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Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water and are less than half an inch in size. They are partially white and clear and do not sting. This article focuses on scyphomedusae. These are the large, often colorful, jellyfish that are common in coastal zones worldwide.


Etymology and taxonomic history

Since jellyfish are not actually fish, the term "jellyfish" is a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead. The word "jellyfish" is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish).

In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term "gelatinous zooplankton", when referring to these, together with other soft-bodied animals in the water column.


A group of jellyfish is sometimes called a bloom or a swarm. "Bloom" is usually used for a large group of jellyfish that gather in a small area. Blooms of jellyfish reproduce very quickly.Using "swarm" implies some kind of active ability to stay together, which a few species like Aurelia, the moon jelly, demonstrate.

Many jellyfish have a second part of their life cycle, which is called the polyp phase. When single polyps, arising from a single egg, develop into a multiple-polyp cluster, connected to each other by strands of tissue called stolons, they are said to be "colonial." A few polyps never proliferate and are referred to as "solitary" species.

Body systems

Jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They digest using the gastrodermal lining of the gastrovascular cavity, where nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to accomplish movement through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are passive much of the time. Jellyfish are composed of more than 90% water; most of their umbrella mass is a gelatinous material — the jelly — called mesoglea which is surrounded by two layers of epithelial cells which form the umbrella (top surface) and subumbrella (bottom surface) of the bell, or body.

Jellyfish do not have a brain or central nervous system, but rather have a loose network of nerves, located in the epidermis, which is called a "nerve net." A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells. Some jellyfish also have ocelli: light-sensitive organs that do not form images but which can detect light, and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented.

Jellyfish blooms

The presence of ocean blooms is usually seasonal, responding to prey availability and increasing with temperature and sunshine. Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. In addition to sometimes being concentrated by ocean currents, blooms can result from unusually high populations in some years. Bloom formation is a complex process that depends on ocean currents, nutrients, temperature and oxygen concentrations. Jellyfish are better able to survive in oxygen-poor water than competitors, and thus can thrive on plankton without competition. Jellyfish may also benefit from saltier waters, as saltier waters contain more iodine, which is necessary for polyps to turn into jellyfish. Rising sea temperatures caused by climate change may also contribute to jellyfish blooms, because many species of jellyfish are better able to survive in warmer waters. Jellyfish are likely to stay in blooms that are quite large and can reach up to 100,000 in each.

There is very little data about changes in global jellyfish populations over time, besides "impressions" in the public memory. Scientists have little quantitative data historic or current jellyfish populations. Recent speculation about increases in jellyfish populations are based on no "before" data.

The global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches formerly occupied by now overfished creatures, but notes that this hypothesis lack supporting data. Jellyfish researcher Marsh Youngbluth further clarifies that "jellyfish feed on the same kinds of prey as adult and young fish, so if fish are removed from the equation, jellyfish are likely to move in."

Some jellyfish populations that have shown clear increases in the past few decades are "invasive" species, newly arrived from other habitats: examples include the Black Sea and the Caspian Sea, the Baltic Sea, the eastern Mediterranean coasts of Egypt and Israel, and the American coast of the Gulf of Mexico. Populations of invasive species can expand rapidly because there are often no natural predators in the new habitat to check their growth. Such blooms would not necessarily reflect overfishing or other environmental problems.


By sampling sea life in a heavily fished region off the Namibianmarker coast, total jellyfish biomass has overtaken that of fish, following intense fishing in the area in the last few decades..

Areas which have been seriously affected by jellyfish blooms include the northern Gulf of Mexicomarker, about which, Graham states, "Moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf."

Detrimental Effects

Jellyfish blooms cause severe problems for mankind. The most obvious are human stings (sometimes deadly) and tourism declines on coasts.

However, the most severe effects are those to fish; severe or complete declines in commercial fish stocks, destroying fish nets, poisoning or crushing captured fish, consuming fish eggs and young fish.

Clogging also causes many problems including stoppage of nuclear power plants and desalination plants, as well clogging engines of ships and even overturning of boats by one of the largest species, the Nomura's jellyfish.

Life cycle

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; this polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish. Polyps generally have a mouth surrounded by upward-facing tentacles like miniatures of the closely-related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. Polyps may be solitary or colonial, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters.

In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the bell's border 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 are dioecious; that is, they are either male or female. In most cases, both release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber.

After fertilization and initial growth, a larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, resembling a tiny sea anemone. After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyrae may be formed. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few of species of hydromedusae reproduce by fission (splitting in half.)

Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles and at least one species of Pacific salmon. Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.

Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. One unusual species is reported to live as long as 30 years. Another species, Turritopsis dohrnii as T. nutricula, may be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. They feed continuously and grow to adult size fairly rapidly. After reaching adult size, jellyfish spawn daily if there is enough food. 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.

Importance to humans

Culinary uses

Only scyphozoan jellyfish belonging to the order Rhizostomeae are harvested for food; about 12 of the approximately 85 species are harvested and sold on international markets. Most of the harvest takes place in southeast Asia. Rhizostomes, especially Rhopilema esculentum in China (Chinese name: hǎizhē, meaning "sea sting") and Stomolophus meleagris (cannonball jellyfish) in the United States, are favored because of their larger and more rigid bodies and because their toxins are harmless to humans.

Traditional processing methods, carried out by a Jellyfish Master, involve a 20 to 40 day multi-phase procedure in which after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed. Processing reduces liquidation, off-odors and the growth of spoilage organisms, and makes the jellyfish drier and more acidic, producing a "crunchy and crispy texture." Jellyfish prepared this way retain 7-10% of their original weight, and the processed product contains approximately 94% water and 6% protein. Freshly processed jellyfish has a white, creamy color and turns yellow or brown during prolonged storage.



In biotechnology

In 1961, Osamu Shimomura of Princeton University extracted green fluorescent protein (GFP) and another bioluminescent protein, called aequorin from Aequorea victoria, while studying photoproteins which cause jellyfish's bioluminescence. Three decades later, Douglas Prasher, a post-doctoral scientist at Woods Hole Oceanographic Institution, sequenced and cloned the gene for GFP. Martin Chalfie of Columbia University figured out how to use GFP as a fluorescent marker of genes inserted into other cells or organisms. Roger Tsien of University of California, San Diego, chemically manipulated GFP in order to get other colors of fluorescence to use as markers. In 2008, Shimomura, Chalfie, and Tsien won the Nobel Prize in Chemistry for their work with GFP.

Man-made GFP is used as a fluorescent tag to show which cells or tissues express specific genes. The genetic engineering technique fuses the gene of interest to the GFP gene. The fused DNA is then put into a cell, to generate either a cell line or (via IVF techniques) an entire animal bearing the gene. In the cell or animal, the artificial gene turns on in the same tissues and the same time as the normal gene. But instead of making the normal protein, the gene makes GFP. One can then find out what tissues express that protein—or at what stage of development—by shining light on the animal or cell and observing fluorescence. The fluorescence shows where the gene is expressed.


In captivity

Jellyfish are displayed in aquariums in many countries. 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 feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquariummarker uses a modified version of the kreisel (German for "spinning top") for this purpose. Jellyfish are becoming a popular trend in home aquariums. It is now possible to buy jellyfish aquariums and live jellyfish online. It is also possible to assemble a jellyfish aquarium for personal use.

Toxicity to humans

Scyphozoan jellyfish stings are not generally deadly, though some species of the completely separate class Cubozoa (box jellyfish), such as the famous and especially toxic Irukandji, can be.

Jellyfish sting using microscopic cells called nematocysts, which are capsules full of poison expelled through a microscopic lance. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom.

Jellyfish sting prey and threatening humans using their nematocysts, but only some jellyfish species harm humans. Even beached and dying jellyfish can still sting when touched. Sting effects range from no effect to extreme pain to death.

Stings may cause anaphylaxis, which may result in death. Hence, victims should immediately get out of the water. Medical care may include administration of an antivenom.

The three goals of first aid for uncomplicated jellyfish stings are: prevent injury to rescuers, inactivate the nematocysts, and remove tentacles attached to the patient. Rescuers should wear barrier clothing, such as panty hose, wet suits or full-body sting-proof suits. Inactivating the nematocysts, or stinging cells, prevents further injection of venom.
Like many species of jellyfish, the sting of some species of Mastigias have no discernible effect on humans
Vinegar (3 to 10% aqueous acetic acid) helps with box jellyfish stings, but not Portuguese Man o' War stings. For stings on or around the eyes, dampen a towel with vinegar and dab around the eyes, but avoid the eyeballs. Salt water may also be used if vinegar is unavailable. Do not use fresh water if the sting occurred in salt water, because a change in tonicity can release additional venom. Avoid rubbing the wound, or using alcohol, spirits, ammonia, or urine which also encourage the release of venom. Meat tenderizer efficiently removes the nematocysts from the skin . An extremely hot shower or bath can neutralize stings. However, if hypothermia is suspected this method may cause serious complications.

Clearing the area of jelly, tentacles, and wetness stops further nematocyst firing. Shaving the affected skin with a knife edge, safety razor, or credit card can remove remaining nematocysts.

Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) can control skin irritation (pruritus). To remove the venom in the skin, apply a paste of baking soda and water and a cloth covering on the sting. If possible, reapply paste every 15–20 minutes. Ice can stop the spread of venom until either of these is available.

Recent discoveries

Jellyfish research has increased due to their rapidly rising populations and resulting contact with humans.

Among the latest discoveries, some of which contradict previous understandings, are:
  • The box jellyfish has no brain, but does have four clusters of six well-developed eyes. These eyes allow them to hunt prey, as well as to use landmarks outside the water for navigation.
  • Box jellies have colour perception
  • The Turritopsis nutricula jellyfish is immortal, rejuvenating itself after it becomes an adult.

Jellyfish in the media

The new discoveries about jellyfish and their popularity as symbol of the beauty and fragility of the oceans are reflected on television in programs such as in "Jellyfish Invasion," which is an episode of the National Geographic Channel documentary series Explorer, which includes research conducted by scientists in Australia, Hawaiimarker and Japanmarker.

Taxonomic classification systematics

Taxonomic classification systematics within the Cnidaria, as with all organisms, are always in flux. Many scientists who work on relationships between these groups are reluctant to assign ranks, although there is general agreement on the different groups, regardless of their absolute rank. Presented here is one scheme, which includes all groups that produce medusae (jellyfish), derived from several expert sources:

Phylum Cnidaria
Subphylum Medusozoa
:Class Hydrozoa
::Subclass Hydroidolina
::::Order Anthomedusae (= Anthoathecata or Athecata)
:::::Suborder Filifera - see for families
:::::Suborder Capitata - see for families
::::Order Leptomedusae (= Leptothecata or Thecata)
:::::Suborder Conica - see for families
:::::Suborder Proboscoida - see for families
::::Order Siphonophorae
:::::Suborder Physonectae
::::::Families: Agalmatidae, Apolemiidae, Erennidae, Forskaliidae, Physophoridae, Pyrostephidae, Rhodaliidae
:::::Suborder Calycophorae
::::::Families: Abylidae, Clausophyidae, Diphyidae, Hippopodiidae, Prayidae, Sphaeronectidae
:::::Suborder Cystonectae
::::::Families: Physaliidae, Rhizophysidae
::Subclass Trachylina
:::Order Limnomedusae
::::::Families: Olindiidae, Monobrachiidae, Microhydrulidae, Armorhydridae
:::Order Trachymedusae
::::::Families: Geryoniidae, Halicreatidae, Petasidae, Ptychogastriidae, Rhopalonematidae
:::Order Narcomedusae
::::::Families: Cuninidae, Solmarisidae, Aeginidae, Tetraplatiidae
:::Order Actinulidae
::::::Families: Halammohydridae, Otohydridae
:Class Staurozoa (= Stauromedusae)
:::Order Eleutherocarpida
::::::Families: Lucernariidae, Kishinouyeidae, Lipkeidae, Kyopodiidae
:::Order Cleistocarpida
::::::Families: Depastridae, Thaumatoscyphidae, Craterolophinae
:Class Cubozoa
::::::Families: Carybdeidae, Alatinidae, Tamoyidae, Chirodropidae, Chiropsalmidae
:Class Scyphozoa
:::Order Coronatae
::::::Families: Atollidae, Atorellidae, Linuchidae, Nausithoidae, Paraphyllinidae, Periphyllidae
:::Order Semaeostomeae
::::::Families: Cyaneidae, Pelagiidae, Ulmaridae
:::Order Rhizostomeae
::::::Families: Cassiopeidae, Catostylidae, Cepheidae, Lychnorhizidae, Lobonematidae, Mastigiidae, Rhizostomatidae, Stomolophidae


Image:Chrysaora quinquecirrha.JPG|Pacific sea nettle jellyfish Chrysaora fuscescens.Image:Cassiopeia andromeda (Upside-down jellyfish).jpg|Upside-down jellyfish harbor algae in their tentacles which they turn up to the sun to promote photosynthesis.Image:Chrysaora Colorata.jpg|Chrysaora colorata, the purple-striped jellyfish, lives off the coast of Southern California.Image:Flower hat jellyfishes.jpg| Olindias sp.Image:Cyanea kils.jpg|The Lion's mane jellyfish, Cyanea capillata, is known for its painful, but rarely fatal, sting.Image:mediterranean-jellyfish-af.jpg|A species of Mediterranean jellyfish, Cotylorhiza tuberculata, on display at the Monterey Bay Aquariummarker.Image:5733_aquaimages.jpg| Scrippsia pacifica, a fist-sized jellyfish from the coast of California.File:Moonjellyfish2500ppx.JPG|Aurelia sp.

See also


  1. Marques, A.C. and A. G. Collins, 2004. Cladistic analysis of Medusozoa and cnidarian evolution. Invertebrate Biology 123: 23-42.
  2. Kramp, P.L. 1961. Synopsis of the Medusae of the World. Journal of the Marine Biological Association of the United Kingdom 40: 1-469 and many subsequent descriptions of new species.
  3. Flower Hat Jelly, New York Aquarium, retrieved Aug 2009.
  4. {{cite web |last=Shubin |first=Kristie |accessdate=19 November 2009 |date=10 December 2008 |title=Anthropogenic Factors Associated with Jellyfish Blooms - Final Draft II |url=
  6. Lynam, C. and six other authors, 2006. Jellyfish overtake fish in a heavily fished ecosystem. Current Biology 16, no. 13: R492-R493.
  7. Jellyfish Gone Wild — Text-only
  8. Piraino, S. et al. 1996. Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). Biological Bulletin 190: 302-312.
  9. Omori, M. and E. Nakano, 2001. Jellyfish fisheries in southeast Asia. Hydrobiologia 451: 19-26.
  15. A test with coloured objects placed in the water showed the jellyfish bumping into white-coloured objects, navigating around black-coloured ones and shying away from red.
  16. Turritopsis nutricula: the world's only 'immortal' creature, The Times, 26 Jan 2009, retrieved Feb 2009.
  17. Jellyfish Invasion, National Geographic, retrieved Feb 2009.
  18. Jellyfish Invasion, YouTube, retrieved Feb 2009.
  19. Killer jellyfish population explosion warning, The Daily Telegraph, 11 Feb 2008, retrieved Feb 2009.
  20. Mills, C.E., D.R. Calder, A.C. Marques, A.E. Migotto, S.H.D. Haddock, C.W. Dunn and P.R. Pugh, 2007. Combined species list of Hydroids, Hydromedusae, and Siphonophores. pp. 151-168. In Light and Smith's Manual: Intertidal Invertebrates of the Central California Coast. Fourth Edition (J.T. Carlton, editor). University of California Press, Berkeley.

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