Jellyfish are planktonic marine invertebrates belonging to the class Scyphozoa (phylum Cnidaria), with about 200 described species. The term is also used for other cnidarians with a medusoid body form, like hydrozoans, and even unrelated organisms such as comb jellies and salps. Most jellyfish are free-swimming and found in all oceans, typically ranging from 1 to 16 inches in diameter, though some can be much larger. Their bodies are composed of nearly 99 percent water, with a thick gelatinous layer called mesoglea between two cellular layers. They capture prey, such as copepods and fish larvae, using tentacles armed with stinging cells called nematocysts. Their life cycle usually involves a sessile polyp stage that buds off young medusae, which then grow into adults.

Where do jellyfish live?

Jellyfish inhabit all of the world's oceans, from surface waters to the deep sea. While most jellyfish are marine, a few species, like those in the genus Craspedacusta , can be found in freshwater environments. The "immortal jellyfish" ( Turritopsis dohrnii ), though native to the Mediterranean Sea, has been introduced to marine habitats globally. They are found in various regions, including coastal areas of North America and the waters around Japan.

How do jellyfish move through the water?

Jellyfish move through the water using a combination of passive drifting and active propulsion. They possess a nerve net, a decentralized nervous system that coordinates their movements. By contracting and relaxing their bell-shaped bodies, jellyfish can expel water, creating a jet-propulsion effect that allows them to move. This process is aided by a rudimentary internal circulatory system that passes water through canals within their gelatinous bells. While they can actively swim, jellyfish also drift with ocean currents, making them planktonic, or "sea-flea". Their movement is also influenced by sensory organs like rhopalia, which help them detect environmental cues for navigation.

What do jellyfish eat and how do they catch their food?

Jellyfish are carnivores that capture their food using stinging cells called nematocysts located on their tentacles. While they don't actively pursue prey, they ensnare organisms that come into contact with their tentacles. Their diet primarily consists of small animals such as copepods, fish larvae, and zooplankton. Some jellyfish also suspension feed, extracting minute animals and algae from the water. Once prey is captured, tentacles move it to the mouth for consumption. The lion's mane jellyfish, for example, uses its numerous tentacles to immobilize prey with neurotoxins before specialized oral arms transport the food to its mouth.

How do jellyfish sting and protect themselves?

Jellyfish sting and protect themselves using specialized stinging cells called nematocysts . These microscopic capsules, found on their tentacles, contain a coiled, barbed thread. When stimulated, the thread rapidly everts, injecting venom into prey or potential predators. This venom can paralyze or kill, serving as both a defense mechanism and a tool for capturing food. Some species, like the box jellyfish, possess potent venom that can be extremely painful and even deadly to humans. Jellyfish do not have brains but use a nerve net to coordinate movements and respond to stimuli, aiding in navigation and hunting.

What role do jellyfish play in the ocean ecosystem?

Jellyfish play a dual role in marine ecosystems as both predators and prey. They capture small animals like copepods, fish larvae, and other small organisms using their tentacles, which are equipped with stinging cells called nematocysts. Some jellyfish also function as suspension feeders, extracting phytoplankton and minute animals from the water. However, jellyfish are also a food source for various marine animals, including fish, sea turtles, sea anemones, penguins, and other jellyfish. This positions them as an important link in the ocean food web.

jellyfish

Introduction References & Edit History Quick Facts & Related Topics

Images & Videos

Study a jellyfish's muscular contractions and learn how the carnivore uses its tentacles to catch prey

A deep red jellyfish of the genus Crossota with long, thin tentacles extending from its bell-shaped body, shown against a black background.

Jellyfish belonging to the genus Cassiopea viewed from above.

Crystal jellyfish, Aequorea victoria, float against a black background, their translucent bells showing glowing radial canals and long trailing tentacles.

Translucent box jellyfish with long trailing tentacles, illuminated in blue light against a dark aquarium background.

An ornate Cassiopeia jellyfish with a translucent bell and ruffled orange oral arms floats in clear blue water in Jellyfish Lake, Kakaban, Indonesia.

A sea nettle jellyfish, Chrysaora fuscescens, with a translucent golden bell and long, trailing tentacles against a blue background.

Translucent jellyfish in the medusa stage with a rounded bell and long, trailing tentacles against a dark background.

At a Glance

jellyfish summary

Quick Facts

Key People:: Oka Asajirō

Related Topics:: Portuguese man-of-war; box jellyfish; lion’s mane jellyfish; Do jellyfish have a heart?; Scyphozoa

On the Web:: World Animal Rescue Network - Jellyfish (Nov. 08, 2026)

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jellyfish

marine invertebrate

Also known as: jelly

Written and fact-checked by

Britannica Editors

Last updated

Nov. 8, 2026 •History

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A School of Baby Fish Was Seen Riding a 6-Meter 'Phantom' Jellyfish in The Deep • Oct. 19, 2026, 6:01 AM ET (ScienceAlert)

Top Questions

What is a jellyfish?
Jellyfish are planktonic marine invertebrates belonging to the class Scyphozoa (phylum Cnidaria), with about 200 described species. The term is also used for other cnidarians with a medusoid body form, like hydrozoans, and even unrelated organisms such as comb jellies and salps.

Most jellyfish are free-swimming and found in all oceans, typically ranging from 1 to 16 inches in diameter, though some can be much larger. Their bodies are composed of nearly 99 percent water, with a thick gelatinous layer called mesoglea between two cellular layers. They capture prey, such as copepods and fish larvae, using tentacles armed with stinging cells called nematocysts. Their life cycle usually involves a sessile polyp stage that buds off young medusae, which then grow into adults.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where do jellyfish live?
Jellyfish inhabit all of the world's oceans, from surface waters to the deep sea. While most jellyfish are marine, a few species, like those in the genus Craspedacusta, can be found in freshwater environments. The "immortal jellyfish" (Turritopsis dohrnii), though native to the Mediterranean Sea, has been introduced to marine habitats globally. They are found in various regions, including coastal areas of North America and the waters around Japan.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do jellyfish move through the water?
Jellyfish move through the water using a combination of passive drifting and active propulsion. They possess a nerve net, a decentralized nervous system that coordinates their movements. By contracting and relaxing their bell-shaped bodies, jellyfish can expel water, creating a jet-propulsion effect that allows them to move. This process is aided by a rudimentary internal circulatory system that passes water through canals within their gelatinous bells. While they can actively swim, jellyfish also drift with ocean currents, making them planktonic, or "sea-flea". Their movement is also influenced by sensory organs like rhopalia, which help them detect environmental cues for navigation.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What do jellyfish eat and how do they catch their food?
Jellyfish are carnivores that capture their food using stinging cells called nematocysts located on their tentacles. While they don't actively pursue prey, they ensnare organisms that come into contact with their tentacles.

Their diet primarily consists of small animals such as copepods, fish larvae, and zooplankton. Some jellyfish also suspension feed, extracting minute animals and algae from the water. Once prey is captured, tentacles move it to the mouth for consumption. The lion's mane jellyfish, for example, uses its numerous tentacles to immobilize prey with neurotoxins before specialized oral arms transport the food to its mouth.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do jellyfish sting and protect themselves?
Jellyfish sting and protect themselves using specialized stinging cells called nematocysts. These microscopic capsules, found on their tentacles, contain a coiled, barbed thread. When stimulated, the thread rapidly everts, injecting venom into prey or potential predators. This venom can paralyze or kill, serving as both a defense mechanism and a tool for capturing food. Some species, like the box jellyfish, possess potent venom that can be extremely painful and even deadly to humans. Jellyfish do not have brains but use a nerve net to coordinate movements and respond to stimuli, aiding in navigation and hunting.
7 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What role do jellyfish play in the ocean ecosystem?
Jellyfish play a dual role in marine ecosystems as both predators and prey. They capture small animals like copepods, fish larvae, and other small organisms using their tentacles, which are equipped with stinging cells called nematocysts. Some jellyfish also function as suspension feeders, extracting phytoplankton and minute animals from the water.

However, jellyfish are also a food source for various marine animals, including fish, sea turtles, sea anemones, penguins, and other jellyfish. This positions them as an important link in the ocean food web.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

jellyfish, any planktonic marine member of the class Scyphozoa (phylum Cnidaria), a group of invertebrate animals composed of about 200 described species, or of the class Cubozoa (approximately 20 species). The term is also frequently applied to certain other cnidarians (such as members of the class Hydrozoa) that have a medusoid (bell- or saucer-shaped) body form, as, for example, the hydromedusae and the siphonophores (including the Portuguese man-of-war). Unrelated forms such as comb jellies (phylum Ctenophora) and salps (phylum Chordata) are also referred to as jellyfish. Scyphozoan jellyfish can be divided into two types, those that are free-swimming medusae and those that are sessile (i.e., stem animals that are attached to seaweed and other objects by a stalk). The sessile polyplike forms constitute the order Stauromedusae.

Free-swimming scyphozoan jellyfish occur in all oceans and include the familiar disk-shaped animals that are often found drifting along the shoreline. Most live for only a few weeks, but some are known to survive a year or longer. The bodies of most range in size from about 2 to 40 cm (1 to 16 inches) in diameter; some species are considerably larger, however, with diameters of up to 2 metres (6.6 feet). Scyphozoan medusae consist of almost 99 percent water as a result of the composition of the jelly that forms the bulk in nearly all species. Most feed on copepods, fish larvae, and other small animals that they catch in their tentacles, which have stinging cells (nematocysts). Some, however, simply suspension feed, extracting minute animals and algae (phytoplankton) from the water. Like all cnidarians, their bodies are made up of two cellular layers, the ectoderm and the endoderm, between which lies the gelatinous mesoglea. In jellyfish the transparent mesoglea layer is quite thick.

The life cycle of free-swimming scyphozoan jellyfish typically consists of three stages. A sessile polyp (scyphistoma) stage asexually buds off young medusae from its upper end, with each such ephyra growing into an adult. The adults are either male or female, but in some species they change their sex as they age. In many species, normal fusion of egg and sperm results in an embryo that is brooded in the gut of the adult until it becomes a ciliated planula larva, but in some this development takes place in the sea. After the planula larva leaves its parent, it lives for a time in the plankton and eventually attaches to a rock or other solid surface, where it grows into a new scyphistoma. Such a life cycle characterizes the order Semaeostomeae, which contains some 50 species of mainly coastal-water jellyfish, several of which have very wide geographic ranges. Included among these are members of the genera Aurelia and Chrysaora and the big red jellyfish, Tiburonia granrojo (subfamily Tiburoniinae), one of only three species of jellyfish that lack tentacles.

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The order Coronatae includes about 30 species of mostly deep-sea jellyfish, often maroon in colour. A deep circular groove delimits the central part of the bell-shaped body from the periphery, which is divided into broad flaps, or lappets. The marginal tentacles are large and solid. Some species are known to have a scyphistoma stage, but the life cycle of most of the forms has yet to be described. The coronate jellyfish are the most primitive of the present-day scyphozoans and are thought to be descended directly from the fossil form Conulata, which flourished between about 180 and 600 million years ago. Some of the known sessile stages form branched colonies, which were once separately identified under the name Stephanoscyphus.

The order Rhizostomeae includes some 80 described species. In these jellyfish the frilly projections (oral arms) that extend down from the underside of the body are fused, obliterating the mouth and forming a spongy area used in filter feeding. Marginal tentacles are lacking, and the gelatinous bell is firm and warty. In species whose life cycles are known, there is a typical benthic (bottom-dwelling) scyphistoma stage. Most members of the order are vigorous swimmers. Species of Cassiopea, the upside-down jellyfish, however, swim infrequently and sit inverted in tropical shallows, exposing their photosynthetic symbiotic algae to sunlight. The group Rhizostomeae is found mainly in shallow tropical to subtropical seas in the Indo-Pacific region, but members of the genus Rhizostoma, also called football jellyfish, often inhabit cooler waters, and Cotylorhiza is common in the Mediterranean.

The fourth order, Stauromedusae, comprises some 30 described species of nonswimming, stalked jellies. These species occur chiefly in cooler waters. They are goblet-shaped and fixed by a basal stalk; the mouth is situated at the upper end. Ranging from 1 to 10 cm (0.4 to 4 inches) in diameter, the body has a tetradiate design and typically bears eight clusters of tentacles. Some species can detach and resettle. Stauromedusae usually feed on small marine animals and live for several years. Development is direct from a larva into an adult. The polyp stage is suppressed.

The class Cubozoa contains two orders, Carybdeida and Chirodropida. Together, both orders comprise about 20 described species. Although some reach a diameter of 25 cm (10 inches), most range between 2 to 4 cm (1 to 2 inches). The jelly is rather spherical but squared off along the edges, giving rise to the common name of box jellies. The genera Chironex and Chiropsalmus, commonly called sea wasps, occur widely from Queensland northward to about Malaya. These forms have remarkably sophisticated eyes, and they are dangerously venomous; a moderate sting can cause death within a few minutes. In all the box jellies so far studied, the polyp stage produces but a single medusa. Through the process of budding, polyps emerge from a medusa or from another polyp. Essentially, a single planula larva may produce numerous, genetically identical medusae.

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The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.

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