ocean,” jellyfish in vast (really vast) numbers are now showing up all over the world, from the Black Sea to the coasts of Britain, Israel and Brazil.

Jellyfish blooms are a lot more than a nuisance to beachgoers not inclined to swim in waves teeming with gelatinous blobs and tentacles that can sting and poison. What’s going on now, as Tim Flannery writes in reviewing Gershwin’s book in the New York Review of Books, is nothing less than the jellification (a term used even by scientists) of the ocean with far-reaching consequences and in no small part due to human activity.

Fossils of this gelatinous marine animal are the oldest ever found. The notable upsurge in their numbers is a very recent development and downright alarming for several reasons.

1. For all that they lack backbones, a heart, blood, a brain or gills and are about 95 water, jellyfish can kill. The box jellyfish, Chironex fleckeri, has a bell (the jellyfish’s head) that’s about a foot across, attached to 550 feet of tentacles. 76 people have died from contact since with the box jellyfish since 1884. As Flannery writes, “if just six yards of tentacle contact your skin, you have, on average, four minutes to live — though you might die in just two.”

2. Jellyfish are invertebrates but some, such as box jellyfish, can hunt medium-sized fish and crustaceans. The ox jellyfish does have some unique features that set it apart from other jellyfish: it has eyes with retinas, corneas, and lenses and a brain that can learn, remember and direct complex behaviors (like swimming 21 feet per minute.)

3. Get enough jellyfish together and they can bring down a ship. That’s just what happened in November of 2009, when a net of gigantic jelly fish (the largest was 450 pounds) capsized a Japanese trawler and knocked its crew of three overboard. Millions of jellyfish also caused a major coal-fired power plant in the Philippines to shut down in December of 1999. They’ve also been clogging up the cooling systems of nuclear power plants in Japan and India since the 1960s.

4. Jellyfish are intrepid travelers. Stowed away in the ballast water of ships, jellyfish have made the journey from the U.S. east coast to the Black Sea and been responsible for the vanishing of anchovies and sturgeon in Bulgaria, Romania and Georgia. Jellyfish eat the eggs and young of anchovies as well as the same food of adult anchovies, who then starve to death.


5. A 30,000 square mile “curtain of death” now exists off the coast of southern Africa. In 2006, a biomass of 13 million jellyfish far exceeded the 3.9 million tons of fish.

6. Jellyfish have been around since about 550 million years ago. As indeed “survivors of an earlier, less hospitable world,” they can thrive in conditions that other species would find challenging or deathly. For instance, thanks to having a low metabolic rate, their oxygen requirements are low.

7. Accordingly, jellyfish — including those only found in tropical waters such as the box jellyfish and its smaller cousin, the Irukandjis – are poised to extend their ranges as the ocean warms up and oxygen levels decline, “thanks” to phosphorus and nitrogen from agriculture and industrial human activities. Those two substances and whatever else is in fertilizer runoff from farms are now filling our oceans, to jellyfish’s benefit.

8. Jellyfish add to the carbon content of the ocean via the feces and mucus they release into the water and due to their feeding on copepods and plankton. The latter creature helps to take carbon dioxide out of the oceans and atmosphere by consuming carbon-rich food at the surface and then expelling it as pellets that fall to the ocean floor. With jellyfish eating so many of them, the amount of “carbon reducing” creatures has been severely reduced.

9. Jellyfish can be male, female or hermaphrodites, and some can reproduce all on their own. One type, Mnemiopsis, starts layings eggs at the tender age of 13 days and can soon lay more than 10,000 a day.

10. Jellyfish “can eat anything, and often do,” Gershwin notes. They can eat a lot (ten times their body weight in food), too, and are even by nourished just by absorbing “dissolved organic matter through their epidermis.”

11.  That said, jellyfish can get by fine without eating; their bodies become smaller but remain in proportion. One type, Turritopsis dohrnii, could be called immortal. After it dies, some cells leave its rotting body and recombine into a polyp, a small creature like a sea anemone which, after affixing itself to a stable surface, develops into a stack of small jellyfish.

Credit: Bart Heird via flickr

12. They sting and they can be poisonous, but — the last “terrifying fact” about jellyfish I’ll note — you can eat them. Not the box jellyfish but some types, including rhizostomes, are harvested by the Japanese, Chinese and others in Asia. The tentacles can be dried and stored for weeks and then cured with vinegar and cooked or eaten raw.


21,000 tons of jellyfish are harvested a year and consumed mostly in China and Japan; Gershwin suggests that eating at least some jellyfish could help control their numbers.

Flannery is dubious about jellyfish as the next big culinary trend. Figuring out how to stem the global jellification of the ocean should be our concern first and foremost. Gershwin urges us to take action, push for policies and legislation to protect our oceans and keep them hospitable for all the creatures of the sea and not just millions of jellyfish.

Photos from Thinkstock, Photo Credit: Bart Heird, Son of Groucho


Maria R
Maria R12 days ago

Thanks for sharing

Marija M
Marija M13 days ago


Peggy B
Peggy B13 days ago


ellie d
Ellie M15 days ago


Glennis W
Glennis W15 days ago

Great information and advice Thank you for caring and sharing

Glennis W
Glennis W15 days ago

Very interesting article Thank you for caring and sharing

Mark Donners
Mark Donner1 years ago

Hopefully the jellyfish will learn to sink all the fishing trawlers. They are better than humans.

Melania Padilla
Melania P2 years ago

They are so pretty!

Joseph Glackin
Joseph Glackin2 years ago

Christine J.--

The problem is man usually has only one way to stop predation of any species------

Christine Jones
Christine J2 years ago

Very worrying. Perhaps if we stopped decimating the large marine predators (tuna, sharks, etc), the problem would be solved. Such imbalances are usually caused by human interference. We have some really deadly jellyfish here in Australia so it's a real issue.