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A jellyfish is a very simple animal.

But what exactly is a jellyfish? A jellyfish isn’t a fish but an invertebrate, which means it doesn’t have a backbone. In fact, it doesn’t have much of anything. Jellyfish don’t have a brain, a heart, or even blood, and have a very simple digestive cavity with a single opening for eating and expelling waste. What they do have is water – lots of it. Jellyfish are about 95% water. This makes them highly camouflaged in the ocean. Going a little deeper, the body of the jellyfish is divided into three main parts: the bell, the oral arms (long appendages that move captured prey into their mouths), and the stinging tentacles.

Jellyfish come in a variety of shapes, colors, and sizes.

Though the basic parts of the jellyfish are fixed, the bells, oral arms, and tentacles can be different shapes, sizes, and colors. Thought to be the smallest jellyfish in the world, the Irukandji jellyfish has a bell that only reaches a maximum of 25 millimeters across, about the size of a quarter. A species of box jellyfish, they are one of the most venomous jellyfish in the world despite their tiny size.

The lion’s mane jellyfish (featured in the Sherlock Holmes story “The Adventure of the Lion’s Mane”) is the largest known species of jellyfish by length, reaching up to 120 feet from the top of the bell to the bottom of the tentacles. This is about the length of 4 school buses, making it longer than a blue whale! Nomura's jellyfish are the largest by weight; they can weigh up to 450 pounds. That’s the weight of a piano!

Are jellyfish older than dinosaurs?

Jellyfish have been around for more than 500 million years. That means they appeared more than 250 million years before the first dinosaurs. However, because jellyfish are soft-bodied and almost all water, jellyfish fossils are incredibly rare. Of those that do exist, the oldest-known jellyfish fossils, found in Utah, date to 505 million years ago and have enough detail to show clear relationships with some modern species of jellyfish.

Like butterflies, jellyfish undergo metamorphosis.

Have you ever wondered what a baby jellyfish looks like? What we usually think of as a jellyfish is called a medusa. Medusas lay eggs. Eggs grow into larva called planula – which have been described as looking like furry tic-tacs – and they start swimming until they find somewhere to stick themselves. Once a planula finds a rock, dock, or other place to attach itself to, it stretches into a tube called a polyp. When a polyp is ready, the tube becomes longer and separates out into a series of snowflake-like discs. Each disc will begin to wiggle then pop off the stack. The disc, called an ephyra, is a baby medusa. It pumps its body to swim away. It can grow an inch every few days until it becomes mature medusa. 

There is a species of jellyfish that is basically immortal.

Now that we’ve talked about the jellyfish life cycle, the real fun can begin. Turritopsis dohrnii, a species of tiny jellyfish discovered in the Mediterranean Sea, can turn from medusas into polyps when damaged or starving. This would be like a butterfly turning back into a caterpillar or a frog turning back into a tadpole. T. dohrnii can go back and forth between its polyp and medusa stages, leading to it being known as “the immortal jellyfish.” Further research shows that other species of jellyfish may be able to reserve-age, too. Studying the cells of these jellyfish has potential uses for medicine.

Some jellyfish get sleepy.

Since they lack a brain, jellyfish have a very different kind of nervous system from many animals. Jellyfish have what are called “nerve nets,” which are loose networks of neurons and sensors spread out across their bodies. Even with this very simple nervous system, jellyfish can carry out a variety of behaviors, including some once thought impossible. A 2017 study showed that one type of jellyfish (Cassiopea, or the upside-down jellyfish) enters a sleep-like state at night and were sluggish when they didn’t get a full night of sleep. This was the first time an animal without a brain was observed sleeping!

Glow-in-the-dark jellyfish revolutionized biotechnology.

Bioluminescence is the ability of living things to make light through chemical processes. The jellyfish species Aequorea victoria, also known as the crystal jelly, glows bright green due to both bioluminescent and fluorescent proteins. The green fluorescent protein (GFP), which glows green under blue light, has since been cloned and inserted into other organisms’ genetic codes, allowing scientists to literally see how genes and cells work. According to Juli Berwald, scientists have used fluorescent proteins made from GFP to see how bacteria divide; how cancer, Alzheimer’s and HIV affect cells; to trace neurological pathways in the brain; to test for diseases like malaria and ebola; to build solar cells, and to make low-temperature, energy-efficient lasers.

Scientists are making robo-jellyfish.

Scientists and engineers have created robotic jellyfish to assist them in studying the ocean. In 2018, Erik Engeberg, an associate professor of engineering at Florida Atlantic University, and his team tested prototypes of a soft-bodied robot that moves like a jellyfish. The robot can monitor and study the underwater environments of coral reefs without harming them, since these robo-jellyfish are quieter and safer for marine life than underwater drones. Dr. Edie Widder developed an electronic jellyfish as a lure to attract large, deep-sea predators. Her e-jelly, which used the bioluminescent patterns of the jellyfish Atolla wyvillei, was used to capture the first video footage of a living giant squid in 2012.