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

Editor’s note: This Care2 favorite was originally posted 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 , who may not be 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 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.

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. They can be deadly

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. They have unique body structures

Jellyfish are invertebrates but some, such as box jellyfish, can hunt medium-sized fish and crustaceans. The box 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.


Photo Credit: Justine/Unsplash

3. They’re powerful en masse

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.

Photo Credit: Thinkstock

4. They’re 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.

Photo Credit: Thinkstock

5. They’ve formed a ‘curtain of death’

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.


Photo Credit: naomi tamar/Unsplash

6. They’re ancient

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. Their range is expanding with climate change

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.


Photo Credit: Joel Filipe/Unsplash

8. They impact the world’s carbon cycle

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.


Photo Credit: naomi tamar/Unsplash

9. They may have both male and female sex characteristics

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

Photo Credit: Thinkstock

10. They’re not picky eaters

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.”

Photo Credit: Thinkstock

11. But they don’t require regular food

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.

Photo Credit: Bart Heird/Flickr

12. They’re edible

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.

Photo Credit: Brittany Rockwell/Unsplash


Lesa DiIorio
Past Member 8 months ago

thank you Kristina...

Marie W
Marie W11 months ago

Thanks for sharing.

Paulo R
Paulo Rabout a year ago

very interesting. ty

Stephanie s
Stephanie Yabout a year ago

Fascinating creatures. The human race should be banned from fishing and eating your typical fish, instead Jellyfish should be the only fish consumed worldwide. This would restore all fish species to normal numbers while saving several from extinction.

Debra Tate
Debra Tabout a year ago

Thanks. I will be staying out of the water!

One Heart i
One Heart incabout a year ago


Patrice Z
Patrice Zabout a year ago

Very interesting. Thanks for sharing.

Marge F
Marge Fabout a year ago

Thank you for posting this informative article.

Janis K
Janis Kabout a year ago

Thanks for sharing.

Morgan I
Morgan Iabout a year ago

Jellyfish are thriving because the sea turtles are declining. The ecosystem works if humans stay out of the cycles and don't ruin it..