15 Astonishing Insights About Jellyfish

Vibrant, perilous, and utterly captivating—jellyfish are far more complex than they seem. Discover 15 shocking facts about these fascinating ocean dwellers.

1. Dead jellyfish can still deliver a sting.

Even after death, the venomous cells in a jellyfish’s tentacles can remain active. For instance, in 2010, around 150 beach visitors in New Hampshire were stung by the detached tentacles of a deceased lion’s mane jellyfish.

2. Jellyfish consist of 95 percent water.

Jellyfish may appear as soft, translucent blobs, and their composition reflects this. They are composed of 95 percent water, with the remaining 5 percent consisting of minerals and proteins. Between their two skin layers lies a gel-like, water-based material known as mesoglea, which houses muscle cells, nerve cells, and structural proteins.

3. Jellyfish have a knack for disrupting nuclear power plants.

Nuclear facilities in Scotland, Sweden, California, Israel, and Japan have all experienced shutdowns due to massive jellyfish invasions. These plants rely on external water sources to cool their reactor cores, and when jellyfish infiltrate these systems, they can block the flow, prompting emergency shutdowns.

4. A certain jellyfish species can reverse its aging process.

The Turritopsis dohrnii, often referred to as the “immortal jellyfish” or the “Benjamin Button jellyfish,” has the unique ability to revert its life cycle at any point, transforming back into a polyp and restarting its development. Researchers speculate that this backward aging mechanism might hold the secret to eternal life.

5. Jellyfish predate dinosaurs.

Jellyfish have thrived in the world’s oceans for more than 650 million years, making them older than both sharks and dinosaurs.

6. A jellyfish species bears Frank Zappa’s name.

The Phialella zappai was christened in tribute to Frank Zappa, the beloved musician of the scientist who identified it. Zappa reportedly remarked, “There’s nothing I’d cherish more than having a jellyfish named after me.”

7. Jellyfish have ventured into space.

In 1991, NASA launched 2478 jellyfish polyps into space as part of an experiment titled “The Effects of Microgravity-Induced Weightlessness on Aurelia Ephyra Differentiation and Statolith Synthesis.” The jellyfish were housed in flasks and bags filled with artificial seawater, and astronauts introduced chemicals to stimulate reproduction. By the experiment’s conclusion, around 60,000 jellyfish were orbiting Earth.

8. Jellyfish lack traditional organs.

Jellyfish don’t possess lungs, intestines, or stomachs. Instead, they rely on a basic yet efficient system to function. Their bodies consist of two cell layers—the outer epidermis and the inner gastrodermis. The gastrodermis features a single opening for eating, excreting waste, and reproductive activities. Oxygen and nutrients are absorbed directly through the walls of both their inner and outer cell layers.

9. Jellyfish vary dramatically in size.

The lion’s mane jellyfish holds the title for the largest species, with a diameter reaching up to 6 meters (nearly 20 feet) and tentacles stretching as long as 50 meters (164 feet). On the other end of the spectrum, the common kingslayer is smaller than a fingernail and ranks among the most venomous organisms on the planet.

10. Jellyfish play a crucial role in medical research.

Researchers at the Mayo Clinic once injected unfertilized cat eggs with a green fluorescent protein derived from crystal jellies, along with a gene from rhesus monkeys known to inhibit the feline AIDS virus. The jellyfish protein served as a marker to confirm the gene’s successful transfer. When the kittens were born, they emitted a bright green glow under black light, proving the experiment’s success.

11. A collection of jellyfish was historically termed a “smack.”

While this term has fallen out of favor, it’s now more common to refer to a large group of jellyfish as a "swarm."

12. Jellyfish can reproduce both sexually and asexually.

Jellyfish engage in sexual reproduction by releasing sperm and eggs into the water, forming free-swimming larvae. These larvae develop into polyps that attach to surfaces and can bud off multiple young jellyfish, enabling asexual reproduction.

13. Scientists have created a human-sized robotic jellyfish.

Inspired by jellyfish, researchers at Virginia Tech developed a 170-pound aquatic robot named Cyro to explore self-powered underwater robotics. Jellyfish’s energy-efficient propulsion system makes them an ideal blueprint for future autonomous underwater machines.

14. Jellyfish are a source of food.

While sea turtles are known to snack on jellyfish, humans also enjoy them. Blubber jellies, for instance, are a prized dish in some Asian regions. In one creative example, students at a Japanese high school used powdered jellyfish to craft salted caramels.

15. Jellyfish possess the simplest nervous system among multicellular animals.

Jellyfish lack brains and instead rely on a “nerve net” to interpret sensory input. Structures like statocysts help them determine orientation, while rhopalia detect light, chemicals, and water movement. This basic nervous system, also found in hydras and anemones, is the most primitive form in multicellular organisms.

Originally published in 2015, this article has been refreshed for 2021.