Jellyfish, also known as jellies, inhabit all the world’s oceans and even some freshwater lakes. They have been around for over 600 million years, even before the dinosaurs!
But did you know jellyfish are not actually fish? They are gelatinous animals without backbones. In fact, jellyfish have no bones, brains, hearts, blood, or teeth!
What is a Jellyfish Made of?
A whopping 95% of a jellyfish is water. Their bell-shaped body consists of three layers: the epidermis (outer layer), the mesoglea (a thick, jelly-like substance in the middle), and the gastrodermis (inner layer).
Jellies have a very basic nervous system called a nerve net, which can smell, sense light, and react to stimuli. They also have transparent muscles which allow them to pulse and move forward in the water. They have one digestive cavity, which acts as both their stomach and intestine.
In addition to their bell-shaped body, jellyfish also have tentacles.
How Big are Jellyfish?
Jellyfish range in size depending on the species. They can be as small as one centimeter (under .5 in.) like the Irukandji jellyfish, or they can be over 30 m (100 ft.) long like the lion’s mane jelly. That’s as long as a blue whale!
Jellyfish Colors
Most jellies are transparent, but they can be pink, red, orange, yellow, blue or purple as well. Some jellyfish are red because it provides camouflage. You might not think red would blend in very well in the water, but red cannot be seen in deep, dark water.
Jellyfish Bioluminescence
Some jellyfish have the ability to glow in the dark. This ability to produce light is called bioluminescence. These jellyfish have special proteins that emit blue or green light. In general, jellyfish use bright flashes of light as a way to defend against predators.
Jellyfish Stings
If you’ve ever been swimming in the ocean, you probably know about jellyfish stings. Most jellyfish have stinging cells on their tentacles, which they use to stun their prey. But sometimes they also sting people when they come in contact.
Usually, jellyfish stings are mild for people. But certain kinds of jellyfish, such as box jellyfish, have very deadly stings. In fact, the Australian box jelly, also known as the sea wasp, is the most venomous animal in the world! Their toxins attack the heart, nervous system, and skin cells.
What Jellyfish Eat
Jellyfish eat plankton, larval shrimp, crabs, fish, and other jellies. Some jellyfish use their stinging cells to inject venom into their prey. Other jellies, such as comb jellies, use sticky cells called colloblasts to capture prey. And some jellies secrete a net of mucus to catch prey.
Jellyfish Reproduction
Jellyfish have two body forms. The adult, or medusa, form is what we think of when we think of jellyfish: a bell-shaped body with tentacles. But they have another body form as well: the polyp.
One way jellyfish reproduce is by collectively releasing eggs and sperm into the ocean. The fertilized eggs grow into larvae called planulae. The planulae then attach to a surface and become flower-like polyps.
When the time and conditions are right, the polyps either metamorphose into adult (medusa) jellyfish or they release infant jellies called ephyrae, which then turn into adult jellyfish.
But jellyfish can also reproduce in another way. Sometimes, a polyp divides in half and creates a new polyp. This is called budding.
Adult jellyfish generally only live a few months, but some can live 2-3 years in captivity.
Immortal Jellyfish
There is one jellyfish, however, that is able to transform back into a polyp if it gets injured as an adult.
The Turritopsis dohrnii is known as the immortal jellyfish because it can keep transforming back into a polyp, and then bud genetically identical medusae. In this way, it could potentially keep living forever.
Conservation Status
Jellyfish play an important role in the ocean ecosystem. They help control algae blooms and zooplankton populations. In addition, some fish use jellyfish and their stinging tentacles for protection. And finally, many animals rely on jellyfish for food, such as blue rockfish, ocean sunfish, dogfish, salmon, and sea turtles.
Some species of jellyfish are considered endangered or vulnerable of extinction by the IUCN Red List. But other jellyfish populations are actually increasing due to a decline in jellyfish predators.
One big threat to jellyfish and marine life in general is the proliferation of microplastics in the ocean.
What You Can Do to Help
If you would like to help jellyfish, you can share information about the jellyfish you see at a beach or ocean near you at Jelly Watch. You can also cut back on using single-use plastics, as these contribute to microplastic pollution in the ocean.
More Jellyfish Resources
- National Ocean Service, NOAA: What are jellyfish made of?
- Monterey Bay Aquarium: Jellies
- Smithsonian National Museum of Natural History: Jellyfish and Comb Jellies
- National Geographic Kids: Jellyfish
- American Museum of Natural History: The Immortal Jellyfish
Quick Fact Sheet
Common Name(s): Jellyfish, Jelly
Scientific Name: Cnidaria and Ctenophora
Animal Type: invertebrate
Size: 1 cm-30 m (under .5 in.-100 ft.)
About the Author
Abi Cushman is a contributing editor of Animal Fact Guide and My House Rabbit. She is also an award-winning author and illustrator of funny picture books for kids, including Wombats Are Pretty Weird and Flamingos Are Pretty Funky.
To learn more and to download free activity sheets, visit AbiCushman.com.
