kids encyclopedia robot

Jellyfish facts for kids

Kids Encyclopedia Facts
Quick facts for kids
Pacific sea nettle ("Chrysaora fuscescens")
Pacific sea nettle (Chrysaora fuscescens)
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Subphylum: Medusozoa
Groups included
Cladistically included but traditionally excluded taxa

Jellyfish are animals of the phylum Cnidaria. They are a monophyletic clade, the Medusozoa.

Types of jellyfish

The Medusozoa are four classes of the Cnidaria:

Phyllorhiza punctata (White-spotted jellyfish) edit
White-spotted jellyfish

There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 20 species of Cubozoa, and the Hydrozoa includes about 1000–1500 species that produce medusae, but many more species that do not.


The name jellyfish has been in use since 1796. The term jellies or sea jellies is more recent, having been introduced to avoid use of the word "fish" with its modern connotation of an animal with a backbone, though shellfish, cuttlefish and starfish are not vertebrates either. In scientific literature, "jelly" and "jellyfish" have been used interchangeably. Many sources refer to only scyphozoans as "true jellyfish".

A group of jellyfish is called a "smack" or a "smuck". An occurrence of many jellyfish simultaneously is sometimes called a bloom.


They have soft bodies and long, stinging, venomous tentacles that they use to catch their prey, usually small plankton animals or small crustaceans or tiny fish.

They move by contracting their bodies, but they do not have much control over where they go: most of the time, they drift with the water current. A few jellysish species are anchored to the seabed by stalks and are not mobile.


Jellyfish are found all over the world, from surface waters to the deep sea. Most of them live in the oceans, in salt water, where they eat small sea animals like plankton and little fish, and float in the sea.

Only a few jellyfish live in fresh water. The best known freshwater example is the cosmopolitan hydrozoan jellyfish, Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and does not sting.


Jellyfish eat plankton, crustaceans, small fish, fish eggs and larvae, and other jellyfish, which they catch using their venomous tentacles. Jellyfish may live in symbiosis with algae. The jellyfish transports them into sunlight and get nutrients from the algae's photosynthesis.


Many animals eat jellyfish, including sea turtles and some fish (including the sun fish). Sea anemones may eat jellyfish that drift into their range. Other predators include tunas, sharks, swordfish, and penguins. Jellyfish washed up on the beach are consumed by foxes, other terrestrial mammals and birds. In general however, few animals prey on jellyfish.


Some small fish are immune to the stings of the jellyfish and live among the tentacles, serving as bait in a fish trap; they are safe from potential predators and are able to share the fish caught by the jellyfish. The cannonball jellyfish has a symbiotic relationship with ten different species of fish, and with the longnose spider crab, which lives inside the bell, sharing the jellyfish's food and nibbling its tissues.

Life cycle

Polypen einer Gorgonie

Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage, when the animal takes the form of a small stalk with feeding tentacles. Very often, this polyp is attached to the sea floor, or to another hard surface; it rarely moves around. A polyp that lives that way is called sessile. In some cases, the polyp is free-floating. Polyps generally have a mouth surrounded by upward-facing tentacles. Polyps may be on their own or in groups, and some bud asexually, making more polyps. Most are very small, measured in millimeters.

In the second stage, the tiny polyps asexually produce jellyfish, each of which is known as a medusa. Tiny jellyfish swim away from the polyp and then grow and feed in the plankton. Jellyfish reproduce both sexually and asexually. Well-fed adult jellyfish spawn daily. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn. Jellyfish are usually either male or female (with occasional hermaphrodites). In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms.

Box jellyfish at Bakoven Rock DSC11031
Box jellyfish at Bakoven Rock

Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs. Jellyfish at the medusa stage usually lives only up to six months, after which it dies.

Relation to humans

Jellyfish production time series
Global harvest of jellyfish in thousands of tonnes as reported by the FAO

As food

Jellyfish are eaten by humans in certain cultures. They are considered a delicacy in some Asian countries, where species in the Rhizostomae order are pressed and salted to remove excess water. Australian researchers have described them as a "perfect food", sustainable, and protein-rich but relatively low in food energy.

Jellyfish spoil very quickly after they are caught. Sometimes they are dried to preserve them. There is a different process in which they are cleaned, which can take up to 40 days. They are often eaten in a kind of salad, with soy sauce or vinegar.

In research

The green fluorescent protein used by some species to cause bioluminescence has been adapted as a fluorescent marker for genes inserted into other cells or organisms.

Aquarium display

Jellyfish aqurium
Pacific sea nettles (Chrysaora fuscescens) in an aquarium exhibit

Jellyfish are displayed in many public aquariums. Often the tank's background is blue and the animals are illuminated by side light, increasing the contrast between the animal and the background. In natural conditions, many jellies are so transparent that they are nearly invisible.

Jellyfish are not adapted to closed spaces. They depend on currents to transport them from place to place. Professional exhibits as in the Monterey Bay Aquarium feature precise water flows, typically in circular tanks to avoid trapping specimens in corners. As of 2009, jellyfish were becoming popular in home aquariums, where they require similar equipment.


The stinging cells (nematocysts) used by jellyfish to huntfor their prey can injure humans. Thousands of swimmers worldwide are stung every year, with effects ranging from mild discomfort to serious injury or even death.

Most jellyfish stings are not deadly, but stings of some box jellyfish (Irukandji jellyfish), such as the sea wasp, can be deadly.

Vinegar (3–10% aqueous acetic acid) may help with box jellyfish stings but not the stings of the Portuguese man o' war. Salt water may help if vinegar is unavailable. Immersing the sting in hot water may be the most effective way to reduce the pain from a Physalia sting. Covering it with an iced pack may help significantly relieve pain as well.

Scraping the affected skin, such as with the edge of a credit card, may remove remaining nematocysts. Once the skin has been cleaned of nematocysts, hydrocortisone cream could be applied locally in order to reduce pain and inflammation. Antihistamines may help to control itching. Immunobased antivenins are used for serious box jellyfish stings.

Mechanical issues

Jellyfish in large quantities can fill and split fishing nets and crush captured fish. They can clog cooling equipment, having disabled power stations in several countries; jellyfish caused a cascading blackout in the Philippines in 1999, as well as damaging the Diablo Canyon Power Plant in California in 2008. They can also stop desalination plants and ships' engines.

Interesting facts about jellyfish

  • Jellyfish have existed for at least 500 million years, and possibly 700 million years or more. They are the oldest multi-organ animal group.
  • The smallest jellyfish are just a few inches across.
  • The largest type of jellyfish is the Lion's mane jellyfish. It body can be over 3 feet (1 m) across and its tentacles are as long as 60 meters.
  • Some of the most dangerous jelly fish include the box jelly (Genuses Chironex, Chiropsalmus and Carybdea) and the tiny, two-cm-across Irukandji jelly (Carukia barnesi); the venomous sting of these jellyfish can kill a person.
  • Some jellyfish glow in the dark (this is called phosphorescence).
  • Box jellyfish have more advanced vision than the other groups. Each individual has 24 eyes, two of which are capable of seeing colour. Supposedly, they are one of the few kinds of animal to have a 360-degree view of its environment.
  • Jellyfish kill 20 to 40 people a year in the Philippines alone.
  • In 2006 the Spanish Red Cross treated 19,000 stung swimmers along the Costa Brava.

Images for kids

See also

  • Jellyfish dermatitis
  • List of prehistoric medusozoans
  • Ocean sunfish, a significant jellyfish predator
National Hispanic Heritage Month on Kiddle
Outstanding Hispanic athletes
Oscar De La Hoya
Michael Carbajal
Lee Trevino
Lorena Ochoa
kids search engine
Jellyfish Facts for Kids. Kiddle Encyclopedia.