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{{About|the aquatic animal-form|similar animals|gelatinous zooplankton|other uses|Jellyfish (disambiguation)}}
{{Automatic taxobox
| fossil_range = {{Fossil range|505|0}} <small>[[Cambrian]]&nbsp;– [[Recent]]</small>
| taxon = Medusozoa
| name = Jellyfish
| image = Jelly cc11.jpg
| image_width =
| image_caption = <center>Pacific sea nettle<br>(''[[Chrysaora fuscescens]]'')</center>
| authority = Petersen, 1979
| display children = 1
}}
[[File:Olindias formosa1.jpg|thumb|A [[flower hat jelly]] (''Olindias formosa'') photographed at the [[Osaka Aquarium Kaiyukan]]]]
[[File:Chrysaora Colorata.jpg|thumb|A [[Chrysaora colorata|purple-striped jelly]] at the [[Monterey Bay Aquarium]]]]
[[File:Aurelia aurita 1.jpg|thumb|A [[Aurelia aurita|moon jelly]] shown in false color at the [[Pairi Daiza]]]]
[[File:Phyllorhiza punctata (White-spotted jellyfish) edit.jpg|thumb|A [[Phyllorhiza punctata|white-spotted jellyfish]] off the [[Geography of Haiti|north coast of Haiti]]]]

'''Jellyfish''' or '''jellies'''<ref name = Sting>Van Patten, Peg. [http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1010&context=wracklines&sei-redir=1&referer=http%3A%2F%2Fscholar.google.com%2Fscholar%3Fstart%3D80%26q%3Djellyfish%2Bjellies%26hl%3Den%26as_sdt%3D0%2C5#search=%22jellyfish%20jellies%22 "Sting"]. (2004). Wrack Lines. Paper 11.</ref> are the major non-[[polyp]] form of individuals of the [[phylum]] [[Cnidaria]]. They are typified as free-swimming marine animals consisting of a gelatinous [[umbrella]]-shaped bell and trailing tentacles. The bell can pulsate for [[motion (physics)|locomotion]], while stinging tentacles can be used to capture prey.
Jellyfish are found in every ocean, from the surface to the deep sea. A few jellyfish inhabit [[freshwater]]. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years,<ref>[http://www.sciencedaily.com/releases/2007/10/071030211210.htm Fossil Record Reveals Elusive Jellyfish More Than 500 Million Years Old]. ScienceDaily (2007-11-02).</ref> and possibly 700 million years or more, making them the oldest multi-organ animal.<ref name=angier2dec>{{cite news|last=ANGIER|first=NATALIE|title=So Much More Than Plasma and Poison|url=http://www.nytimes.com/2011/06/07/science/07jellyfish.html?_r=1|accessdate=2 December 2011|newspaper=[[The New York Times]]|date=June 6, 2011}}</ref>
{{toclimit|3}}
== Terminology ==
The English popular name ''jellyfish'' has been in use since 1796.<ref name=etymol/> It has traditionally also been applied to other animals sharing a superficial resemblance, for example [[ctenophores]] (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as ''comb jellies'') were included as "jellyfishes".<ref>{{Cite book|last=Kelman|first=Janet Harvey|coauthors=Rev. Theodore Wood|title=The Sea-Shore, Shown to the Children|publisher=T. C. & E. C. Jack|location=London|year=1910|page=146|url=http://openlibrary.org/books/OL7043926M/The_sea-shore}}</ref> Even some scientists include the phylum ctenophora when they are referring to jellyfish.<ref>{{Cite book
| last1 = Kaplan | first1 = Eugene H.
| last2 = Kaplan | first2 = Susan L.
| last3 = Peterson | first3 = Roger Tory
| title = A Field Guide to Coral Reefs: Caribbean and Florida
| url= http://books.google.com/?id=OLYPWMoBkccC&pg
| accessdate = 2009-08-31
| year= 1999
| month = August
| publisher = Boston : Houghton Mifflin
| isbn = 0-618-00211-1
| page = 55 }}
</ref> 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.<ref>{{Cite journal
| author=Haddock, S.H.D., and Case, J.F.
| title=Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores
| journal=Marine Biology | date=April 1999| volume=133 | doi=10.1007/s002270050497
| url=http://www.lifesci.ucsb.edu/~haddock/abstracts/haddock_spectra.pdf | accessdate=2009-09-09
| page=571
| issue=3
}}</ref>

As jellyfish are not [[vertebrates]], let alone true [[fish]], the word ''jellyfish'' is considered by some to be a [[misnomer]]. Public aquariums may use the terms ''jellies'' or ''sea jellies'' instead.<ref>[http://web.archive.org/web/20090406102702/http://www.nyaquarium.com/look-and-learn/animal-profiles/flower-hat-jelly.aspx ''Flower Hat Jelly''], New York Aquarium.</ref> Indeed, it may be said that the term "jellies" has become more popular than "jellyfish".<ref name = Sting /> In scientific literature, "jelly" and "jellyfish" are often used interchangeably.<ref>See, e.g., Brotz, Lucas. [https://circle.ubc.ca/bitstream/id/142376/ubc_2011_fall_brotz_lucas.pdf Changing Jellyfish Populations: Trends in Large Marine Ecosystems]. 2011. p.1.</ref> Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate.<ref>{{cite book|last=Coulombe|first=Deborah A.|title=Seaside Naturalist: A Guide to Study at the Seashore|url=http://books.google.com/books?id=VOoqKMdI0ekC&lpg=PP1&pg=PA60#v=onepage|publisher=[[Simon & Schuster]]|accessdate=20 March 2013|page=60|date=14 February 1990|isbn=9780671765033}}</ref>

Many textbooks and sources refer to only [[scyphozoa]] as "true jellyfish".<ref>{{cite web|url=http://animals.about.com/od/cnidarians/a/tenfactsjellyfi.htm|title=Ten Facts about Jellyfish|last=Klappenbach|first=Laura|accessdate=24 January 2010}}</ref><ref>{{cite web|url=http://qanda.encyclopedia.com/question/some-determining-characteristics-jellyfish-class-scyphozoa-97854.html|title=What are some determining characteristics of jellyfish in the class, Scyphozoa?|accessdate=24 January 2010}}</ref>

A group of jellyfish is sometimes called a bloom or a swarm.<ref>{{cite web
|url= http://www.nsf.gov/news/special_reports/jellyfish/textonly/intro.jsp
|title= Jellyfish Gone Wild
|accessdate= 17 November 2009
|date= 3 March 2009
|format= Text of Flash
|publisher= [[National Science Foundation]]
|quote= In recent years, massive blooms of stinging jellyfish and jellyfish-like creatures have overrun some of the world’s most important fisheries and tourist destinations.... Jellyfish swarms have also damaged fisheries, fish farms, seabed mining operations, desalination plants and large ships.
}}</ref> "[[Algal bloom|Bloom]]" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected.<ref>{{cite web
|url=http://www.npr.org/templates/story/story.php?storyId=5573968
|title= Jellyfish Take Over an Over-Fished Area
|accessdate= 19 November 2009
|date= 21 July 2006
}}</ref>
Another collective name for a group of jellyfish is a ''smack'',<ref>"''Smack: a smattering; a taste; a small quantity. Examples: smack of jellyfish – Lipton, 1970; of knowledge; of my muse, 1766; ...''" from [http://www.thefreedictionary.com/smack The Free Dictionary]. For more detail see [http://books.google.com.au/books?id=8a_0tZp--EMC An exaltation of larks] by [[James Lipton]].</ref> although this term is not commonly used by scientists who study jellyfish. Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic [[polyp]]s usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance.<ref name="Mills">{{cite journal | doi = 10.1023/A:1011888006302 | last1 = Mills | first1 = C.E. | year = 2001 | title = Jellyfish blooms: are populations increasing globally in response to changing ocean conditions? | url = http://faculty.washington.edu/cemills/jellyblooms2001.pdf | journal = Hydrobiologia | volume = 451 | issue = | pages = 55–68 }}</ref> Using "[[swarm]]" usually implies some kind of active ability to stay together, which a few species such as ''[[Aurelia (genus)|Aurelia]]'', the [[moon jelly]], demonstrate.<ref>{{Cite journal|last=Hamner|first=W. M.|coauthors=P. P. Hamner, S. W. Strand|year=1994|title=Sun-compass migration by Aurelia aurita (Scyphozoa): population retention and reproduction in Saanich Inlet, British Columbia|journal=Marine Biology|volume=119|pages=347–356.|doi=10.1007/BF00347531|issue=3}}</ref>

Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), [[stauromedusae]] (stalked jellyfish), [[Box jellyfish|cubomedusae]] (box jellyfish), or [[hydromedusae]], according to which clade their species belongs.<ref>{{cite web|author=|url=http://www.youtube.com/watch?v=3HzFiQFFQYw&feature=channel_video_title |title=‪There's no such thing as a jellyfish‬‏ |publisher=YouTube |date= |accessdate=2011-07-05}}</ref>

The term ''medusa'' was coined by [[Linnaeus]] in 1752, alluding to the tentacled head of [[Medusa]] in [[Greek mythology]].<ref name=etymol>[http://www.etymonline.com/index.php?search=jellyfish jellyfish]. Online Etymology Dictionary. Etymonline.com. Retrieved on 2013-01-10.</ref> This term refers exclusively to the non-polyp life-stage which occurs in many cnidarians, which is typified by a large pulsating gelatinous bell with long trailing tentacles. All medusa-producing species belong to the sub-phylum Medusozoa.

In [[biology]], a ''medusa'' (plural: medusae) is a [[Form (zoology)|form]] of [[cnidaria]]n in which the body is shaped like an umbrella, in contrast with [[polyp]]s. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the ''exumbrella'' and the lower surface is called the ''subumbrella''; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the ''velum''). The digestive cavity consists of the [[gastrovascular cavity]] and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and [[tentacle]]s as its said.

German biologist [[Ernst Haeckel]] popularized medusae through his vivid illustrations, particularly in ''[[Kunstformen der Natur]]''.

== Anatomy ==
[[File:Radiate Oral-aboral Axes.JPG|thumb|right|The major surfaces and axes of a jellyfish]]

Most jellyfish do not have specialized [[Gastrointestinal tract|digestive]], [[Osmoregulation|osmoregulatory]], [[Central nervous system|central nervous]], [[Respiratory system|respiratory]], or [[Circulatory system|circulatory]] systems. The [[manubrium]] is a stalk-like structure hanging down from the centre of the underside, with the mouth at its tip. This opens into the [[gastrovascular cavity]], where digestion takes place and nutrients are absorbed. It is joined to the radial canals which extend to the margin of the bell.<ref>[http://www.britannica.com/EBchecked/topic/372811/medusa?anchor=ref100538 Medusa] Encyclopedia Britannica. Retrieved 2011-12-08.</ref> Jellyfish 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 navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive.{{Citation needed|date=July 2009}} Depending on the species, the body contains between 95 and 98% water.<ref>Hsieh, Yun-Hwa and Rudloe, Jack (1994) [http://www.sciencedirect.com/science/article/pii/0924224494902534 "Potential of utilizing jellyfish as food in Western countries"] ''Trends in Food Science & Technology'', '''5''' (7): 225–229. {{doi|10.1016/0924-2244(94)90253-4}}</ref> Most of the umbrella mass is a gelatinous material — the jelly — called [[mesoglea]] which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut.

=== Nervous system ===

Jellyfish employ a loose network of nerves, located in the [[Squamous epithelium|epidermis]], which is called a "[[nerve net]]".<ref>Satterlie RA (2002) [http://www.biochem.uci.edu/steele/Satterlie.pdf "Neuronal control of swimming in jellyfish: a comparative story"], ''Canadian Journal of Zoology'', '''80''': 1654–1669. {{doi|10.1139/z02-138}}</ref> Although traditionally thought not to have a [[central nervous system]], nerve net concentration and [[ganglion]]-like structures could be considered to constitute one in most species.<ref>{{cite journal|last=Satterlie|first=Richard A.|title=Do jellyfish have central nervous systems?|journal=Journal of Experimental Biology|year=2011|month=April|volume=214|pages=1215–1223|doi=10.1242/jeb.043687|url=http://jeb.biologists.org/content/214/8/1215.long|accessdate=7 May 2013|issue=8}}</ref> 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.

Another counter to the "brainless jellyfish" hypothesis{{clarify|date=May 2012}}<!--Either this is a fact or it's a hypothesis. And why "another", What is the first one?--> is that some species explicitly adapt to [[tide|tidal]] flux to control their location. In [[Roscoe Bay Provincial Park|Roscoe Bay]], jellyfish ride the current at ebb tide until they hit a [[gravel bar]], and then descend below the current. They remain in still waters waiting for the tide to rise, ascending and allowing it to sweep them back into the bay. They monitor salinity to avoid fresh water from mountain snowmelt, again by diving until they find enough salt.<ref name=angier2dec/>

=== Vision ===

Some jellyfish 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.

Certain species of jellyfish, such as the [[box jellyfish]], have been revealed to be more advanced than their counterparts. The box jellyfish has 24 eyes, two of which are capable of seeing [[color]], and four parallel information processing areas or [[Rhopalium|rhopalia]] that act in competition,<ref>{{cite journal|author=Wehner R |year=2005|url=http://www.imls.uzh.ch/static/CMS_publications/wehner/literatur/pdf05/wehner200510.pdf |title=Sensory physiology: brainless eyes|doi=10.1038/435157a|journal=Nature|volume=435|issue=7039|pages=157–9|pmid=15889076}}</ref> supposedly making it one of the few creatures to have a 360-degree view of its environment.<ref>[http://www.newscientist.com/article/mg18624995.700-multieyed-jellyfish-helps-with-darwins-puzzle.html Multi-eyed jellyfish helps with Darwin's puzzle]. Newscientist.com (2005-05-14). Retrieved on 2013-01-10.</ref>

The eyes are suspended on stalks with heavy [[crystal]]s on one end, acting like a [[gyroscope]] to orient the eyes skyward. They look upward to navigate from roots in [[mangrove]] swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.<ref name=angier2dec/>

=== Size ===
[[File:Medusozoa Scyphozoa SA.jpg|thumb|right|A relative comparison of the size of a [[Scyphozoa]] jellyfish with the size of a human being]]

Jellyfish range from about one millimeter in bell height and diameter to nearly two meters in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension.

The smallest jellyfish are the peculiar creeping jellyfish in the genera ''[[Staurocladia]]'' and ''[[Eleutheria (Hydrozoa)|Eleutheria]]'', which have bell disks from 0.5&nbsp;mm to a few mm diameter, with short tentacles that extend out beyond this, on which these tiny jellyfish crawl around on seaweed or the bottoms of rocky pools.<ref>{{cite book|last=Mills|first=C.E.|coauthors=Hirano, Y.M.|title==Encyclopedia of Tidepools and Rocky Shores: Hydromedusae|year=2007|pages=286–288|isbn=0520251180|publisher=University of California Press}}</ref> Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by splitting in half (called fission). Other very small jellyfish, which have bells about one mm, are the hydromedusae of many species that have just been released from their parent polyps;<ref>{{cite journal|last=Mills|first=C.E.|title=Podocoryne selena, a new species of hydroid from the Gulf of Mexico, and a comparison with Hydractinia echinata|journal=Biological Bulletin|year=1976|volume=151|pages=214–224|doi=10.2307/1540715}}</ref> some of these live only a few minutes before shedding their gametes in the plankton and then dying, while others will grow in the plankton for weeks or months. The hydromedusae ''Cladonema radiatum'' and ''Cladonema californicum'' are also very small, living for months, yet never growing beyond a few mm in bell height and diameter.<ref>{{cite journal|last=Costello|first=J.|title=Laboratory culture and feeding of the hydromedusa Cladonema californicum Hyman (Anthomedusa: Cladonemidae)|journal=Journal of Experimental Marine Biology and Ecology|year=1988|volume=123|issue=2|pages=177–188|doi=10.1016/0022-0981(88)90168-2}}</ref> Another small species of jellyfish is the Australian [[Irukandji jellyfish|Irukandji]], which is about the size of a fingernail.<ref name=angier2dec/>

The [[lion's mane jellyfish]], ''[[Cyanea (jellyfish)|Cyanea]] capillata'', was long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles that may extend up to {{convert|36.5|m}} long (though most are nowhere near that large).<ref>{{cite web|url=http://www.waterford-today.ie/index.php?option=com_content&task=view&id=933&Itemid=10177&ed=68 |archiveurl=http://web.archive.org/web/20100530112100/http://www.waterford-today.ie/index.php?option=com_content&task=view&id=933&Itemid=10177&ed=68 |archivedate=2010-05-30 |title=Rare sighting of a lion’s mane jellyfish in Tramore Bay |publisher=Waterford Today |date= 2007-08-01|accessdate=2010-10-18}}</ref><ref>{{cite web|url=http://www.redorbit.com/education/reference_library/cnidaria/lions_mane_jellyfish/4326/index.html |title=Lion’s Mane Jellyfish – Reference Library |publisher=redOrbit |date= |accessdate=2010-10-18}}</ref> They have a moderately painful, but rarely fatal, sting. Claims that this jellyfish may be the longest animal in the world are likely exaggerated; some other planktonic cnidarians called [[siphonophores]] may typically be tens of meters long and seem a more legitimate candidate for longest animal.{{citation needed|date=April 2013}}<!--siphonophores are not particular animals, but colonies of animals-->

The increasingly common giant Nomura's jellyfish, ''[[Nemopilema nomurai]]'', found in some, but not all years in the waters of Japan, Korea and China in summer and autumn is probably a much better candidate for "largest jellyfish", since the largest Nomura's jellyfish in late autumn can reach {{convert|200|cm}} in bell (body) diameter and about {{convert|200|kg}} in weight, with average specimens frequently reaching {{convert|90|cm}} in bell diameter and about {{convert|150|kg}} in weight.<ref>{{cite journal|last=Omori|first=Makoto|coauthors=Kitamura, Minoru|title=Taxonomic review of three Japanese species of edible jellyfish (Scyphozoa: Rhizostomeae)|journal=Plankton Biology and Ecology|year=2004|volume=51|issue=1|pages=36–51|url=http://www.plankton.jp/PBE/issue/vol51_1/vol51_1_036.pdf | format=PDF}}</ref><ref>{{cite journal|last=Uye|first=Shin-Ichi|title=Blooms of the giant jellyfish Nemopilema nomurai: a threat to the fisheries sustainability of the East Asian Marginal Seas|url=http://www.plankton.jp/PBR/issue/vol03_suppl/03suppl_125.pdf|journal=Plankton & Benthos Research|year=2008|volume=3 (Supplement)|pages=125–131|doi=10.3800/pbr.3.125}}</ref> The large bell mass of the giant Nomura's jellyfish<ref>{{cite news|title=Giant Echizen jellyfish off Japan coast|url=http://news.bbc.co.uk/2/hi/science/nature/8385953.stm|publisher=BBC | date=2009-11-30}}</ref> can dwarf a diver and is nearly always much greater than the up-to-{{convert|100|cm}} bell diameter Lion's Mane.<ref>{{cite journal|last=Kramp|first=P.L.|title=Synopsis of the medusae of the world|journal=[[Journal of the Marine Biological Association of the United Kingdom]]|year=1961|volume=40|pages=1–469}}</ref>

The rarely encountered deep-sea jellyfish ''[[Stygiomedusa gigantea]]'' is another solid candidate for "largest jellyfish", with its thick, massive bell up to {{convert|100|cm}} wide, and four thick, "strap-like" oral arms extending up to {{convert|6|m}} in length,<ref>{{cite news|title=Giant deep sea jellyfish filmed in Gulf of Mexico|url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8638000/8638527.stm|work=BBC Earth News | date=2010-04-23 | first=Jody | last=Bourton}}</ref> very different from the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane.

== Taxonomy ==
[[File:Largelionsmanejellyfish.jpg|thumb|The [[lion's mane jellyfish]] (''Cyanea capillata'') is one of the larger species of jellyfish.]]
[[File:Sea Nettle Jelly 1.jpg|thumb|A [[purple-striped jellyfish]] at the [[Monterey Bay Aquarium]]]]
{{See also|alpha taxonomy|Linnean taxonomy|taxon|binomial nomenclature}}

Jellyfish belong to Medusozoa, the clade of cnidarians which excludes [[Anthozoa]] (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps.<ref name="treeoflife">[http://tolweb.org/Cnidaria/ Cnidaria], Tree of Life. ''A comprehensive morphological cladistic analysis by Schuchert (1993) supports the basal position of Anthozoa with the Scyphozoa and Cubozoa being more closely related to each other than to Hydrozoa. Morphological, mtDNA, and 18S rDNA data separately and together also support the basal position of Anthozoa but do not resolve the relationships among Scyphozoa, Cubozoa and Hydrozoa (Bridge et al. 1995).''</ref>

The phylogenetics of this group are complex and evolving. The Medusozoa appear to be a sister group to [[Octocorallia]].<ref name=Zou2012>Zou H, Zhang J, Li W, Wu S, Wang G (2012) Mitochondrial Genome of the Freshwater Jellyfish Craspedacusta sowerbyi and Phylogenetics of Medusozoa. PLoS One 7(12):e51465. doi: 10.1371/journal.pone.0051465</ref> [[Staurozoa]] appears to be the earliest diverging; Cubozoa and the coronate Scyphozoa form a clade that is the sister group of [[Hydrozoa]] plus discomedusan Scyphozoa. The Hydrozoa are the sister group of discomedusan Scyphozoa. [[Limnomedusae]] (Trachylina) is the sister group of hydroidolinans. This group may be the earliest diverging lineage among Hydrozoa. [[Semaeostomeae]] is a paraphyletic clade with [[Rhizostomeae]].

The four major classes of medusozoan Cnidaria are:
*[[Scyphozoa]] are often called true jellyfish. They have tetra-radial symmetry. They have tentacles around the outer margin of the bowl-shaped bell, and oral arms around the mouth.
*[[Cubozoa]] (box jellyfish) have a box-shaped bell, and their velarium assists them to swim more quickly. Box jellyfish may be related more closely to "true jellyfish" than either are to hydrozoa.<ref name="treeoflife"/>
*[[Hydrozoa]] may form medusa which resemble scyphozoans (but generally with a velum) and are distinguished by an absence of cells in the mesoglea. However, many hydrozoa species do not form medusa at all (such as [[Hydra (genus)|hydra]], which is hence not considered a jellyfish).
*[[Staurozoa]] (stalked jellyfish) do not have a polyp stage, however the medusa is generally sessile, oriented upside down and with a stalk from the "bell" planted to the substrate. Until recently, staurozoa was classified within scyphozoa.

Some other animals are frequently associated with or mistaken for medusa jellyfish.
*[[Siphonophorae]] are an order of hydrozoa which generally live as colonies (for example, free-swimming chains of repeated units, some units similar to polyps or to medusa). They are not considered medusa jellyfish. A well-known example is the [[Portuguese Man o' War]].
*[[Ctenophora|Ctenophores]] (comb jellies) are a separate phylum from Cnidaria. Their method of propulsion is cilia paddles rather than a pulsating bell.
*[[Salp]]s are transparent, gelatinous marine organisms that form pelagic colonies like siphonophores. Salps are [[chordates]], and as such are actually more closely related to humans than they are to cnidarians and comb jellies.<ref>[http://www.antarctica.gov.au/about-antarctica/wildlife/animals/salps Salps] (wildlife), Antartic Division, Australian Government website.</ref>

There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 20 species of Cubozoa, and in Hydrozoa there are about 1000–1500 species that produce medusae (and many more hydrozoa species that do not).<ref>{{Cite journal|doi=10.1111/j.1744-7410.2004.tb00139.x|last=Marques|first=A.C.|coauthors=A. G. Collins|year=2004|title=Cladistic analysis of Medusozoa and cnidarian evolution|journal=Invertebrate Biology|volume=123|pages=23–42}}</ref><ref>{{Cite journal|last=Kramp|first=P.L.|year=1961|title=Synopsis of the Medusae of the World|journal=Journal of the Marine Biological Association of the United Kingdom|volume=40|pages=1–469}}</ref>

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. Here is one scheme, which includes all groups that produce jellyfish, derived from several expert sources:

{| class="wikitable"
|-
! colspan="5" style="text-align:center;"| Jellyfish taxonomy (phylum [[Cnidaria]]: subphylum Medusozoa)
|-
! style="width:90px;"| Class
! style="width:85px;"| Subclass
! style="width:115px;"|Order
! style="width:95px;"|Suborder
! Families
|-
| rowspan="11"| [[Hydrozoa]] <ref name="schuchert-hydrozoa">{{cite web|url=http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm|title=The Hydrozoa Directory|last=Schuchert|first=Peter|accessdate=2008-08-11}}</ref><ref>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 ISBN 0520239393.</ref>
| rowspan="7"| [[Hydroidolina]]
| rowspan="2"| [[Anthomedusae]]
| [[Filifera]]
| ''see <ref name="schuchert-hydrozoa" />''
|-
| [[Capitata (Hydrozoa)|Capitata]]
| ''see'' <ref name="schuchert-hydrozoa" />
|-
| rowspan="2"| [[Leptomedusae]]
| [[Conica (Hydrozoa)|Conica]]
| ''see'' <ref name="schuchert-hydrozoa" />
|-
| [[Proboscoida]]
| ''see'' <ref name="schuchert-hydrozoa" />
|-
| rowspan="3"| [[Siphonophorae]]
| [[Physonectae]]
| [[Agalmatidae]], [[Apolemiidae]], [[Erennidae]], [[Forskaliidae]], [[Physophoridae]], [[Pyrostephidae]], [[Rhodaliidae]]
|-
| [[Calycophorae]]
| [[Abylidae]], [[Clausophyidae]], [[Diphyidae]], [[Hippopodiidae]], [[Prayidae]], [[Sphaeronectidae]]
|-
| [[Cystonectae]]
| [[Physaliidae]], [[Rhizophysidae]]
|-
| rowspan="4"| [[Trachylina]]
| colspan=2 | [[Limnomedusae]]
| [[Olindiidae]], [[Monobrachiidae]], [[Microhydrulidae]], [[Armorhydridae]]
|-
| colspan=2 | [[Trachymedusae]]
| [[Geryoniidae]], [[Halicreatidae]], [[Petasidae]], [[Ptychogastriidae]], [[Rhopalonematidae]]
|-
| colspan=2 | [[Narcomedusae]]
| [[Cuninidae]], [[Solmarisidae]], [[Aeginidae]], [[Tetraplatiidae]]
|-
| colspan=2 | [[Actinulidae]]
| [[Halammohydridae]], [[Otohydridae]]
|-
| colspan="2" rowspan="2" style="text-align:left;"| [[Staurozoa]]<ref>{{cite web|url=http://faculty.washington.edu/cemills/Staurolist.html|title=Stauromedusae: List of all valid species names|last=Mills|first=Claudia E|accessdate=2008-08-11}}</ref>
| colspan=2 | [[Eleutherocarpida]]
| [[Lucernariidae]], [[Kishinouyeidae]], [[Lipkeidae]], [[Kyopodiidae]]
|-
| colspan=2 | [[Cleistocarpida]]
| [[Depastridae]], [[Thaumatoscyphidae]], [[Craterolophidae]]
|-
| colspan="4" style="text-align:left;"| [[Cubozoa]] <ref name="thescyphozoan.ucmerced.edu">{{cite web|url=http://thescyphozoan.ucmerced.edu/|title=The Scyphozoan|last=Dawson|first=Michael N|accessdate=2008-08-11}}</ref>
| [[Carybdeidae]], [[Alatinidae]], [[Tamoyidae]], [[Chirodropidae]], [[Chiropsalmidae]]
|-
| colspan="2" rowspan="3"| [[Scyphozoa]]<ref>{{cite journal|author=Daly M, Brugler MR, Cartwright P, Collins AG, Dawson MN, Fautin DG, France SC, Mcfadden CS, Opresko DM, Rodriguez E, Romano SL, Stake JL|year=2007|url=http://www.mapress.com/zootaxa/2007f/zt01668p182.pdf |title=The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus|journal=Zootaxa |volume=1668|pages=127–182}}</ref><ref>{{cite journal|author=Bayha, K. M., and M. N. Dawson |year=2010|title=New family of allomorphic jellyfishes, Drymonematidae (Scyphozoa, Discomedusae), emphasizes evolution in the functional morphology and trophic ecology of gelatinous zooplankton|journal=The Biological Bulletin|volume= 219|issue=3|pages= 249–267|pmid=21183445}}</ref><ref>{{cite journal|author=Straehler-Pohl, I., C. L. Widmer, and A. C. Morandini |year=2011|title=Characterizations of juvenile stages of some semaeostome Scyphozoa (Cnidaria), with recognition of a new family (Phacellophoridae)|journal=Zootaxa|volume= 2741|pages= 1–37|url=http://www.ib.usp.br/~acmorand/2011_Straehler-Pohl_et_al_%28Phacellophoridae%29.pdf}}</ref>
| colspan=2 | [[Coronatae]]
| [[Atollidae]], [[Atorellidae]], [[Linuchidae]], [[Nausithoidae]], [[Paraphyllinidae]], [[Periphyllidae]]
|-
| colspan=2 | [[Semaeostomeae]]
| [[Cyaneidae]], [[Drymonematidae]], [[Pelagiidae]], [[Phacellophoridae]], [[Ulmaridae]]
|-
| colspan=2 | [[Rhizostomeae]]
| [[Cassiopeidae]], [[Catostylidae]], [[Cepheidae]], [[Lobonematidae]], [[Lychnorhizidae]], [[Mastigiidae]],[[Rhizostomatidae]], [[Stomolophidae]], [[Thysanostomatidae]], [[Versurigidae]]
|}

== Life history/behavior ==
{{See also|Biological life cycle|Developmental biology}}

[[File:Schleiden-meduse-2.jpg|thumb|right|The developmental stages of [[scyphozoan]] jellyfish's life cycle:<br>'''1–3''' Larva searches for site<br>'''4–8''' Polyp grows<br>'''9–11''' Polyp [[strobilation|strobilates]]<br>'''12–14''' Medusa grows|alt=Illustration of two life stages of seven jelly species]]

=== Phases ===

Jellyfish development occurs in multiple phases. Eggs and sperm develop into larval planulae, become polyps, bud into ephyrae and then transform into adult medusae. In some species, specimens may skip some phases.<ref>{{cite web|url=http://www.scientificamerican.com/article.cfm?id=how-do-jellyfish-reproduc |title=How do jellyfish reproduce? What effect does their sting have on humans? What's the difference between red and translucent jellyfish? |publisher=Scientific American |date=2013-10-15 |accessdate=2013-10-22}}</ref>

The ''[[planula]]'' is a small [[larva]] covered with [[cilium|cilia]]. It settles onto a firm surface and develops into a [[polyp]].

The polyp is generally a small stalk with a mouth that is ringed by upward-facing tentacles. The polyps resemble the closely related Cnidaria [[anthozoa]]n ([[sea anemone]]s and [[coral]]s) polyps. The jellyfish polyp may be [[Sessility (zoology)|sessile]], living on the bottom or on another substrate such as floats or boat hulls, or it may be free-floating or attached to tiny bits of free-living plankton<ref name="Mills 1987">{{Cite journal|last=Mills|first=C. E.|year=1987|title=In situ and shipboard studies of living hydromedusae and hydroids: preliminary observations of life-cycle adaptations to the open ocean|journal=Modern Trends in the Systematics, Ecology, and Evolution of Hydroids and Hydromedusae|publisher=Clarendon Press|location=Oxford|isbn=0198571909}}</ref> or rarely, fish<ref>{{Cite journal|last=Fewkes|first=J. Walter|year=1887|title=A hydroid parasitic on a fish|journal=Nature|volume=36|pages=604–605|doi=10.1038/036604b0 | issue=939}}</ref><ref>{{cite web|url=http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm|title=The Hydrozoa|last=Schuchert|first=Peter|accessdate=24 January 2010}}</ref> or other invertebrates. Polyps may be solitary or colonial. Polyp colonies form by [[strobilation]], in which multiple polyps share a common stomach cavity.<ref>[http://animals.about.com/od/cnidarians/a/lifecyclejellyf.htm Jellyfish – The Life Cycle of a Jellyfish]. Animals.about.com. Retrieved on 2013-01-10.</ref> Most polyps are only millimeters in size. They feed continuously. The polyp stage may last for years.

The next stage is the ephyra, which is a free-swimming precursor of the final adult stage.

The ephyra then develops into a [[medusa]]. The medusa is the life stage that is typically identified as a jellyfish.

=== Reproduction ===

Jellyfish reproduce both sexually and asexually.

==== Spawning ====

Upon reaching adult size, jellyfish [[Spawn (biology)|spawn]] daily given 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.<ref>{{Cite journal|last=Mills|first=Claudia|year=1983|title=Vertical migration and diel activity patterns of hydromedusae: studies in a large tank|journal=Journal of Plankton Research|volume=5|pages=619–635|doi=10.1093/plankt/5.5.619|issue=5}}</ref> Jellyfish are usually either male or female (with occasional [[hermaphrodites]]). In most cases, adults release [[spermatozoon|sperm]] and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms.

==== Budding ====

After a growth interval, the polyp begins reproducing asexually by [[budding]] and, in the [[Scyphozoa]], is called a''segmenting polyp'', or a scyphistoma. Budding produces more scyphistomae and also ephyrae. Budding sites vary by species; from the [[tentacle]] bulbs, the manubrium (above the mouth), or the [[gonad]]s of hydromedusae.<ref name="Mills 1987"/>

Polyps asexually produce free-swimming ephyra, which then become a ''[[medusa (biology)|medusa]].'' New specimens (usually only a millimeter or two across) swim away from the polyp and then grow.{{Citation needed|date=July 2009}}

==== Other development paths ====

A few species can produce new medusae by budding directly from the medusan stage. Some hydromedusae reproduce by ''[[fission (biology)|fission]]'' (splitting in half).<ref name="Mills 1987"/> A few omit the planula, polyp and ephyra phases and produce new medusae directly from eggs.{{Citation needed|date=December 2012}}

Some polyps can asexually produce a creeping ''frustule'' larval form, which then develops into another polyp.

=== Brooding ===

In a few species, the sperm swim into the female's mouth, fertilizing the eggs within her body, where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing [[planula]] larvae.

=== Lifespan ===

Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life generally ends after the medusa has begun spawning. Life span varies by species. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter.

Aquarium jellyfish that are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, may live several years, though this would be very unusual in the wild.

One unusual species is reported to live as long as 30 years.{{citation needed|date=April 2012}} Another unusual species, ''[[Turritopsis nutricula|T. nutricula]]'', falsely reported as ''Turritopsis dohrnii'', might be effectively immortal because of its ability under certain circumstances to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other ocean organism. So far this reversal has been observed only in the laboratory.<ref>{{cite journal | doi = 10.2307/1543022 | last1 = Piraino | first1 = S. | last2 = Boero | year = 1996 | first2 = F. | last3 = Aeschbach | first3 = B. | last4 = Schmid | first4 = V. | title = Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa) | jstor = 1543022| journal = Biological Bulletin | volume = 190 | issue = 3| pages = 302–312 }}</ref>

===Movement===
Jellyfish move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion to create a [[vortex]] that pushes them forward. The displaced water creates a “hole” that when re-filled, propels them forward. The vortex allows the animal to travel 30 percent farther on each stroke. This reduces the energy required. Jellyfish achieved a 48 percent lower cost of transport (the amount of food and oxygen consumed, versus energy spent in movement) than other animals in similar studies..<ref>{{cite web|url=http://www.kurzweilai.net/jellyfish-energy-efficiency-to-improve-bio-inspired-robotic-designs-for-navy |title=Jellyfish energy efficiency to improve bio-inspired robotic designs for Navy |doi=10.1073/pnas.1306983110 |publisher=KurzweilAI |date= |accessdate=2013-10-22}}</ref><ref>{{cite doi|10.1073/pnas.1306983110}}</ref>

== Ecology ==

=== Diet ===

Jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same hole in the middle of the bell. Jellies hunt passively using their tentacles as drift nets.

=== Predation ===

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]].

=== Blooms ===
[[File:Moon jelly - adult (rev2).jpg|thumb|right|''[[Aurelia (genus)|Aurelia sp.]]'' occurs in large quantities in most of the world's coastal waters. Members of this genus are nearly identical to each other.|alt=Photo of translucent moon jelly on black blackground. The jelly contains a white gamsana mass extending through about 2/3 of its body]]
[[File:Jellyfish population trends by LME.jpg|thumb|Map of population trends of native and invasive species of jellyfish<ref>{{cite journal|journal=Hydrobiologia|title=Increasing jellyfish populations: trends in Large Marine Ecosystems|year=2012|volume=688|url=http://www.springerlink.com/content/h2m74376448540r8/?MUD=MP|author=Brotz, Lucas; Cheung, William W. L.; Kleisner Kristin; Pakhomov, Evgeny; Pauly, Daniel|doi=10.1007/s10750-012-1039-7|pages=3}}</ref>
{{legend|#E51A1D|Increase (high certainty)}}
{{legend|#F1A341|Increase (low certainty)}}
{{legend|#4DAF4A|Stable/variable}}
{{legend|#377CB5|Decrease}}
{{legend|#CCCCCA|No data}}]]

Jellyfish bloom formation is a complex process that depends on [[ocean current]]s, [[nutrient]]s, sunshine, temperature, season, prey availability, reduced predation and [[oxygen]] concentrations. Ocean currents tend to congregate jellyfish into large [[swarm]]s or "blooms", consisting of hundreds or thousands of individuals. Blooms can also result from unusually high populations in some years. Jellyfish are better able to survive in nutrient-rich, oxygen-poor water than competitors, and thus can feast 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 relatively better able to survive in warmer waters.<ref>
{{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=http://jrscience.wcp.muohio.edu/fieldcourses08/PapersMarineEcologyArticles/AnthropogenicFactorsAssocA.html}}</ref>

Scientists have little historic data about jellyfish populations.<ref name="Mills">
{{Cite journal
|last=Mills
|first=C.E.
|year=2001
|url=http://faculty.washington.edu/cemills/jellyblooms2001.pdf
|title=Jellyfish blooms: are populations increasing globally in response to changing ocean conditions?
|journal=Hydrobiologia
|volume=451
|pages=55–68
|doi=10.1023/A:1011888006302
}}</ref>

One hypothesis is that the global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling [[ecological niche]]s formerly occupied by now [[overfishing|overfished]] creatures, but this hypothesis lacks supporting data.<ref name="Mills"/> Youngbluth states 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."<ref name="WaPost">The [[Washington Post]], republished in the European Cetacean Bycatch Campaign, [http://www.eurocbc.org/page727.html ''Jellyfish “blooms” could be sign of ailing seas''], May 6, 2002. Retrieved November 25, 2007.</ref>

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]], [[Caspian Sea]], [[Baltic Sea]], central and eastern [[Mediterranean Sea|Mediterranean]], [[Hawaii]], and tropical and subtropical parts of the West Atlantic (including the [[Caribbean Sea|Caribbean]], [[Gulf of Mexico]] and Brazil).<ref>{{cite journal|author=Abed-Navandi, D., and R. Kikinger |year=2007|title=First record of the tropical scyphomedusa Phyllorhiza punctata von Lendenfeld, 1884 (Cnidaria: Rhizostomeae) in the Central Mediterranean Sea|journal=Aquatic Invasions |volume=2|issue=4|pages= 391–394|url=http://elnais.ath.hcmr.gr/PDF/Abel_Navanti_Phylorizapunctata.pdf|doi=10.3391/ai.2007.2.4.7}}</ref><ref>[http://www.haaretz.com/news/world-s-most-invasive-jellyfish-spreading-along-israel-coast-1.278097 World's most invasive jellyfish spreading along Israel coast] [[Haaretz]] article from June 15, 2009</ref> Invasive populations can expand rapidly because they often face no predators in the new habitat.

Increased nutrients, ascribed to agricultural [[Runoff (water)|runoff]], have been cited as contributing to jellyfish proliferation. Graham states, "ecosystems in which there are high levels of nutrients ... provide nourishment for the small organisms on which jellyfish feed. In waters where there is [[eutrophication]], low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem."<ref name="WaPost"/>

=== Population ===
Jellyfish populations may be expanding globally as a result of [[overfishing]] of their [[predation|natural predators]] and the availability of excessive nutrients due to [[land runoff]].<ref>Hays GC, T Bastian, TK Doyle, S Fossette, AC Gleiss, MB Gravenor, VJ Hobson, NE Humphries, MKS Lilley, NG Pade and DW Sims (2011) [http://www.swan.ac.uk/bs/turtle/reprints/Hays_etal_PRSB_doi_2011.pdf "High activity and Lévy searches: jellyfish can search the water column like fish"] ''Proc. R. Soc. B,'' [Epub ahead of print] {{doi|10.1098/rspb.2011.0978}}</ref><ref>Pauly, D., Christensen, V., Dalsgaard, J., Froese, R. & Torres Jr, F. (1998) [http://umanitoba.ca/institutes/natural_resources/pdf/pauly_fishing_down_marine_food_webs.pdf "Fishing down marine food webs"] ''Science'', '''279''': 860–863. {{doi|10.1126/science.279.5352.860}}</ref> When marine ecosystems become disturbed jellyfish can proliferate. For example, jellyfish reproduce rapidly and have fast growth rates; they predate many species, while few species predate them; and they feed via touch rather than visually, so they can feed effectively at night and in turbid waters.<ref>Richardson, A. J., Bakun, A., Hays, G. C. & Gibbons, M. J. (2009) [http://www.swan.ac.uk/bs/turtle/reprints/Richardson%20et%20al%202009%20TREE%20-%20The%20Jellyfish%20Joyride.pdf "The jellyfish joyride: causes, consequences and management responses to a more gelatinous future"] ''Trends Ecol.Evol.,'' '''24''': 312–322. {{doi|10.1016/j.tree. 2009.01.010}}</ref><ref>Aksnes, D. L., Nejstgaard, J., Sædberg, E. & Sørnes, T. (2004) [http://www.new.aslo.org/lo/toc/vol_49/issue_1/0233.pdf "Optical control of fish and zooplankton populations"] ''Limnol. Oceanogr.'' '''49''': 233–238. {{doi|10.4319/lo. 2004.49.1.0233}}</ref> It may become difficult for [[fish stock]]s to reestablish themselves in marine ecosystems once they have become dominated by jellyfish, because jellyfish feed on plankton, which includes [[fish eggs]] and [[fish larvae|larvae]].<ref>Lynam, C. P., Gibbons, M. J., Axelsen, B. E., Sparks, C. A. J., Coetzee, J., Heywood, B. G. & Brierley, A. S. (2006) [https://www.st-andrews.ac.uk/~perg/Lynam_et_al_Current_Biology_16_2006.pdf "Jellyfish overtake fish in a heavily fished ecosystem"] ''Curr. Biol.'' '''16''': 492–493. {{doi|10.1016/j.cub.2006.06.018}}</ref><ref>Pauly, D., Graham, W., Libralato, S., Morissette, L. & Palomares, M. L. D. (2009) [http://filaman.ifm-geomar.de/home/pages/JellyfishInEcosystems_publication.pdf "Jellyfish in ecosystems, online databases, and ecosystem models"] ''Hydrobiologia,'' '''616''': 67–85.{{doi|10.1007/s10750-008-9583-x}}</ref>

=== Habitats ===

Although most jellyfish are marine animals, some inhabit [[freshwater]]. This is most common for hydromedusae (in fact many hydrozoa inhabit freshwater). The best known example is the [[Cosmopolitan distribution|cosmopolitan]] freshwater jellyfish, [[Craspedacusta sowerbii]]. It is less than an inch (2.5&nbsp;cm) in diameter, colorless and it does not sting.

Some other jellyfish populations have also become restricted into lakes, such as [[Jellyfish Lake]] in [[Palau]].

Although what first comes to mind as the common domain of jellyfish is living well up off the ocean floor in the plankton, a few species of jellyfish are closely associated with the bottom for much of their lives (that is, they can be considered [[benthic]]). The upside-down jellyfish in the genus ''[[Cassiopea]]'' typically lie on the bottom of shallow lagoons where they sometimes pulsate gently with their umbrella top facing down. The tiny creeping jellyfish ''[[Staurocladia]]'' and ''[[Eleutheria]]'' (see section on Size, above) cannot swim and "walk" around on seaweed fronds or rocky bottoms on their tentacles. Most [[hydromedusae]] and [[scyphomedusae]] that live in coastal habitats find themselves on the bottom periodically, where they may stop swimming for a while, and certain box jellyfish species also rest on the sea bed in shallow water.<ref>{{cite journal|url=http://homepage.psy.utexas.edu/homepage/class/Psy355D/jellyfish.pdf |doi=10.1016/j.mehy.2005.08.036|title=Is sleep's 'supreme mystery' unraveling? An evolutionary analysis of sleep encounters no mystery; nor does life's earliest sleep, recently discovered in jellyfish|year=2006|last1=Kavanau|first1=J. Lee|journal=Medical Hypotheses|volume=66|pages=3–9|pmid=16213664|issue=1}}</ref> Even some deep-sea species of hydromedusae and scyphomedusae are usually collected on or near the bottom. All of the [[stauromedusae]] are found attached to either seaweed or rocky or other firm material on the bottom.<ref>{{cite journal|last=Mills|first=C.E.|coauthors=Hirano, Y.M.|title=Stauromedusae|journal=Encyclopedia of Tidepools and Rocky Shores|year=2007|pages=541–543}}</ref>
[[File:A relaxed Jelly Fish.jpg|thumb|A relaxed jellyfish]]

== Relation to humans ==

=== Gastronomy ===
[[File:Cannonball Jellyfish.jpg|thumb|right|[[Cannonball jellyfish]], ''Stomolophus meleagris'', are harvested for culinary purposes|alt=Photo of whole jellyfish]]

In some countries, such as Japan and Korea, jellyfish are known as a delicacy. "Dried jellyfish" has become increasingly popular throughout the world. The jellyfish is dried to prevent spoiling; if not dried they can spoil within a matter of hours. Once dried, they can be stored for weeks at a time.
Only scyphozoan jellyfish belonging to the order [[Rhizostomeae]] are harvested for food; about 12 of the approximately 85 species. Most of the harvest takes place in southeast Asia.<ref>{{cite journal | doi = 10.1023/A:1011879821323 | last1 = Omori | first1 = M. | last2 = Nakano | first2 = E. | author-separator =, | author-name-separator= | year = 2001 | title = Jellyfish fisheries in southeast Asia | url = | journal = Hydrobiologia | volume = 451 | issue = | pages = 19–26 }}</ref> Rhizostomes, especially ''[[Rhopilema]] esculentum'' in China ({{lang|zh|海蜇}} ''hǎizhē'', "sea stings") 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.<ref name="Hydrobiologia">
{{Cite journal
| author = Y-H. Peggy Hsieh, Fui-Ming Leong, and Jack Rudloe
| title = Jellyfish as food
| journal = Hydrobiologia
| volume = 451
| issue = 1–3
| pages = 11–17
| year = 2001
| doi = 10.1023/A:1011875720415
| url = http://www.springerlink.com/content/x7204250k4174gwt/
}}</ref>

[[File:CantoneseJellyfish.jpg|thumb|left|Rehydrated jellyfish strips prepared with [[soy sauce]] and [[sesame oil]]|alt=Photo of gold-colored jellyfish strips on plate]]
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 membrane]]s, the umbrella and oral arms are treated with a mixture of [[table salt]] and [[alum]], and compressed. Processing reduces liquefaction, odor, 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 China, processed jellyfish are desalted by soaking in water overnight and eaten cooked or raw. The dish is often served shredded with a dressing of oil, soy sauce, vinegar and sugar, or as a salad with vegetables. In Japan, cured jellyfish are rinsed, cut into strips and served with vinegar as an appetizer.<ref name="Hydrobiologia"/><ref>{{Cite book|title=The Encyclopedia of Marine Resources |last=Firth |first=F.E. |authorlink= |coauthors= |year=1969 |publisher=Van Nostrand Reinhold Co. |location=New York |pages=New York |isbn=0-442-22399-4 |nopp=true }}</ref> Desalted, ready-to-eat products are also available.<ref name="Hydrobiologia"/>
{{clear}}

=== Fisheries ===
[[File:Jellyfish production time series.png|thumb|right|<center>Global harvest of jellyfish in thousands of tonnes as reported by the [[FAO]]<ref name=faostat>Based on data extracted from the [http://faostat.fao.org/site/629/default.aspx FishStat database]</ref></center>]]

Fisheries have begun harvesting the American cannonball jellyfish, ''Stomolophus meleagris'', along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asia.<ref name="Hydrobiologia"/>

Jellyfish are also harvested for their [[collagen]], which can be used for a variety of applications including the treatment of [[rheumatoid arthritis]].

=== Biotechnology ===
[[File:Aequorea victoria.jpg|thumb|right|The hydromedusa ''[[Aequorea victoria]]''|alt=Photo of live jelly in the sea]]

In 1961, [[Osamu Shimomura]] extracted [[green fluorescent protein]] (GFP) and another bioluminescent protein, called [[aequorin]], from the large and abundant hydromedusa ''[[Aequorea victoria]]'', while studying [[photoprotein]]s that cause [[bioluminescence]] in this species. Three decades later, [[Douglas Prasher]] sequenced and cloned the gene for GFP. [[Martin Chalfie]] figured out how to use GFP as a fluorescent marker of genes inserted into other cells or organisms. [[Roger Tsien]] later chemically manipulated GFP to produce other fluorescent colors to use as markers. In 2008, Shimomura, Chalfie and Tsien won the [[Nobel Prize in Chemistry]] for their work with GFP.

Man-made GFP became commonly 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, making GFP instead of the normal protein. Illuminating the animal or cell reveals what tissues express that protein—or at what stage of development. The fluorescence shows where the gene is expressed.<ref>
{{Cite book
| author = Pieribone, V. and D.F. Gruber
| title =Aglow in the Dark: The Revolutionary Science of Biofluorescence
| publisher = Harvard University Press
| year =2006
| isbn=0674024133
}}</ref>

=== Aquariums ===
[[File:Jellyfish aqurium.jpg|thumb|right|A group of Pacific sea nettle jellyfish, ''[[Chrysaora fuscescens]]'', in an aquarium exhibit|alt=Photo of downward-swimming jellies]]

Jellyfish are displayed in many [[public aquarium]]s. 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 avoid trapping specimens in corners. The [[Monterey Bay Aquarium]] uses a modified version of the ''kreisel'' (German for "spinning top") for this purpose. The outflow is spread out over a large surface area and the inflow enters as a sheet of water in front of the outflow, so the jellyfish don't get sucked into it.<ref>US Patent for Jellyfish Tank

https://www.google.com/patents/USD669229?dq=jellyfish+tank&hl=en&sa=X&ei=ydcOUujaPKa0igL35oHQAw&sqi=2&pjf=1&ved=0CEQQ6AEwAg</ref> As of 2009, jellyfish were becoming popular in home aquariums.<ref>{{Cite news| url=http://bits.blogs.nytimes.com/2009/03/14/how-to-avoid-liquefying-your-jellyfish/ | work=The New York Times | title=How to Avoid Liquefying Your Jellyfish | first=Matt | last=Richtel | date=2009-03-14 | accessdate=6 May 2010}}</ref><ref>{{cite web|url=http://www.airtranmagazine.com/features/2009/08/garage-brands |title=Garage brands|publisher=Airtran Magazine |date= |accessdate=2010-10-18}}</ref><ref>{{cite web|url=http://www.jellyfishart.com/ |title=Jellyfish Art}}</ref><ref>{{cite web|url=http://www.wikihow.com/Start-a-Jellyfish-Tank |title=How to Start a Jellyfish Tank |publisher=wikiHow |date=2010-10-07 |accessdate=2010-10-18}}</ref> These home aquariums generate this special water flow pattern using an air-lift pump <ref>US Patent on Air-Lift Jellyfish Tank

https://www.google.com/patents/US8393298?dq=jellyfish+tank&hl=en&sa=X&ei=ydcOUujaPKa0igL35oHQAw&sqi=2&pjf=1&ved=0CEsQ6AEwAw</ref> and require special food for the jellyfish, which can be shipped from suppliers to their final destination.

=== Toxicity ===

Jellyfish sting their prey using [[nematocyst]]s, also called cnidocysts, stinging structures located in specialized cells called [[cnidocytes]], which are characteristic of all Cnidaria. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject [[venom]],<ref>Purves WK, Sadava D, Orians GH, Heller HC. 1998. Life. The Science of Biology. Part 4: The Evolution of Diversity. Chapter 31</ref> yet only some species' venom cause an adverse reaction in humans. When a nematocyst is triggered by contact by predator or prey, pressure builds up rapidly inside it up to {{convert|2000|psi}} until it bursts. A [[lance]] inside the nematocyst pierces the victim's skin, and poison flows through into the victim.<ref>[http://www.jellyfishart.com/kb_results.asp?ID=11 KB Results<!-- Bot generated title -->]</ref> Touching or being touched by a jellyfish can be very uncomfortable, sometimes requiring medical assistance; sting effects range from no effect to extreme pain to death. Even beached and dying jellyfish can still sting when touched.

Scyphozoan jellyfish stings range from a twinge to tingling to agony.<ref name="smith" /> Most jellyfish stings are not deadly, but stings of some species of the class ''Cubozoa'' and the [[Box jellyfish]], such as the famous and especially toxic [[Irukandji jellyfish]], can be deadly. Stings may cause [[anaphylaxis]], which can be fatal. Medical care may include administration of an [[antivenom]].

In 2010, at a [[New Hampshire]] beach, pieces of a single dead [[lion's mane jellyfish]] stung between 125 and 150 people.<ref>[http://www.boston.com/news/local/breaking_news/2010/07/ouch_jellyfish.html Ouch! Jellyfish stings 150 on N.H. beach], Boston Globe, July 21, 2010</ref><ref>[http://www.nytimes.com/2010/07/23/us/23jelly.html?hp Death Does Not Deter Jellyfish Sting], ''The New York Times'', July 22, 2010</ref> Jellyfish kill 20 to 40 people a year in the Philippines alone.<ref name="smith" /> In 2006 the Spanish Red Cross treated 19,000 stung swimmers along the [[Costa Brava]].<ref name="smith" />

The [[sea wasp]], a box jellyfish found in [[Australia]]n waters, can kill an adult human within a few minutes. However the stings are so short that wearing [[pantyhose]] is sufficient to prevent the stings from being able to penetrate the skin.<ref name=angier2dec/>

=== Treatment ===
[[File:Palau stingless jellyfish.jpg|thumb|right|Like many species of jellyfish, the sting of some species of ''[[Mastigias]]'' have little or no discernible effect on humans.|alt=Photo of umbrella jelly in water]]
<!-- Note: This section is NOT meant to be a how-to guideline or instructional medical guide. Please refer to suitable medical sites for further information -->

The three goals of first aid for uncomplicated stings are to prevent injury to rescuers, deactivate the [[cnidocyte|nematocysts]], and remove tentacles attached to the patient. Rescuers usually wear barrier clothing, such as [[pantyhose]], wet suits or full-body sting-proof suits while removing jellies or tentacles from injured. Deactivating the nematocysts (stinging cells) prevents further injection of [[venom (poison)|venom]].

[[Vinegar]] (3–10% aqueous [[acetic acid]]) may be used as a common remedy to help with [[box jellyfish]] stings,<ref name="MJA1993-fenner">{{Cite journal| author = Fenner P, Williamson J, Burnett J, Rifkin J | title = First aid treatment of jellyfish stings in Australia. Response to a newly differentiated species | journal = Med J Aust | volume = 158 | issue = 7 | pages = 498–501 | year = 1993 | pmid = 8469205 | doi = }}</ref><ref>{{Cite journal| author = Currie B, Ho S, Alderslade P | title = Box-jellyfish, Coca-Cola and old wine | journal = Med J Aust | volume = 158 | issue = 12 | page = 868 | year = 1993 | pmid = 8100984 | doi = }}</ref> but not the stings of the [[Portuguese Man o' War]] (which is not a true jellyfish, but a colony).<ref name="MJA1993-fenner"/> For stings on or around the eyes, a towel dampened with vinegar may be used to dab around the eyes, with care taken to avoid the eyeballs. Salt water may be used as an alternative if vinegar is unavailable;<ref name="MJA1993-fenner"/><ref>{{Cite journal| author = Yoshimoto C | title = Jellyfish species distinction has treatment implications | journal = Am Fam Physician | volume = 73 | issue = 3 | page = 391 | year = 2006|pmid = 16477882 | doi = | last2 = Leong | first2 = Fui-Ming | last3 = Rudloe | first3 = Jack}}</ref> and may be preferred over vinegar.<ref>{{cite web| url= http://www.uamshealth.com/?id=11935&sid=1 | title= Does urine help a jellyfish sting? | work= Health Library > Medial Myths| publisher= [[University of Arkansas for Medical Sciences]] | accessdate= 2013-03-16| quote= Dr. Pait suggests the best thing to do is to immediately get out of the water and wash the affected area with salt water. Salt water will deactivate the stinging cells while fresh or tap water can reactive the stinging cells. It is also helpful to try to remove the cells carefully with something such as a credit card.}}</ref> Fresh water is not usually used if the sting occurs in salt water, as changes in [[tonicity]]<ref>{{cite web|author=Paul Auerbach, M.D. |url=http://www.healthline.com/blogs/outdoor_health/2008/01/meat-tenderizer-for-jellyfish-sting.html |title=Meat Tenderizer for a Jellyfish Sting |publisher=Healthline.com |date=2008-01-19 |accessdate=2010-10-18}}</ref> can release additional venom. Rubbing wounds, or using [[alcohol]], spirits, [[ammonia]], or [[urine]] may have strongly negative effects as these can encourage the release of venom.<ref name="MJA1980-hartwick">{{Cite journal| author = Hartwick R, Callanan V, Williamson J | title = Disarming the box-jellyfish: nematocyst inhibition in Chironex fleckeri | journal = Med J Aust | volume = 1 | issue = 1 | pages = 15–20 | year = 1980 | pmid = 6102347 }}</ref>

Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing.<ref name="MJA1980-hartwick"/> Scraping the affected skin with a knife edge, safety razor, or credit card may remove remaining nematocysts.<ref name="AFP2004-Perkins">{{Cite journal| author = Perkins R, Morgan S | title = Poisoning, envenomation, and trauma from marine creatures | journal = Am Fam Physician | volume = 69 | issue = 4 | pages = 885–90 | year = 2004 | pmid = 14989575 | doi = }}</ref>

Beyond initial first aid, [[antihistamine]]s such as [[diphenhydramine]] ([[Histamine antagonist|Benadryl]]) may control skin irritation ([[itch|pruritus]]).<ref name="AFP2004-Perkins"/> Ice or fresh water is not usually applied to stings, since they may cause nematocysts to continue to release toxin.<ref>{{cite web|author=|url=http://www.emedicinehealth.com/jellyfish_stings/page4_em.htm |title=Jellyfish Stings Causes, Symptoms, Treatment – Jellyfish Stings Treatment on eMedicineHealth |publisher=Emedicinehealth.com |date=2011-04-07 |accessdate=2011-07-05}}</ref><ref>{{cite web|url=http://www.healthcareask.com/first-aid/first-aid-5-7563.html |title=Jellyfish sting treatment? |publisher=Healthcareask.com |date= |accessdate=2010-10-18}}</ref> Immunobased antivenins have been available since the 1970s;<ref>{{cite journal|last=Baxter|first=E.H.|coauthors=A.G.M. Marr|title=Sea wasp (''Chironex fleckeri'') antivenene: Neutralizing potency against the venom of three other jellyfish species|journal=Toxicon|year=1974|month=May|volume=12|series=Toxicon|issue=3|pages=223–225|doi=10.1016/0041-0101(74)90062-2}}</ref> administration requires medical personnel and refrigeration and are used in extreme cases as with regard to the box jellyfish, Chironex.<ref>{{cite web|title=Jellyfish Stings: Treatment and Drugs|url=http://www.mayoclinic.com/health/jellyfish-stings/DS01119/DSECTION=treatments%2Dand%2Ddrugs|work=Mayo Clinic|publisher=Mayo Foundation for Medical Education and Research|accessdate=2013-04-15|date=2011-09-01}}</ref>

=== Hazards ===

Jellyfish adversely affect humanity by interfering with public systems and harming swimmers.<ref name="smith" /> The most obvious consequences are human injury or death and reduced coastal tourism. Jellies destroy fish nets, poison or crush captured fish, and consume fish eggs and young fish.<ref name="nsf">{{cite web|url=http://www.nsf.gov/news/special_reports/jellyfish/textonly/swarms.jsp |title=Jellyfish Gone Wild — Text-only |publisher=Nsf.gov |date= |accessdate=2010-10-18}}</ref>

Jellyfish can clog cooling equipment, disabling [[power plant]]s in several countries. Jellyfish caused a cascading blackout in the Philippines in 1999,<ref name="smith">{{Cite journal
| last=Tucker
| first=Abigail
| title=The New King of the Sea
| journal=Smithsonian
| year=2010
| month=July }}
</ref> as well as damaging the [[Diablo Canyon Power Plant]] in California in 2008.<ref>{{cite web | author= | title=Current Event Notification Report | url=http://www.nrc.gov/reading-rm/doc-collections/event-status/event/2008/20081022en.html#en44588 | publisher=NRC | date=October 22, 2008 | accessdate=14 July 2010}}</ref> Clogging can stop [[desalination plants]], as well as clogging ship engines<ref name="nsf"/> and infesting fishing nets.<ref>
{{cite news
| url = http://www.telegraph.co.uk/earth/6483758/Japanese-fishing-trawler-sunk-by-giant-jellyfish.html
| title = Japanese fishing trawler sunk by giant jellyfish
| first = Julian
| last = Ryall
| date = 2 November 2009
| publisher = Telegraph.co.uk
| accessdate = 3 November 2009
| location=London
}}</ref>

== Haeckel's medusae ==

Illustrations of medusae by German biologist [[Ernst Haeckel]]:

<gallery>
File:Haeckel Narcomedusae.jpg|Narcomedusae
File:Haeckel Discomedusae 18.jpg|Discomedusae
File:Haeckel Trachomedusae.jpg|Trachomedusae
File:Haeckel Discomedusae 28.jpg|Discomedusae
File:Haeckel Leptomedusae.jpg|Leptomedusae
File:Haeckel Peromedusae.jpg|Peromedusae
File:Haeckel Anthomedusae.jpg|Anthomedusae
File:Haeckel Stauromedusae.jpg|[[Stauromedusae]]
File:Haeckel Cubomedusae.jpg|Cubomedusae (modern [[Box jellyfish|Cubozoa]])
File:Haeckel Discomedusae 88.jpg|Discomedusae
File:Haeckel Discomedusae 98.jpg|Discomedusae
</gallery>

== See also ==
* [[Jellyfish dermatitis]]
* [[List of prehistoric medusozoans]]
* [[Ocean sunfish]], a significant jellyfish predator

== References ==
{{Reflist|30em}}

== External links ==
{{Commons|Jellyfish}}
{{Commons category|Scyphozoa}}
{{Wikivoyage|Jellyfish}}
*[http://ocean.si.edu/jellyfish-and-comb-jellies Jellyfish and Comb Jellies – Smithsonian Ocean Portal]
*[http://jellieszone.com/ Jellyfish and Other Gelatinous Zooplankton]
*[http://www.jellyfishfacts.net/ Jellyfish Facts – Information on Jellyfish and Jellyfish Safety]
*[http://www.malawicichlidhomepage.com/other/cotylorhiza_tuberculata.html Cotylorhiza tuberculata]
*"[https://www.youtube.com/watch?v=3HzFiQFFQYw There's no such thing as a jellyfish]" from The [[Monterey Bay Aquarium Research Institute|MBARI]] YouTube channel
* [http://www.youtube.com/watch?v=AT4RAb2zrOk "Vicious beauties - Jellyfish"] - a documentary about jellyfish
* [http://www.nybooks.com/articles/archives/2013/sep/26/jellyfish-theyre-taking-over/ They’re Taking Over!]|nybooks.com |September 26, 2013 |Tim Flannery
'''Photos:'''
*[http://picasaweb.google.com/sridhar.saraf.pictures/JellyfishExhibitionAtNationalAquariumBaltimoreMarylandUSA/ Jellyfish Exhibition At National Aquarium, Baltimore, Maryland (USA) – Photo Gallery]

[[Category:Cnidarians]]
[[Category:Hydrozoa| ]]
[[Category:Scyphozoa| ]]

{{Link FA|sh}}

Revision as of 18:00, 14 November 2013

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