Question:

Can jellyfishes breath underwater or do they have to come up for air?

Answer:

No jellyfishes have lungs and breath through all the surface of their body. If you touch them they sting you so be careful in the sea if you are ever swimming.

More Info:


Aurelia aurita
Aurelia aurita (also called the moon jelly, moon jellyfish, common jellyfish, or saucer jelly) is a widely studied species of the genus Aurelia. All species in the genus are closely related, and it is difficult to identify Aurelia medusae without genetic sampling; most of what follows applies equally to all species of the genus. The jellyfish is translucent, usually about 25–40 cm in diameter, and can be recognized by its four horseshoe-shaped gonads, easily seen through the top of the bell. It feeds by collecting medusae, plankton, and mollusks with its tentacles, and bringing them into its body for digestion. It is capable of only limited motion, and drifts with the current, even when swimming. The genus Aurelia is found throughout most of the world's oceans, from the tropics to as far north as latitude 70°N and as far south as 40°S. The species Aurelia aurita is found along the eastern Atlantic coast of Northern Europe and the western Atlantic coast of North America in New England and Eastern Canada. In general, Aurelia is an inshore genus that can be found in estuaries and harbors. It lives in ocean water temperatures ranging from 6 °C to 31 °C; with optimum temperatures of 9 °C to 19 °C. A. aurita prefers temperate seas with consistent currents. It has been found in waters with salinity as low as 6 parts per thousand. A. aurita and other Aurelia species feed on plankton that includes organisms such as mollusks, crustaceans, tunicate larvae, rotifers, young polychaetes, protozoans, diatoms, eggs, fish eggs, and other small organisms. Occasionally, they are also seen feeding on gelatinous zooplankton such as hydromedusae and ctenophores. Both the adult medusae and larvae of Aurelia have nematocysts to capture prey and also to protect themselves from predators. The food is caught with its nematocyst-laden tentacles, tied with mucus, brought to the gastrovascular cavity, and passed into the cavity by ciliated action. There, digestive enzymes from serous cell break down the food. There is little known about the requirements for particular vitamins and minerals, but due to the presence of some digestive enzymes, we can deduce in general that A. aurita can process carbohydrates, proteins and lipids. The invention of the Kreisel tank has made it possible to maintain these jellies in captivity. Aureliaauritakils1.jpg
High resolution in situ image of an undulating live Aurelia in the Baltic showing the grid of tentacles which are slowly pulled through the water. The motion is so slow that copepods can not sense it and don't react with an escape response Aureliaauritakils2.jpg Higher magnification showing a prey item, probably a copepod Aureliaauritakils3.jpg The prey is then drawn to the body by contracting the tentacles in a corkscrew fashion (image taken with an ecoSCOPE). Aurelia does not have respiratory parts such as gills, lungs, or trachea. Since it is a small organism][, it respires by diffusing oxygen from water through the thin membrane covering its body. Within the gastrovascular cavity, low oxygenated water can be expelled and high oxygenated water can come in by ciliated action, thus increasing the diffusion of oxygen through cell. The large surface area membrane to volume ratio helps Aurelia to diffuse more oxygen and nutrients into the cells. The basic body plan of Aurelia consists of several parts. The animal lacks respiratory, excretory, and circulatory systems. The adult medusa of Aurelia, with a transparent look, has an umbrella margin membrane and tentacles that are attached to the bottom. It has four bright gonads that are under the stomach. Food travels through the muscular manubrium while the radial canals help disperse the food. There is a middle layer of mesoglea, gastrodervascular cavity with gastrodermis, and epidermis. There is a nerve net that is responsible for contractions in swimming muscles and feeding responses. Adult medusa can have a diameter up to 40 cm. The medusae are either male or female. The young larval stage, a planula, has small ciliated cells and after swimming freely in the plankton for a day or more, settles on an appropriate substrate, where it changes into a special type of polyp called a "scyphistoma", which divides by strobilation into small ephyrae that swim off to grow up as medusae. There is an increasing size from starting stage planula to ephyra, from less than 1 mm in the planula stage, up to about 1 cm in ephyra stage, and then to several cm in diameter in the medusa stage. Aurelia aurita is known to be eaten by a wide variety of predators including the Ocean Sunfish (Mola mola), the Leatherback Sea Turtle (Dermochelys coriacea), the scyphomedusa Phacellophora camtschatica, and a very large hydromedusa (Aequorea victoria). Moon jellies are also fed upon by sea birds, which may be more interested in the amphipods and other small arthropods that frequent the bells of Aurelia, but in any case, birds do some substantial amount of damage to these jellyfish that often are found just at the surface of bays. Aurelia jellyfish naturally die after living and reproducing for several months. It is probably rare for these moon jellies to live more than about six months in the wild, although specimens cared for in public aquarium exhibits typically live several to many years. In the wild, the warm water at the end of summer combines with exhaustive daily reproduction and lower natural levels of food for tissue repair, leaving these jellyfish more susceptible to bacterial and other disease problems that likely lead to the demise of most individuals. Such problems are responsible for the demise of many smaller species of jellyfish. In 1997, Arai summarized that seasonal reproduction leaves the gonads open to infection and degradation. Some metazoan parasites attack Aurelia aurita, as well as most other species of jellyfish. Aurelia sp. from the Monterey Bay Aquarium Aurelia sp. An adult Aurelia aurita

Box jellyfish
Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their cube-shaped medusae. Box jellyfish are known for the extremely potent venom produced by some species. Chironex fleckeri, Carukia barnesi and Malo kingi are among the most venomous creatures in the world. Stings from these and a few other species in the class are extremely painful and sometimes fatal to humans. "Box jellyfish" or "sea wasp" are common names for the notoriously dangerous Chironex fleckeri. However, these terms are ambiguous, seeing as "sea wasp" and "marine stinger" are sometimes used to refer to a wide array of other box jellyfish. Box jellyfish most visibly differ from the Scyphozoan jellyfish in that they are umbrella shaped, rather than domed or crown-shaped. The underside of the umbrella includes a flap, or velarium, concentrating and increasing the flow of water expelled from the umbrella. As a result, box jellyfish can move more rapidly than other jellyfish. In fact, speeds of up to six meters per minute have been recorded. The box jellyfish's nervous system is also more developed than that of many other jellyfish. Notably, they possess a nerve ring around the base of the umbrella that coordinates their pulsing movements; a feature found elsewhere only in the crown jellyfish. Whereas some other jellyfish do have simple pigment-cup ocelli, box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are located on each of the four sides of their bell in clusters called rhopalia. This enables them to see specific points of light, as opposed to simply distinguishing between light and the dark. Box jellies also retain the lesser type of eye][, because the strong eyes][ are only one of four subsets. They therefore have 24 eyes. A box jellyfish has the closest thing a known jellyfish has to a brain. Box jellyfish also display complex, probably visually guided behaviors such as obstacle avoidance and fast directional swimming. Tests have shown that they have a limited memory, and have a limited ability to learn. Research indicates that, owing to the number of rhopalial nerve cells and their overall arrangement, visual processing and integration at least partly happen within the rhopalia of box jelly fish. Some species have tentacles that can reach up to 3 metres in length. Box jellyfish can weigh up to 2 kg. Although the notoriously dangerous species of box jellies are largely, or entirely, restricted to the tropical Indo-Pacific, various species of box jellies can be found widely in tropical and subtropical oceans, including the Atlantic and east Pacific, with species as far north as California, the Mediterranean (e.g., Carybdea marsupialis) and Japan (e.g., Chironex yamaguchii), and as far south as South Africa (e.g., Carybdea branchi) and New Zealand (e.g., Carybdea sivickisi). The box jellyfish actively hunts its prey (zooplankton and small fish), rather than drifting as do true jellyfish. They are capable of achieving speeds of up to 1.8 m/s. Each tentacle has about 500,000 cnidocytes, containing nematocysts, a harpoon-shaped microscopic mechanism that injects venom into the victim. Many different kinds of nematocysts are found in cubozoans. The venom of cubozoans is distinct from that of scyphozoans, and is used to catch prey (small fish and invertebrates, including shrimp and bait fish) and for defense from predators, which include the butterfish, batfish, rabbitfish, crabs (Blue Swimmer Crab) and various species of sea turtles (hawksbill turtle, flatback turtle). Sea turtles, however, are apparently unaffected by the sting and eat box jellyfish. Although the box jellyfish has been called "the world's most venomous creature", only a few species in the class have been confirmed to be involved in human deaths, and some species pose no serious threat at all. For example, the sting of Chiropsella bart only results in short-lived itching and mild pain. In Australia, fatalities are most often perpetrated by the largest species of this class of jellyfish Chironex fleckeri. In December 2012, Angel Yanagihara of the University of Hawaii's Department of Tropical Medicine found the venom causes cells to become porous enough to allow potassium leakage, causing hyperkalemia which can lead to cardiovascular collapse and death as quickly as within 2 to 5 minutes. She postulates a zinc compound may be developed as an antidote. The recently discovered and very similar Chironex yamaguchii may be equally dangerous, as it has been implicated in several deaths in Japan. It is unclear hence which of these species is the one usually involved in fatalities in the Malay Archipelago. In 1990, a 4-year-old child died after being stung by Chiropsalmus quadrumanus at Galveston Island in the Gulf of Mexico, and either this species or Chiropsoides buitendijki are considered the likely perpetrators of two deaths in West Malaysia. At least two deaths in Australia have been attributed to the thumbnail-sized Irukandji jellyfish. Those who fall victim to these may suffer severe physical and psychological symptoms known as Irukandji syndrome. Nevertheless, most victims do survive, and out of 62 people treated for Irukandji envenomation in Australia in 1996, almost half could be discharged home with few or no symptoms after 6 hours, and only two remained hospitalized approximately a day after they were stung. In Australia, C. fleckeri has caused at least 64 deaths since the first report in 1883, but even in this species most encounters appear to only result in mild envenoming. Most recent deaths in Australia have been in children, which is linked to their smaller body mass. In parts of the Malay Archipelago, the number of lethal cases is far higher (in the Philippines alone, an estimated 20-40 die annually from Chirodropid stings), likely due to limited access to medical facilities and antivenom, and the fact that many Australian beaches are enclosed in nets and have vinegar placed in prominent positions allowing for rapid first aid. Vinegar is also used as treatment by locals in the Philippines. Box jellyfish are known as the "suckerpunch" of the sea not only because their sting is rarely detected until the venom is injected, but also because they are almost transparent. In northern Australia, the highest risk period for the box jellyfish is between October and May, but stings and specimens have been reported all months of the year. Similarly, the highest risk conditions are those with calm water and a light, onshore breeze; however, stings and specimens have been reported in all conditions. In Hawaii, box jellyfish numbers peak approximately 7 to 10 days after a full moon, when they come near the shore to spawn. Sometimes the influx is so severe that lifeguards have closed infested beaches, such as Hanauma Bay, until the numbers subside. Once a tentacle of the box jellyfish adheres to skin, it pumps nematocysts with venom into the skin, causing the sting and agonizing pain. Domestic vinegars have been confirmed as an effective treatment as they disable the sea wasp's nematocysts not yet discharged into the bloodstream. Pressure immobilisation can also be used on limbs to slow down the spreading of the deadly venom. Common practice is to apply generous amounts of vinegar prior to and after the stinging tentacle is removed. Removal of additional tentacles is usually done with a towel or gloved hand, to prevent secondary stinging. Tentacles will still sting if separated from the bell, or after the creature is dead. Removal of tentacles without prior application of vinegar may cause unfired nematocysts to come into contact with the skin and fire, resulting in a greater degree of envenomation.][ Although commonly recommended in folklore and even some papers on sting treatment, there is no scientific evidence that urine, ammonia, meat tenderizer, sodium bicarbonate, boric acid, lemon juice, fresh water, steroid cream, alcohol, cold packs, papaya, or hydrogen peroxide will disable further stinging, and these substances may even hasten the release of venom. Pressure immobilization bandages, methylated spirits, or vodka should never be used for jelly stings. In severe Chironex fleckeri stings cardiac arrest can occur quickly, so cardiopulmonary resuscitation (CPR) can be life-saving and takes priority over all other treatment options.][ In 2011, University of Hawaii Assistant Research Professor Angel Yanagihara announced that she had developed an effective treatment by "deconstructing" the venom contained in the box jellyfish tentacles. Its effectiveness was demonstrated in the PBS NOVA documentary Venom: Nature's Killer, originally shown on North American television in February 2012. Wearing pantyhose during diving (both by women and men, also under scuba-diving suit) is an effective protection against box jellyfish stings. The pantyhose were formerly thought to work because of the length of the box jellyfish's stingers (nematocysts), but it is now known to be related to the way the stinger cells work. The stinging cells on a box jellyfish's tentacles are not triggered by touch, but are instead triggered by the chemicals found on skin. As of 2007, at least 36 species of box jellyfish were known. These are grouped into two orders and seven families. A few new species have been described since then, and it is likely undescribed species remain. Class Cubozoa

Jellyfish
Jellyfish or jellies 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 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, and possibly 700 million years or more, making them the oldest multi-organ animal. The English popular name jellyfish has been in use since 1796. 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". Even some scientists include the phylum ctenophora when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column. 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. Indeed, it may be said that the term "jellies" has become more popular than "jellyfish". In scientific literature, "jelly" and "jellyfish" are often used interchangeably. Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate. Many textbooks and sources refer to only scyphozoa as "true jellyfish". A group of jellyfish is sometimes called a bloom or a swarm. "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Another collective name for a group of jellyfish is a smack, 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 polyps 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. Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate. Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs. The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology. 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 of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. 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 tentacles as its said. German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur. Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or 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. 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.][ The body is composed of over 95% water; 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. Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net". 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 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][ is that some species explicitly adapt to tidal flux to control their location. 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 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. 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 rhopalia that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment. The eyes are suspended on stalks with heavy crystals 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. 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, which have bell disks from 0.5 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. 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; 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. Another small species of jellyfish is the Australian Irukandji, which is about the size of a fingernail. 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. 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.][ 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 200 centimetres (79 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight, with average specimens frequently reaching 90 centimetres (35 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 up-to-100 centimetres (39 in) bell diameter Lion's Mane. The rarely encountered deep-sea jellyfish Stygiomedusa gigantea is another solid 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. Medusa jellyfish are a life stage exhibited in some species of the phylum Cnidaria. Medusa jellyfish belong exclusively to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps. The phylogenetics of this group are complex and still being worked out. The Medusozoa appear to be a sister group to Octocorallia. 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. There are four major classes of medusozoan Cnidaria: Some other animals are frequently associated with or mistaken for medusa jellyfish. 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). Most jellyfish alternate between polyp and medusa generations during their life cycle. Additionally, there are several possible larval life-stages. After fertilization a primitive free-swimming larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. Some polyps can also asexually produce a creeping frustule larval form, which then also develops into a new polyp. The polyp is generally a small planted stalk with a mouth that is ringed by upward-facing tentacles. The polyps are like miniatures of the closely related anthozoan (sea anemones and corals) polyps, which are also members of Cnidaria. The jellyfish polyp may be sessile, living on the bottom or another substrate such as floats or boat hulls, 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. Polyp colonies form by strobilation, resulting in multiple polyps which share a common stomach cavity. Most polyps are very small, measured in millimeters. They feed continuously. The polyp stage may last for years. Eventually the polyp gives rise to the medusa stage. New medusae are usually created asexually by strobilation or budding from the polyp. The medusa is the life stage which is most typically identified as a jellyfish. Jellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily if there is enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn. Jellyfish are usually either male or female (hermaphroditic specimens are occasionally found). In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body 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. After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or form new, immature jellies called ephyrae. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few species of hydromedusae reproduce by fission (splitting in half). In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton.][ Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles – fringe-like protrusions from the bell's border that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.][ Most jellyfish have a second stage to their life cycle, the planula larvae phase, following the initial egg and sperm phase. Although this is a short lived stage for jellyfish, it is an important phase when the fertilized eggs that had previously undergone embryonic development, hatch, and planulae emerge from the females mouth or brood pouch and are off on their own. Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. Jellyfish held in public aquariums are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, so may live several years, though this would be very unusual in the sea. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. One unusual species is reported to live as long as 30 years][. Another unusual species, T. nutricula, falsely reported as Turritopsis dohrnii, might be effectively immortal because of its ability under certain circumstances in the laboratory 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 transdifferentian of life form has been observed only in the laboratory and it is not known if it actually occurs in wild Turritopsis populations. 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. Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles and at least one species of Pacific salmon. Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp. Jellyfish bloom formation is a complex process that depends on ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentrations. Ocean currents tend to congregate jellyfish into large swarms 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. Scientists have little historic data about jellyfish populations. One hypothesis is that the global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches formerly occupied by now overfished creatures, but this hypothesis lacks supporting data. 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." 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). Invasive populations can expand rapidly because they often face no predators in the new habitat. Increased nutrients, ascribed to agricultural 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." Jellyfish populations may be expanding globally as a result of overfishing of their natural predators and the availability of excessive nutrients due to land runoff. 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. It may become 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. 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 freshwater jellyfish, Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and 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 awhile, 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. In some countries, such as Japan, 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. Rhizostomes, especially Rhopilema esculentum in China ( 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. Traditional processing methods, carried out by a Jellyfish Master, involve a 20- to 40-day multi-phase procedure in which after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed. Processing reduces 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. Desalted, ready-to-eat products are also available. 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 can be used for a variety of applications including the treatment of rheumatoid arthritis. 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 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. Jellyfish are displayed in many public aquaria. 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. As of 2009, jellyfish were becoming popular in home aquaria. Jellyfish sting their prey using nematocysts, 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, 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 2,000 pounds per square inch (14,000 kPa) until it bursts. A lance inside the nematocyst pierces the victim's skin, and poison flows through into the victim. 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. 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. 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. An Australian box jellyfish called the sea wasp can kill a grown man in a matter of seconds or minutes. Because the harpoons are so shallow, however, Australians have learned that they can protect themselves while swimming in sea wasp waters simply by covering their exposed skin with pantyhose. The three goals of first aid for uncomplicated stings are to prevent injury to rescuers, deactivate the 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. Vinegar (3–10% aqueous acetic acid) may be used as a common remedy to help with box jellyfish stings, but not the stings of the Portuguese Man o' War (which is not a true jellyfish, but a colony). 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; and may be preferred over vinegar. Fresh water is not usually used if the sting occurs in salt water, as changes in tonicity 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. Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing. Scraping the affected skin with a knife edge, safety razor, or credit card may remove remaining nematocysts. Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may control skin irritation (pruritus). Ice or fresh water is not usually applied to stings, since they may cause nematocysts to continue to release toxin. Immunobased antivenins have been available since the 1970s; administration requires medical personnel and refrigeration and are used in extreme cases as with regard to the box jellyfish, Chironex. Jellyfish adversely affect humanity by interfering with public systems and harming swimmers. 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. Jellyfish can clog cooling equipment, disabling power plants 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. Clogging can stop desalination plants, as well as clogging ship engines and infesting fishing nets. Taxonomic classification systematics within the Cnidaria, as with all organisms, are always in flux. Many scientists who work on relationships between these groups are reluctant to assign ranks, although there is general agreement on the different groups, regardless of their absolute rank. Presented here is one scheme, which includes all groups that produce medusae (jellyfish), derived from several expert sources: Illustrations of medusae by German biologist Ernst Haeckel: Narcomedusae Discomedusae Trachomedusae Discomedusae Leptomedusae Peromedusae Anthomedusae Stauromedusae Cubomedusae (modern Cubozoa) Discomedusae Discomedusae Photos:

Jellyfish Lake
Jellyfish Lake (Palauan: , "Fifth Lake") is a marine lake located on Eil Malk island in Palau. Eil Malk is part of the Rock Islands, a group of small, rocky, mostly uninhabited islands in Palau's Southern Lagoon, between Koror and Peleliu. There are about 70 other marine lakes located throughout the Rock Islands. Jellyfish Lake is one of Palau's most famous dive (snorkeling only) sites. It is notable for the millions of golden jellyfish which migrate horizontally across the lake daily. Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of an ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish, Mastigias cf. papua etpisoni, and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons. Jellyfish Lake is stratified into two layers, an oxygenated upper layer (mixolimnion) and a lower anoxic layer (monimolimnion). The oxygen concentration in the lake declines from about 5 ppm at the surface to zero at 15 meters (at the chemocline). Stratification is persistent and seasonal mixing does not occur. The lake is one of about 200 saline meromictic lakes that have been identified in the world. However most of these lakes are of freshwater origin. Permanently stratified marine lakes are unusual, but on Eil Malk and on other nearby islands there are eleven other apparently permanent stratified marine lakes. The stratification of the lake is caused by conditions which prevent or restrict the mixing of water vertically. These conditions include:
1. The lake is surrounded by rock walls and trees which substantially block the wind flow across the lake that would cause mixing.
2. The primary water sources for the lake (rain, runoff and tidal flows through tunnels) are all close to the surface.
3. The lake is in the tropics where seasonal temperature variation is small so that the temperature inversion that can cause vertical mixing of lakes in temperate zones does not occur. The oxygenated layer extends from the surface to about 15 metres (49 ft). All organisms that require oxygen live in this layer including the jellyfish, a few species of fish and copepods. This layer is somewhat turbid. Visibility is limited to about 5 metres (16 ft). The salinity of this layer down to about 3 metres (9.8 ft) is affected by rain and runoff, and below this, salinity levels are unaffected by freshwater inputs. The lake is connected to the sea via three tunnels that lie near the surface. The tunnels channel tidal water in and out of the lake. Tide levels in the lake are damped to about a third of the lagoon tidal levels. The tidal peaks are delayed from the lagoon tidal peaks by about 1 hour and forty minutes. Biologist William Hamner estimated that about 2.5% of the lake's volume is exchanged during a tidal flow. However because the tidal water enters at the surface, the lower anoxic layer is largely unaffected by tidal flows. The anoxic layer extends from about 15 metres (49 ft) below the surface to the bottom of the lake. The oxygen concentration in this layer is zero. The hydrogen sulfide concentration rises from about zero at the top of this layer to over 80 mg/liter at the bottom of the lake. The top three meters of this layer contains a dense population of bacteria, at least one species of which is a purple photosynthetic sulfur bacterium. This bacterial layer absorbs all sunlight so that the anoxic layer below the bacterial plate is dark, but transparent. Hamner estimated the visibility to be about 30 meters. The anoxic layer also contains high concentrations of ammonia and phosphate which are almost completely absent from the upper layer. The anoxic layer is potentially dangerous for divers, who can be poisoned through their skin. This risk is mitigated as scuba diving equipment is not allowed in the lake, thus limiting the depths to which individuals may dive. Jellyfish Lake is around 12,000 years old. This age estimate is based on the depth of the lake (about 30 meters), an estimate of the thickness of the sediment (at least 20 meters) and the rising sea level since the end of the last ice age. About 12,000 years ago, the sea level had risen to the point that sea water began to fill the Jellyfish Lake basin. Two species of scyphozoan jellyfish live in Jellyfish Lake, moon jellyfish (Aurelia sp.) and the golden jellyfish (Mastigias sp.). The golden jellyfish are most closely related to the spotted jellyfish (Mastigias papua) that inhabit the nearby lagoons. They are similar to the spotted jellyfish in that they derive part of their nutrition from symbiotic algae (Zooxanthellae) that live in their tissues and part of their nutrition from captured zooplankton. However, the golden jellyfish are morphologically, physiologically, and behaviourally distinct from the spotted jellyfish. They are easily distinguished from the spotted jellyfish by the almost complete loss of spots on the exumbrella and the almost complete loss of their clubs, an appendage attached to the oral arms. Marine biologist Michael Dawson proposed that the golden jellyfish that inhabit Jellyfish Lake be classified as a subspecies (Mastigias cf. papua etpisoni) of the spotted jellyfish living in the nearby lagoons. The species identification is uncertain (denoted by cf. in the name) because the Mastigias papua local to Palauan lagoons may be only one of several cryptic species that make up the M. papua group, and in the future, the M. papua local to Palau may be identified as a separate species from other M. papua. He also proposed that the jellyfish living in four other Palauan marine lakes were distinctive enough to deserve recognition as unique subspecies. The moon jellyfish were identified as Aurelia aurita by Hamner. However, since the release of that report in 1981, genetic testing has been done on specimens of Aurelia collected from locations throughout the world. The results of that testing indicate in addition to the three named species of Aurelia there are at least six other cryptic species in the Aurelia genus. Three of the cryptic species identified were from Palau. One of these cryptic species is common to four of Palau's marine lakes with jellyfish populations including Jellyfish Lake. Hence, the most accurate designation for the moon jellyfish in Jellyfish Lake (as of February 2001) is Aurelia sp. Despite the close proximity of Palau's moon jellyfish cryptic species, Dawson and Jacobs stated that the molecular data suggested that they had not interbred for millions of years. The golden jellyfish migration pattern is as follows: The golden jellyfish rotate counter-clockwise as they swim at the surface, presumably to provide even exposure to the sun for the symbiotic algae in their bodies. The Jellyfish Lake golden jellyfish migration pattern is similar to Mastigias sp. in other Palauan marine lakes and coves which all migrate west to east in the morning. However the migration patterns in other Palauan coves and marine lakes are less well defined than that of the Jellyfish Lake golden jellyfish. The east to west migration in all of these other lakes (except for Clear Lake on Eil Malk) does not begin until the late afternoon. Hamner and Dawson proposed that the difference is caused by evolutionary change driven by the jellyfish-eating anemones (Entacmaea medusivora) that inhabit the eastern regions of Jellyfish Lake and Clear Lake. The jellyfish instinctively avoid shadows and in the morning with the shadows on the eastern end the jellyfish also avoid the anemones. By moving east to west in the early afternoon the jellyfish avoid the time of day when the setting sun would eliminate shadows on the lake in the eastern end and thereby avoid the anemones in the afternoon. The moon jellyfish do not have an organized horizontal migration pattern. At night they migrate to the surface presumably to feed. The copepods that make up a significant portion of the moon jellyfish diet in Jellyfish Lake also migrate to the surface at night. Beginning in about the fall of 1998, a precipitous decline in the golden jellyfish medusa population was detected in Jellyfish Lake. By December 1998, the medusa population had declined to zero. Based on their extensive investigation of the disappearance of the golden jellyfish medusae, Dawson et al. determined that the most likely cause was an El Niño weather event that raised the water temperature so that the symbiotic algae (Zooxanthellae) that live within the golden jellyfish medusae and the syphistomae (scyphozoan polyps) could not survive. In January 2000, the golden jellyfish medusae were observed in Jellyfish Lake for the first time since April 1999. By May, 2012, the medusae population had returned to pre-decline levels. Dawson et al. also surveyed the golden jellyfish populations in three other Palauan marine lakes. They found significant changes in the medusa population in two of these lakes (Clear Lake on Eil Malk and Goby Lake on Koror). The golden jellyfish population in Big Jellyfish Lake, Koror did not seem to be affected. The reason for this was not clear but Big Jellyfish Lake experienced lower temperature increases than the other lakes and there was experimental evidence that the golden jellyfish medusa from Big Jellyfish Lake were more tolerant of higher temperatures. Although Clear Lake did not seem to have experienced the complete medusa population die-off that Jellyfish Lake did in 1998, the golden jellyfish medusae in Clear Lake are not always present. When conditions are not favorable for the short lived medusae stage or for syphistomae strobilation the medusae disappear in Clear Lake. The medusa population is reestablished by syphistomae strobilation when conditions are favorable for strobilation and medusae again. A significant reduction in the Jellyfish Lake medusa population had also been noted in 1987. This was previously attributed to turbulence generated by scuba diving that caused disturbance of the toxic layer. However, given that it occurred within the time frame when an unusually high sea surface temperature had been detected, it might reasonably be surmised to have been caused by a rise in water temperature that was the most likely cause of the 1998 die-off. The moon jellyfish exhibited unusual damage in the 1998 time frame; however, the population seemed no smaller than usual. Snorkeling in Jellyfish Lake is a popular activity for tourists to Palau. Several tour operators in Koror offer trips to the lake. Eil Malk island is approximately a 45 minute boat ride from Koror. The lake is accessed by a short trail from the beach on Eil Malk to the lake. Tourists require a pass to access to Jellyfish Lake. The Rock Islands/Jellyfish Lake pass is $35 and is good for 10 days. Scuba diving by tourists in the lake is not allowed. Two reasons are put forward for this: Jellyfish Lake is currently the only one of Palau's marine lakes open to tourists. Although both species of jellyfish living in the lake have stinging cells (nematocytes), they are not in general powerful enough to cause harm to humans. It has been reported that it is possible to notice the stings on sensitive areas like the area around the mouth. The Fish 'n Fins tour guide recommended that people with allergies to jellyfish consider wearing protective clothing. Saltwater crocodiles are native to Palau but there has only been one death attributed to them in recent times and they are generally not considered a threat to divers. The hydrogen sulfide in the anoxic layer is a serious risk to scuba divers entering this layer. The gas can be absorbed through the skin. In 1977, the maximum safe threshold level for hydrogen sulfide was set at 10 ppm. The concentrations exceed that by eightfold at the bottom of the anoxic layer. However, the hydrogen sulfide concentration down to the chemocline at about 15 metres (49 ft) is reported to be zero, and if the anoxic layer is avoided, the hydrogen sulfide in the lake does not pose a risk for snorkelers.

Breathing gas
Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas. Other artificial gases, either pure gases or mixtures of gases, are used in breathing equipment and enclosed habitats such as SCUBA equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft, medical life support and first aid equipment, high-altitude mountaineering and anaesthetic machines. Most breathing gases are a mixture of oxygen and one or more inert gases. Other breathing gases have been developed to improve on the performance of air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving. A safe breathing gas for hyperbaric use has three essential features: The techniques used to fill diving cylinders with gases other than air are called gas blending. These common diving breathing gases are used: Oxygen (O2) must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes. Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with caution when gas blending. Oxygen has historically been obtained by fractional distillation of liquid air, but is increasingly obtained by non-cryogenic technologies such as pressure swing adsorption (PSA) and vacuum-pressure swing adsorption (VPSA) technologies. The fraction of the oxygen component of a breathing gas mixture is sometimes used when naming the mix: The fraction of the oxygen determines the deepest the mixture gas can safely be used to avoid oxygen toxicity. This depth is called the maximum operating depth. The concentration of oxygen in a gas mix depends on the fraction and the pressure of the mixture. It is expressed by the partial pressure of oxygen (ppO2). The partial pressure of any component gas in a mixture is calculated as: For the oxygen component, The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa (0.16 bar). Below this partial pressure the diver may be at risk of unconsciousness and death due to hypoxia, depending on factors including individual physiology and level of exertion. When a hypoxic mix is breathed in shallow water it may not have a high enough ppO2 to keep the diver conscious. For this reason normoxic or hyperoxic "travel gases" are used at medium depth between the "bottom" and "decompression" phases of the dive. The maximum safe ppO2 in a breathing gas depends on exposure time, the level of exercise and the security of the breathing equipment being used. It is typically between 100 kPa (1 bar) and 160 kPa (1.6 bar); for dives of less than three hours it is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a ppO2 of as much as 180 kPa (1.8 bar). At high ppO2 or longer exposures, the diver risks oxygen toxicity including a seizure. Each breathing gas has a maximum operating depth that is determined by its oxygen content. Oxygen analysers measure the ppO2 in the gas mix. "Divox" is oxygen. In the Netherlands, pure oxygen for breathing purposes is regarded as medicinal as opposed to industrial oxygen, such as that used in welding, and is only available on medical prescription. The diving industry "created" Divox and registered it as a trademark to circumvent the strict rules concerning medicinal oxygen thus making it easier for (recreational) scuba divers to obtain oxygen for blending their breathing gas. In most countries, there is no difference in purity in medical oxygen and industrial oxygen, as they are produced by exactly the same methods and manufacturers, but labeled and tanked differently. The chief difference between them is that the paper record-keeping trail is much more extensive for medical oxygen, to more easily identify the exact manufacturing trail of a "lot" of oxygen, in case problems with its purity are discovered. Nitrogen (N2) is a diatomic gas and the main component of air, the cheapest and most common breathing gas used for diving. It causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness. Equivalent air depth is used to estimate the decompression requirements of a nitrox (oxygen/nitrogen) mixture. Equivalent narcotic depth is used to estimate the narcotic potency of trimix (oxygen/helium/nitrogen mixture). Many divers find that the level of narcosis caused by a 30 m (100 ft) dive, whilst breathing air, is a comfortable maximum. Nitrogen in a gas mix is almost always obtained by adding air to the mix. Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), so it is more suitable for deeper dives than nitrogen. Helium is equally able to cause decompression sickness. At high pressures, helium also causes High Pressure Nervous Syndrome, which is a CNS irritation syndrome which is in some ways opposite to narcosis. Helium fills typically cost ten times more than an equivalent air fill. Helium is not very suitable for dry suit inflation owing to its poor thermal insulation properties – helium is a very good conductor of heat (compared to air which is a rather poor, making it more of an insulator). Helium's low molecular weight (monatomic MW=4, compared with diatomic nitrogen MW=28) increases the timbre of the breather's voice, which may impede communication. This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords. Helium leaks from damaged or faulty valves more readily than other gases because atoms of helium are smaller allowing them to pass through smaller gaps in seals. Helium is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation. Neon (Ne) is an inert gas sometimes used in deep commercial diving but is very expensive. Like helium, it is less narcotic than nitrogen, but unlike helium, it does not distort the diver's voice. Hydrogen (H2) has been used in deep diving gas mixes but is very explosive when mixed with more than about 4 to 5% oxygen (such as the oxygen found in breathing gas). This limits use of hydrogen to deep dives and imposes complicated protocols to ensure that oxygen is cleared from the lungs, the blood stream and the breathing equipment before breathing hydrogen starts. Like helium, it increases the timbre of the diver's voice. The hydrogen-oxygen mix when used as a diving gas is sometimes referred to as Hydrox. Many gases are not suitable for use in diving breathing gases. Here is an incomplete list of gases commonly present in a diving environment: Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is not generally suitable as a diving breathing gas. Argox is used for decompression research. It is sometimes used for dry suit inflation by divers whose primary breathing gas is helium-based, because of argon's good thermal insulation properties. Argon is more expensive than air or oxygen, but considerably less expensive than helium. Carbon dioxide (CO2) is produced by the metabolism in the human body and can cause carbon dioxide poisoning. Carbon monoxide (CO) is produced by incomplete combustion. See carbon monoxide poisoning. Four common sources are: Hydrocarbons (CxHy) are present in compressor lubricants and fuels. They can enter diving cylinders as a result of contamination, leaks, or due to incomplete combustion near the air intake. The process of compressing gas into a diving cylinder removes moisture from the gas. This is good for corrosion prevention in the cylinder but means that the diver inhales very dry gas. The dry gas extracts moisture from the diver's lungs while underwater contributing to dehydration, which is also thought to be a predisposing risk factor of decompression sickness. It is also uncomfortable, causing a dry mouth and throat and making the diver thirsty. This problem is reduced in rebreathers because the soda lime reaction to remove carbon dioxide puts moisture back into the breathing gas. In hot climates, open circuit diving can accelerate heat exhaustion because of dehydration. Another concern with regard to moisture content is the tendency of moisture to condense as the gas is decompressed while passing through the regulator; this coupled with the extreme reduction in temperature, also due to the decompression can cause the moisture to solidify as ice. This icing up in a regulator can cause moving parts to seize and the regulator to fail or free flow. It is for this reason that SCUBA regulators are generally constructed from brass, and chrome plated (for protection). Brass, with its good thermal conductive properties, quickly conducts heat from the surrounding water to the cold, newly decompressed air, helping to prevent icing up. Divers find it difficult to detect most gases that are likely to be present in diving cylinders because they are colourless, odourless and tasteless. Electronic sensors exist for some gases, such as oxygen analysers, helium analyser, carbon monoxide detectors and carbon dioxide detectors. Oxygen analysers are commonly found underwater in rebreathers. Oxygen and helium analysers are often used on the surface during gas blending to determine the percentage of oxygen or helium in a breathing gas mix. Chemical and other types of gas detection methods are not often used in recreational diving.

Free-diving
Freediving (or free-diving) is a form of underwater diving that relies on a diver's ability to hold his or her breath until resurfacing rather than on the use of a breathing apparatus such as scuba gear. Examples include breath-hold spear fishing, freedive photography, recreational breath-hold diving, apnea competitions, and to some degree, snorkeling. The activity that garners the most public attention is the extreme sport of competitive apnea in which competitors attempt to attain great depths, times, or distances on a single breath. Freediving is a technique used with various aquatic activities. Examples of recognized freediving activities are (non-) competitive freediving, (non-) competitive spearfishing, freediving photography and mermaid shows. Less recognized examples of freediving include, but are not limited to, synchronised swimming, underwater rugby, underwater hockey, underwater hunting other than spearfishing, underwater target shooting and snorkeling. The discussion remains whether freediving is only a synonym for breath-hold diving or whether it describes a specific group of underwater activities. The term 'freediving' is often associated with competitive breath-hold diving or competitive apnoea. Competitive freediving is currently governed by two world associations: AIDA International (International Association for Development of Apnea) and CMAS (World Underwater Federation). Most types of competitive freediving have in common that it is an individual sport based on the best individual achievement. An exception to this rule is the bi-annual World Championship for Teams, held by AIDA, where the combined score of the team members makes up the team's total points. There are currently nine disciplines used by official governing bodies and a dozen disciplines that are only practiced locally. In this article, the recognized disciplines of AIDA and CMAS will be described. All disciplines can be done by both men and women and, while done outdoors, no differences in the environment between records are recognized any longer. The disciplines of AIDA can be done both in competition and as a record attempt, with the exception of Variable Weight and No limits, which are both done solely as record attempts. The following official disciplines are recognized by AIDA, CMAS, or both. For all AIDA disciplines, the depth the athlete will attempt is announced before the dive. This is accepted practice for both competitions and record attempts. Each organization has its own rules on recognizing an attempt. These can be found on the website from the respective organizations. As of 25 July 2013[update] the AIDA recognized world records are: As of 9 May 2012[update], the CMAS recognized world records are: Freediving is also a recreational activity, celebrated as a relaxing, liberating and unique experience significantly different from SCUBA diving. Many snorkelers freedive when they hold their breath and swim below the surface. SAFETY TIP: The common practice of letting the snorkel flood when submerging allows water to enter the airways, which are kept open by the snorkel's mouth piece, creating a risk of water entering the lungs. It is therefore recommended that the snorkel be removed from the mouth while under water. The human body has several adaptations under diving conditions, which stem from the mammalian diving reflex. These adaptations enable the human body to endure depth and lack of oxygen far beyond what would be possible without the reflex. The adaptations made by the human body while underwater and at high pressure include: Training for freediving can take many forms and be done on the land. One example is the apnea walk. This consists of a preparation "breathe-up", followed by a short (typically 1 minute) breath hold taken at rest. Without breaking the hold, the participant then initiates a walk for as far as they can, until it becomes necessary to breathe again. Athletes can do close to 400 meters in training this way. This form of training is good for accustoming muscles to work under anaerobic conditions, and for tolerance to 2CO build-up in the circulation. It is also easy to gauge progress, as increasing distance can be measured. Before competition attempt, freedivers perform preparation sequence, which usually consists of physical stretching, mental exercise and breath exercise. It may include sequention of variable length static apnea, special purging deep breaths, hyperventilation. Result of preparation sequence is slower metabolism, lower heart rate and breath rate, lower level of CO2 in bloodstream and overall mental equilibrium. Failing ordinary warning signals or crossing mental barrier by strong will may lead to shallow water blackout or deep water blackout. Trained freedivers are well aware of this and will only dive under strict and first aid competent supervision. However this does not eliminate the risk of deep or shallow water blackout. All safe freedivers have a 'buddy' who accompanies them, observing from within the water at the surface. Due to the nature of the sport, any practice of freediving must include strict adherence to safety measures as an integral part of the activity, and all participants must also be adept in rescue and resuscitation. Without proper training and supervision, freediving/apnea/breath-hold diving is extremely dangerous. Archaeological evidence suggests that people have been freediving since the 5th century BCE. The first known were the haenyeo in Korea who collected shells and sponges to sell to others. The Ama Divers from Japan began to collect pearls 2,000 years ago. Both Plato and Homer mention the sponge as being used for bathing in ancient Greece and this may represent an early reference to commercial freediving to obtain them; the island of Kalymnos was a main centre of diving for sponges. By using weights of as much as 15 kilograms (33 lb) to speed the descent, breath-holding divers would descend to depths up to 30 metres (98 ft) for as long as 5 minutes to collect sponges. Spearfishing around the Mediterranean Sea was important for the historical background for the movement of the apnea sport.

Medusa (biology)
Jellyfish or jellies 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 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, and possibly 700 million years or more, making them the oldest multi-organ animal. The English popular name jellyfish has been in use since 1796. 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". Even some scientists include the phylum ctenophora when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column. 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. Indeed, it may be said that the term "jellies" has become more popular than "jellyfish". In scientific literature, "jelly" and "jellyfish" are often used interchangeably. Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate. Many textbooks and sources refer to only scyphozoa as "true jellyfish". A group of jellyfish is sometimes called a bloom or a swarm. "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Another collective name for a group of jellyfish is a smack, 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 polyps 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. Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate. Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs. The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology. 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 of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. 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 tentacles as its said. German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur. Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or 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. 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.][ The body is composed of over 95% water; 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. Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net". 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 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][ is that some species explicitly adapt to tidal flux to control their location. 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 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. 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 rhopalia that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment. The eyes are suspended on stalks with heavy crystals 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. 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, which have bell disks from 0.5 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. 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; 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. Another small species of jellyfish is the Australian Irukandji, which is about the size of a fingernail. 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. 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.][ 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 200 centimetres (79 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight, with average specimens frequently reaching 90 centimetres (35 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 up-to-100 centimetres (39 in) bell diameter Lion's Mane. The rarely encountered deep-sea jellyfish Stygiomedusa gigantea is another solid 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. Medusa jellyfish are a life stage exhibited in some species of the phylum Cnidaria. Medusa jellyfish belong exclusively to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps. The phylogenetics of this group are complex and still being worked out. The Medusozoa appear to be a sister group to Octocorallia. 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. There are four major classes of medusozoan Cnidaria: Some other animals are frequently associated with or mistaken for medusa jellyfish. 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). Most jellyfish alternate between polyp and medusa generations during their life cycle. Additionally, there are several possible larval life-stages. After fertilization a primitive free-swimming larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. Some polyps can also asexually produce a creeping frustule larval form, which then also develops into a new polyp. The polyp is generally a small planted stalk with a mouth that is ringed by upward-facing tentacles. The polyps are like miniatures of the closely related anthozoan (sea anemones and corals) polyps, which are also members of Cnidaria. The jellyfish polyp may be sessile, living on the bottom or another substrate such as floats or boat hulls, 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. Polyp colonies form by strobilation, resulting in multiple polyps which share a common stomach cavity. Most polyps are very small, measured in millimeters. They feed continuously. The polyp stage may last for years. Eventually the polyp gives rise to the medusa stage. New medusae are usually created asexually by strobilation or budding from the polyp. The medusa is the life stage which is most typically identified as a jellyfish. Jellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily if there is enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn. Jellyfish are usually either male or female (hermaphroditic specimens are occasionally found). In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body 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. After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or form new, immature jellies called ephyrae. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few species of hydromedusae reproduce by fission (splitting in half). In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton.][ Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles – fringe-like protrusions from the bell's border that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.][ Most jellyfish have a second stage to their life cycle, the planula larvae phase, following the initial egg and sperm phase. Although this is a short lived stage for jellyfish, it is an important phase when the fertilized eggs that had previously undergone embryonic development, hatch, and planulae emerge from the females mouth or brood pouch and are off on their own. Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. Jellyfish held in public aquariums are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, so may live several years, though this would be very unusual in the sea. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. One unusual species is reported to live as long as 30 years][. Another unusual species, T. nutricula, falsely reported as Turritopsis dohrnii, might be effectively immortal because of its ability under certain circumstances in the laboratory 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 transdifferentian of life form has been observed only in the laboratory and it is not known if it actually occurs in wild Turritopsis populations. 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. Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles and at least one species of Pacific salmon. Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp. Jellyfish bloom formation is a complex process that depends on ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentrations. Ocean currents tend to congregate jellyfish into large swarms 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. Scientists have little historic data about jellyfish populations. One hypothesis is that the global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches formerly occupied by now overfished creatures, but this hypothesis lacks supporting data. 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." 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). Invasive populations can expand rapidly because they often face no predators in the new habitat. Increased nutrients, ascribed to agricultural 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." Jellyfish populations may be expanding globally as a result of overfishing of their natural predators and the availability of excessive nutrients due to land runoff. 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. It may become 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. 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 freshwater jellyfish, Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and 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 awhile, 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. In some countries, such as Japan, 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. Rhizostomes, especially Rhopilema esculentum in China ( 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. Traditional processing methods, carried out by a Jellyfish Master, involve a 20- to 40-day multi-phase procedure in which after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed. Processing reduces 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. Desalted, ready-to-eat products are also available. 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 can be used for a variety of applications including the treatment of rheumatoid arthritis. 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 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. Jellyfish are displayed in many public aquaria. 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. As of 2009, jellyfish were becoming popular in home aquaria. Jellyfish sting their prey using nematocysts, 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, 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 2,000 pounds per square inch (14,000 kPa) until it bursts. A lance inside the nematocyst pierces the victim's skin, and poison flows through into the victim. 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. 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. 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. An Australian box jellyfish called the sea wasp can kill a grown man in a matter of seconds or minutes. Because the harpoons are so shallow, however, Australians have learned that they can protect themselves while swimming in sea wasp waters simply by covering their exposed skin with pantyhose. The three goals of first aid for uncomplicated stings are to prevent injury to rescuers, deactivate the 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. Vinegar (3–10% aqueous acetic acid) may be used as a common remedy to help with box jellyfish stings, but not the stings of the Portuguese Man o' War (which is not a true jellyfish, but a colony). 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; and may be preferred over vinegar. Fresh water is not usually used if the sting occurs in salt water, as changes in tonicity 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. Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing. Scraping the affected skin with a knife edge, safety razor, or credit card may remove remaining nematocysts. Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may control skin irritation (pruritus). Ice or fresh water is not usually applied to stings, since they may cause nematocysts to continue to release toxin. Immunobased antivenins have been available since the 1970s; administration requires medical personnel and refrigeration and are used in extreme cases as with regard to the box jellyfish, Chironex. Jellyfish adversely affect humanity by interfering with public systems and harming swimmers. 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. Jellyfish can clog cooling equipment, disabling power plants 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. Clogging can stop desalination plants, as well as clogging ship engines and infesting fishing nets. Taxonomic classification systematics within the Cnidaria, as with all organisms, are always in flux. Many scientists who work on relationships between these groups are reluctant to assign ranks, although there is general agreement on the different groups, regardless of their absolute rank. Presented here is one scheme, which includes all groups that produce medusae (jellyfish), derived from several expert sources: Illustrations of medusae by German biologist Ernst Haeckel: Narcomedusae Discomedusae Trachomedusae Discomedusae Leptomedusae Peromedusae Anthomedusae Stauromedusae Cubomedusae (modern Cubozoa) Discomedusae Discomedusae Photos:
Breathing
Human body

The human body is the entire structure of a human organism and comprises a head, neck, torso, two arms and two legs. By the time the human reaches adulthood, the body consists of close to 100 trillion cells, the basic unit of life. These cells are organised biologically to eventually form the whole body.

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Human Interest

In journalism, a human interest story is a feature story that discusses a person or people in an emotional way. It presents people and their problems, concerns, or achievements in a way that brings about interest, sympathy or motivation in the reader or viewer.

Human interest stories may be "the story behind the story" about an event, organization, or otherwise faceless historical happening, such as about the life of an individual soldier during wartime, an interview with a survivor of a natural disaster, a random act of kindness or profile of someone known for a career achievement.

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