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Jellyfish and sea jellies are the informal
common names given to the medusa-phase of
certain gelatinous members of the subphylum
Medusozoa, a major part of the phylum Cnidaria.
Jellyfish are mainly free-swimming marine
animals with umbrella-shaped bells and trailing
tentacles, although a few are not mobile,
being anchored to the seabed by stalks.
The bell can pulsate to provide propulsion
and highly efficient locomotion.
The tentacles are armed with stinging cells
and may be used to capture prey and defend
against predators.
Jellyfish have a complex life cycle; the medusa
is normally the sexual phase, the planula
larva can disperse widely and is followed
by a sedentary polyp phase.
Jellyfish are found all over the world, from
surface waters to the deep sea.
Scyphozoans (the "true jellyfish") are exclusively
marine, but some hydrozoans with a similar
appearance live in freshwater.
Large, often colorful, jellyfish are common
in coastal zones worldwide.
The medusae of most species are fast growing,
mature within a few months and die soon after
breeding, but the polyp stage, attached to
the seabed, may be much more long-lived.
Jellyfish have been in existence for at least
500 million years, and possibly 700 million
years or more, making them the oldest multi-organ
animal group.Jellyfish are eaten by humans
in certain cultures, being considered a delicacy
in some Asian countries, where species in
the Rhizostomae order are pressed and salted
to remove excess water.
They are also used in research, where the
green fluorescent protein, used by some species
to cause bioluminescence, has been adapted
as a fluorescent marker for genes inserted
into other cells or organisms.
The stinging cells used by jellyfish to subdue
their prey can also injure humans.
Many thousands of swimmers are stung every
year, with effects ranging from mild discomfort
to serious injury or even death; small box
jellyfish are responsible for many of these
deaths.
When conditions are favourable, jellyfish
can form vast swarms.
These can be responsible for damage to fishing
gear by filling fishing nets, and sometimes
clog the cooling systems of power and desalination
plants which draw their water from the sea.
== Names ==
The name jellyfish, in use since 1796, has
traditionally been applied to medusae and
all similar animals including the comb jellies
(ctenophores, another phylum).
The term jellies or sea jellies is more recent,
having been introduced by public aquaria in
an effort to avoid use of the word "fish"
with its connotations of an animal with a
backbone, though shellfish, cuttlefish and
starfish are not vertebrates either.
In scientific literature, "jelly" and "jellyfish"
have been used interchangeably.
Many sources refer to only scyphozoans as
"true jellyfish".
== Mapping to taxonomic groups ==
===
Phylogeny ===
====
Definition ====
The term jellyfish broadly corresponds to
medusae, that is, a life-cycle stage in the
Medusozoa.
The American evolutionary biologist Paulyn
Cartwright gives the following general definition:
Typically, medusozoan cnidarians have a pelagic,
predatory jellyfish stage in their life cycle;
staurozoans are the exceptions [as they are
stalked].
The Merriam-Webster dictionary defines jellyfish
as follows:
A free-swimming marine coelenterate that is
the sexually reproducing form of a hydrozoan
or scyphozoan and has a nearly transparent
saucer-shaped body and extensible marginal
tentacles studded with stinging cells.
Given that jellyfish is a common name, its
mapping to biological groups is inexact.
Some authorities have called the comb jellies
and certain salps jellyfish, though other
authorities state that neither of these are
jellyfish, which they consider should be limited
to certain groups within the medusozoa.The
non-medusozoan clades called jellyfish by
some but not all authorities (both agreeing
and disagreeing citations are given in each
case) are indicated with "???" on the following
cladogram of the animal kingdom:
==== Medusozoan jellyfish ====
Jellyfish are not a clade, as they include
most of the Medusozoa, barring some of the
Hydrozoa.
The medusozoan groups included by authorities
are indicated on the following phylogenetic
tree by the presence of citations.
Names of included jellyfish, in English where
possible, are shown in boldface; the presence
of a named and cited example indicates that
at least that species within its group has
been called a jellyfish.
=== Taxonomy ===
The subphylum Medusozoa includes all cnidarians
with a medusa stage in their life cycle.
The basic cycle is egg, planula larva, polyp,
medusa, with the medusa being the sexual stage.
The polyp stage is sometimes secondarily lost.
The subphylum include the major taxa, Scyphozoa
(large jellyfish), Cubozoa (box jellyfish)
and Hydrozoa (small jellyfish), and excludes
Anthozoa (corals and sea anemones).
This suggests that the medusa form evolved
after the polyps.
Medusozoans have tetramerous symmetry, with
parts in fours or multiples of four.The four
major classes of medusozoan Cnidaria are:
Scyphozoa are sometimes called true jellyfish,
though they are no more truly jellyfish than
the others listed here.
They have tetra-radial symmetry.
Most have tentacles around the outer margin
of the bowl-shaped bell, and long, oral arms
around the mouth in the center of the subumbrella.
Cubozoa (box jellyfish) have a (rounded) box-shaped
bell, and their velarium assists them to swim
more quickly.
Box jellyfish may be related more closely
to scyphozoan jellyfish than either are to
the Hydrozoa.
Hydrozoa medusae also have tetra-radial symmetry,
nearly always have a velum (diaphragm used
in swimming) attached just inside the bell
margin, do not have oral arms, but a much
smaller central stalk-like structure, the
manubrium, with terminal mouth opening, and
are distinguished by the absence of cells
in the mesoglea.
Hydrozoa show great diversity of lifestyle;
some species maintain the polyp form for their
entire life and do not form medusae at all
(such as Hydra, which is hence not considered
a jellyfish), and a few are entirely medusal
and have no polyp form.
Staurozoa (stalked jellyfish) are characterized
by a medusa form that is generally sessile,
oriented upside down and with a stalk emerging
from the apex of the "calyx" (bell), which
attaches to the substrate.
At least some Staurozoa also have a polyp
form that alternates with the medusoid portion
of the life cycle.
Until recently, Staurozoa were classified
within the Scyphozoa.There are over 200 species
of Scyphozoa, about 50 species of Staurozoa,
about 20 species of Cubozoa, and the Hydrozoa
includes about 1000–1500 species that produce
medusae, but many more species that do not.
=== Fossil history ===
Since jellyfish have no hard parts, fossils
are rare.
The oldest conulariid scyphozoans appeared
between 635 and 577 mya in the Neoproterozoic
of the Lantian Formation in China; others
are found in the youngest Ediacaran rocks
of the Tamengo Formation of Brazil, c. 505
mya, through to the Triassic.
Cubozoans and hydrozoans appeared in the Cambrian
of the Marjum Formation in Utah, USA, c. 540
mya.
== Anatomy ==
The main feature of a true jellyfish is the
umbrella-shaped bell.
This is a hollow structure consisting of a
mass of transparent jelly-like matter known
as mesoglea, which forms the hydrostatic skeleton
of the animal.
95% or more of the mesogloea (the tissue that
functions as a hydro-static skeleton) consists
of water, but it also contains collagen and
other fibrous proteins, as well as wandering
amoebocytes which can engulf debris and bacteria.
The mesogloea is bordered by the epidermis
on the outside and the gastrodermis on the
inside.
The edge of the bell is often divided into
rounded lobes known as lappets, which allow
the bell to flex.
In the gaps or niches between the lappets
are dangling rudimentary sense organs known
as rhopalia, and the margin of the bell often
bears tentacles.
On the underside of the bell is the manubrium,
a stalk-like structure hanging down from the
centre, with the mouth, which also functions
as the anus, at its tip.
There are often four oral arms connected to
the manubrium, streaming away into the water
below.
The mouth opens into the gastrovascular cavity,
where digestion takes place and nutrients
are absorbed.
This is subdivided by four thick septa into
a central stomach and four gastric pockets.
The four pairs of gonads are attached to the
septa, and close to them four septal funnels
open to the exterior, perhaps supplying good
oxygenation to the gonads.
Near the free edges of the septa, gastric
filaments extend into the gastric cavity;
these are armed with nematocysts and enzyme-producing
cells and play a role in subduing and digesting
the prey.
In some scyphozoans, the gastric cavity is
joined to radial canals which branch extensively
and may join a marginal ring canal.
Cilia in these canals circulate the fluid
in a regular direction.
The box jellyfish is largely similar in structure.
It has a squarish, box-like bell.
A short pedalium or stalk hangs from each
of the four lower corners.
One or more long, slender tentacles are attached
to each pedalium.
The rim of the bell is folded inwards to form
a shelf known as a velarium which restricts
the bell's aperture and creates a powerful
jet when the bell pulsates, allowing box jellyfish
to swim faster than true jellyfish.
Hydrozoans are also similar, usually with
just four tentacles at the edge of the bell,
although many hydrozoans are colonial and
may not have a free-living medusal stage.
In some species, a non-detachable bud known
as a gonophore is formed that contains a gonad
but is missing many other medusal features
such as tentacles and rhopalia.
Stalked jellyfish are attached to a solid
surface by a basal disk, and resemble a polyp,
the oral end of which has partially developed
into a medusa with tentacle-bearing lobes
and a central manubrium with four-sided mouth.Most
jellyfish do not have specialized systems
for osmoregulation, respiration and circulation,
and do not have a central nervous system.
Nematocysts, which deliver the sting, are
located mostly on the tentacles; true jellyfish
also have them around the mouth and stomach.
Jellyfish do not need a respiratory system
because sufficient oxygen diffuses through
the epidermis.
They have limited control over their movement,
but can navigate with the pulsations of the
bell-like body; some species are active swimmers
most of the time, while others largely drift.
The rhopalia contain rudimentary sense organs
which are able to detect light, water-borne
vibrations, odour and orientation.
A loose network of nerves called a "nerve
net" is located in the epidermis.
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.
A jellyfish detects stimuli, and transmits
impulses both throughout the nerve net and
around a circular nerve ring, to other nerve
cells.
The rhopalial ganglia contain pacemaker neurones
which control swimming rate and direction.In
many species of jellyfish, the rhopalia include
ocelli, light-sensitive organs able to tell
light from dark.
These are generally pigment spot ocelli, which
have some of their cells pigmented.
The rhopalia are suspended on stalks with
heavy crystals at one end, acting like gyroscopes
to orient the eyes skyward.
Certain jellyfish look upward at the mangrove
canopy while making a daily migration from
mangrove swamps into the open lagoon, where
they feed, and back again.Box jellyfish have
more advanced vision than the other groups.
Each individual has 24 eyes, two of which
are capable of seeing colour, and four parallel
information processing areas that act in competition,
supposedly making them one of the few kinds
of animal to have a 360-degree view of its
environment.
== Largest and smallest ==
Jellyfish range from about one millimeter
in bell height and diameter, to nearly 2 metres
(6.6 ft) 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, which
have bell disks from 0.5 millimetres (0.02
in) to a few millimeters in diameter, with
short tentacles that extend out beyond this,
which these jellyfish use to move across the
surface of seaweed or the bottoms of rocky
pools.
Many of these tiny creeping jellyfish cannot
be seen in the field without a hand lens or
microscope; they can reproduce asexually by
fission (splitting in half).
Other very small jellyfish, which have bells
about one millimeter, are the hydromedusae
of many species that have just been released
from their parent polyps; 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.
The lion's mane 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 36.5 metres (120 ft) long (though
most are nowhere near that large).
They have a moderately painful, but rarely
fatal, sting.
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 another candidate
for "largest jellyfish", in terms of diameter
and weight, since the largest Nomura's jellyfish
in late autumn can reach 2 metres (6 ft 7
in) in bell (body) diameter and about 200
kilograms (440 lb) in weight, with average
specimens frequently reaching 0.9 metres (2
ft 11 in) in bell diameter and about 150 kilograms
(330 lb) in weight.
The large bell mass of the giant Nomura's
jellyfish can dwarf a diver and is nearly
always much greater than the Lion's Mane,
whose bell diameter can reach 1 metre (3 ft
3 in).The rarely encountered deep-sea jellyfish
Stygiomedusa gigantea is another candidate
for "largest jellyfish", with its thick, massive
bell up to 100 centimetres (39 in) wide, and
four thick, "strap-like" oral arms extending
up to 6 metres (20 ft) in length, very different
from the typical fine, threadlike tentacles
that rim the umbrella of more-typical-looking
jellyfish, including the Lion's Mane.
== Life history and behavior ==
=== Life cycle ===
Jellyfish have a complex life cycle which
includes both sexual and asexual phases, with
the medusa being the sexual stage in most
instances.
Sperm fertilize eggs, which develop into larval
planulae, become polyps, bud into ephyrae
and then transform into adult medusae.
In some species certain stages may be skipped.Upon
reaching adult size, jellyfish spawn regularly
if there is a sufficient supply of food.
In most species, spawning is controlled by
light, with all individuals spawning at about
the same time of day, in many instances this
is at dawn or dusk.
Jellyfish are usually either male or female
(with occasional hermaphrodites).
In most cases, adults release sperm and eggs
into the surrounding water, where the unprotected
eggs are fertilized and develop into larvae.
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.The
planula is a small larva covered with cilia.
When sufficiently developed, it settles onto
a firm surface and develops into a polyp.
The polyp generally consists of a small stalk
topped by a mouth that is ringed by upward-facing
tentacles.
The polyps resemble those of closely related
anthozoans, such as sea anemones and corals.
The jellyfish polyp may be sessile, living
on the bottom, boat hulls or other substrates,
or it may be free-floating or attached to
tiny bits of free-living plankton or rarely,
fish or other invertebrates.
Polyps may be solitary or colonial.
Most polyps are only millimetres in diameter
and feed continuously.
The polyp stage may last for years.After an
interval and stimulated by seasonal or hormonal
changes, the polyp may begin 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 gonads of hydromedusae.
In a process known as strobilation, the polyp's
tentacles are reabsorbed and the body starts
to narrow, forming transverse constrictions,
in several places near the upper extremity
of the polyp.
These deepen as the constriction sites migrate
down the body, and separate segments known
as ephyra detach.
These are free-swimming precursors of the
adult medusa stage, which is the life stage
that is typically identified as a jellyfish.
The ephyrae, usually only a millimeter or
two across initially, swim away from the polyp
and grow.
Limnomedusae polyps can asexually produce
a creeping frustule larval form, which crawls
away before developing into another polyp.
A few species can produce new medusae by budding
directly from the medusan stage.
Some hydromedusae reproduce by fission.
=== Lifespan ===
Little is known of the life histories of many
jellyfish as the places on the seabed where
the benthic forms of those species live have
not been found.
However, an asexually reproducing strobila
form can sometimes live for several years,
producing new medusae (ephyra larvae) each
year.An unusual species, Turritopsis dohrnii,
formerly classified as Turritopsis nutricula,
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.
=== Locomotion ===
Using the moon jelly Aurelia aurita as an
example, jellyfish have been shown to be the
most energy efficient swimmers of all animals.
They 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
phases to create two vortex rings.
Muscles are used for the contraction of the
body, which creates the first vortex and pushes
the animal forward, but the mesoglea is so
elastic that the expansion is powered exclusively
by relaxing the bell, which releases the energy
stored from the contraction.
Meanwhile, the second vortex ring starts to
spin faster, sucking water into the bell and
pushing against the centre of the body, giving
a secondary and "free" boost forward.
The mechanism, called passive energy recapture,
only works in relatively small jellyfish moving
at low speeds, allowing the animal to travel
30 percent farther on each swimming cycle.
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.
One reason for this is that most of the gelatinous
tissue of the bell is inactive, using no energy
during swimming.
== Ecology ==
=== Diet ===
Jellyfish are like other cnidarians generally
carnivorous (or parasitic), feeding on planktonic
organisms, crustaceans, small fish, fish eggs
and larvae, and other jellyfish, ingesting
food and voiding undigested waste through
the mouth.
They hunt passively using their tentacles
as drift lines, or sink through the water
with their tentacles spread widely; the tentacles,
which contain nematocysts to stun or kill
the prey, may then flex to help bring it to
the mouth.
Their swimming technique also helps them to
capture prey; when their bell expands it sucks
in water which brings more potential prey
within reach of the tentacles.A few species
such as Aglaura hemistoma are omnivorous,
feeding on microplankton which is a mixture
of zooplankton and phytoplankton (microscopic
plants) such as dinoflagellates.
Others harbour mutualistic algae (Zooxanthellae)
in their tissues; the spotted jellyfish (Mastigias
papua) is typical of these, deriving part
of its nutrition from the products of photosynthesis,
and part from captured zooplankton.
=== Predation ===
Other species of jellyfish are among the most
common and important jellyfish predators.
Sea anemones may eat jellyfish that drift
into their range.
Other predators include tunas, sharks, swordfish,
sea turtles and penguins.
Jellyfish washed up on the beach are consumed
by foxes, other terrestrial mammals and birds.
In general however, there are few animals
preying on jellyfish.
Jellyfish can broadly be considered to be
top predators in the food chain.
Once jellyfish have become dominant in an
ecosystem, for example through overfishing
which removes predators of jellyfish larvae,
there may be no obvious way for the previous
balance to be restored: they eat fish eggs
and juvenile fish, and compete with fish for
food, preventing fish stocks from recovering.
=== Symbiosis ===
Some small fish are immune to the stings of
the jellyfish and live among the tentacles,
serving as bait in a fish trap; they are safe
from potential predators and are able to share
in the fish caught by the jellyfish.
The cannonball jellyfish has a symbiotic relationship
with ten different species of fish, and with
the longnose spider crab, which lives inside
the bell, sharing the jellyfish's food and
nibbling its tissues.
=== Blooms ===
Jellyfish form large masses or blooms in certain
environmental conditions of ocean currents,
nutrients, sunshine, temperature, season,
prey availability, reduced predation and oxygen
concentration.
Currents collect jellyfish together, especially
in years with unusually high populations.
Jellyfish can detect marine currents and swim
against the current to congregate in blooms.
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.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,
Hawaii, and tropical and subtropical parts
of the West Atlantic (including the Caribbean,
Gulf of Mexico and Brazil).Increased nutrients
from agricultural or urban runoff with nutrients
including nitrogen and phosphorus compounds
increase the growth of phytoplankton, causing
eutrophication and algal blooms.
When the phytoplankton die, they may create
dead zones, so called because they are ahypoxic
(low in oxygen).
This in turn kills fish and other animals,
but not jellyfish, allowing them to bloom.
Jellyfish populations may be expanding globally
as a result of land runoff and overfishing
of their natural predators.
Jellyfish are well placed to benefit from
disturbance of marine ecosystems.
They reproduce rapidly; they prey upon many
species, while few species prey on them; and
they feed via touch rather than visually,
so they can feed effectively at night and
in turbid waters.
It may be difficult for fish stocks to reestablish
themselves in marine ecosystems once they
have become dominated by jellyfish, because
jellyfish feed on plankton, which includes
fish eggs and larvae.Jellyfish form a component
of jelly-falls, events where gelatinous zooplankton
fall to the seafloor, providing food for the
benthic organisms there.
In temperate and subpolar regions, jelly-falls
usually follow immediately after a bloom.
=== Habitats ===
Most jellyfish are marine animals, although
a few hydromedusae inhabit freshwater.
The best known freshwater example is the cosmopolitan
hydrozoan jellyfish, Craspedacusta sowerbii.
It is less than an inch (2.5 cm) in diameter,
colorless and does not sting.
Some jellyfish populations have become restricted
to coastal saltwater lakes, such as Jellyfish
Lake in Palau.
Jellyfish Lake is a marine lake where millions
of golden jellyfish (Mastigias spp.) migrate
horizontally across the lake daily.Although
most jellyfish live well off the ocean floor
and form part of the plankton, a few species
are closely associated with the bottom for
much of their lives and 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 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.
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.Some species explicitly adapt
to tidal flux.
In 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 until the tide
rises, ascending and allowing it to sweep
them back into the bay.
They also actively avoid fresh water from
mountain snowmelt, diving until they find
enough salt.
=== Parasites ===
Jellyfish are hosts to a wide variety of parasitic
organisms.
They act as intermediate hosts of endoparasitic
helminths, with the infection being transferred
to the definitive host fish after predation.
Some digenean trematodes, especially species
in the family Lepocreadiidae, use jellyfish
as their second intermediate hosts.
Fish become infected by the trematodes when
they feed on infected jellyfish.
== Relation to humans ==
===
Fisheries ===
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.Jellyfish are also harvested for their
collagen, which is being investigated for
use in a variety of applications including
the treatment of rheumatoid arthritis.
=== Products ===
Aristotle stated in the Parts of Animals IV,
6 that jellyfish (sea-nettles) were eaten
in winter time in a fish stew.In some countries,
such as China, Japan, and Korea, jellyfish
are a delicacy.
The jellyfish is dried to prevent spoiling.
Only some 12 species of scyphozoan jellyfish
belonging to the order Rhizostomeae are harvested
for food, mostly in southeast Asia.
Rhizostomes, especially Rhopilema esculentum
in China (海蜇 hǎizhé, "sea stingers")
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 makes the jellyfish drier and more
acidic, producing a crispy texture.
Jellyfish prepared this way retain 7–10%
of their original weight, and the processed
product consists of 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.
Desalted, ready-to-eat products are also available.
=== Biotechnology ===
Pliny the Elder reported in his Natural History
that the slime of the jellyfish "Pulmo marinus"
produced light when rubbed on a walking stick.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
photoproteins 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 widely 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.
=== Aquarium display ===
Jellyfish are displayed in many public aquariums.
Often the tank's background is blue and the
animals are illuminated by side light, increasing
the contrast between the animal and the background.
In natural conditions, many jellies are so
transparent that they are nearly invisible.
Jellyfish are not adapted to closed spaces.
They depend on currents to transport them
from place to place.
Professional exhibits as in the Monterey Bay
Aquarium feature precise water flows, typically
in circular tanks to avoid trapping specimens
in corners.
They have a live "Jelly Cam".
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
do not get sucked into it.
As of 2009, jellyfish were becoming popular
in home aquariums, where they require similar
equipment.
=== Stings ===
Jellyfish are armed with nematocysts.
Contact with a jellyfish tentacle can trigger
millions of nematocysts to pierce the skin
and inject venom, but only some species' venom
causes an adverse reaction in humans.
The effects of stings range from mild discomfort
to extreme pain and death.
Most jellyfish stings are not deadly, but
stings of some box jellyfish (Irukandji jellyfish),
such as the sea wasp, can be deadly.
Stings may cause anaphylaxis (a form of shock),
which can be fatal.
Jellyfish kill 20 to 40 people a year in the
Philippines alone.
In 2006 the Spanish Red Cross treated 19,000
stung swimmers along the Costa Brava.Vinegar
(3–10% aqueous acetic acid) may help with
box jellyfish stings, but not the stings of
the Portuguese man o' war.
Salt water may help if vinegar is unavailable.
Rubbing wounds, or using alcohol, ammonia,
fresh water, or urine is not advised as they
can encourage the release of more venom.
Clearing the area of jelly and tentacles reduces
nematocyst firing.
Scraping the affected skin, such as with the
edge of a credit card, may remove remaining
nematocysts.
Once the skin has been cleaned of nematocysts,
hydrocortisone cream applied locally reduces
pain and inflammation.
Antihistamines may help to control itching.
Immunobased antivenins are used for serious
box jellyfish stings.
=== Mechanical issues ===
Jellyfish in large quantities can fill and
split fishing nets and crush captured fish.
They can clog cooling equipment, disabling
power stations in several countries; jellyfish
caused a cascading blackout in the Philippines
in 1999, as well as damaging the Diablo Canyon
Power Plant in California in 2008.
They can stop desalination plants and ships'
engines.
== See also ==
Jellyfish dermatitis
List of prehistoric medusozoans
Ocean sunfish, a significant jellyfish predator

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