Jellyfish (also known as jellies or sea jellies or medusozoa) are free-swimming members of the phylum Cnidaria Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish" (over 200 species), Staurozoa Stauromedusae are the stalked jellyfishes, of the class Staurozoa within the phylum Cnidaria. They are unique in that they do not have an alternation of polyp and medusa life cycle phases, but are interpreted as an attached medusa stage, with a life style more resembling that of polypoid forms. They have a generally trumpet-shaped body, positioned (about 50 species), Cubozoa Box jellyfish 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 species are extremely painful and often fatal to prey animals (about 20 species), and Hydrozoa Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria (about 1000–1500 species that make jellyfish and many more that do not).^[1][2] The jellyfish in these groups are also called, respectively, scyphomedusae Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000–1500 species that make jellyfish and many more that do not), stauromedusae Stauromedusae are the stalked jellyfishes, of the class Staurozoa within the phylum Cnidaria. They are unique in that they do not have an alternation of polyp and medusa life cycle phases, but are interpreted as an attached medusa stage, with a life style more resembling that of polypoid forms. They have a generally trumpet-shaped body, positioned, cubomedusae Carybdeida is an order of box jellyfish. They are distinguished from other box jellyfish by the presence of unbranched muscular bases at the corners of the cubic umbrella. Most species have four tentacles, and hydromedusae Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. A profusion of alternate scientific names exists for this long-known, heavily discussed and spectactular group - it has also been called Gymnoblastea, or, with or without an emended ending "-ae&. All jellyfish are embodied in the Medusozoa subphylum. Medusa In biology, a medusa is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, 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 is another word for jellyfish, and as such is used to refer specifically to the adult stage of the life cycle.

Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water; freshwater species are less than an inch (25 mm) in diameter, are colorless and do not sting. Many of the best-known jellyfish, such as Aurelia Aurelia is a genus of scyphozoan jellyfish . There are at least 13 species in the genus Aurelia including many that are still not formally described. It has been suggested that Aurelia is the most well-studied group of gelatinous zooplankton, with Aurelia aurita the most well studied species in the genus; two other species, Aurelia labiata and, are scyphomedusae Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000–1500 species that make jellyfish and many more that do not). These are the large, often colorful, jellyfish that are common in coastal zones worldwide.

In its broadest sense, the term jellyfish also generally refers to members of the phylum Ctenophora The Ctenophora , (from the Greek ctena/χτένα (comb), phero/φέρω (carry) commonly known as comb jellies, are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia — adults. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are generally transparent or translucent, and exist in shallow to deep portions of all the world's oceans.

Lion's Mane Jellyfish are the largest known jellyfish, and arguably the longest animal in the world.^[3][4][5]

Terminology

Since jellyfish are not actually fish, the word jellyfish is considered a misnomer A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derived their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known, and American public aquariums have popularized use of the terms jellies or sea jellies instead.^[6] Others find jellyfish, which has been in common usage for more than a century,^[7] to be equally useful and picturesque, and prefer it over jellies. The word jellyfish is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans Scyphozoans are members of the phylum Cnidaria and are referred to as the "true jellyfish." Scyphozoans range from the Ediacarian time period through the Recent. In Scyphozoans, the medusa form is dominant life stage, while the polyp form may be a juvenile stage. As medusae, they eat a variety of crustaceans and fish, which they capture, staurozoans Stauromedusae are the stalked jellyfishes, of the class Staurozoa within the phylum Cnidaria. They are unique in that they do not have an alternation of polyp and medusa life cycle phases, but are interpreted as an attached medusa stage, with a life style more resembling that of polypoid forms. They have a generally trumpet-shaped body, positioned (stalked jellyfish), hydrozoans Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria, and cubozoans Box jellyfish, named for their cube-shaped medusae, are a class of invertebrates belonging to the class Cubozoa, as well as being the preferred common name for notoriously dangerous Chironex fleckeri (box jellyfish). Some textbooks and websites refer to scyphozoans as "true jellyfish".^[8][9]

In its broadest usage, some scientists occasionally include members of the phylum Ctenophora The Ctenophora , (from the Greek ctena/χτένα (comb), phero/φέρω (carry) commonly known as comb jellies, are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia — adults (comb jellies) when they are referring to jellyfish.^[10] Other scientists prefer to use the more all-encompassing term "gelatinous zooplankton Gelatinous zooplankton is the term used to describe the fragile animals that live in the water column in the ocean. They have very delicate bodies that are easily damaged or destroyed. Gelatinous zooplankton are often transparent. All jellyfish are gelatinous zooplankton, but not all gelatinous zooplankton are jellyfish. The most commonly", when referring to these, together with other soft-bodied animals in the water column.^[11]

A group of jellyfish is sometimes called a bloom or a swarm.^[12] "Bloom An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of" 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.^[13] 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.^[14] Using "swarm Swarm describes a behaviour of an aggregate of animals of similar size and body orientation, often moving en masse or migrating in the same direction. Swarming is a general term that can be applied to any animal that swarms. The term is applied particularly to insects, but can also be applied to birds, fish, various microorganisms such as bacteria," implies some kind of active ability to stay together, which a few species like Aurelia, the moon jelly, demonstrate.^[15]

Most jellyfish have a second part of their life cycle, which is called the polyp phase. When single polyps, arising from a single fertilized egg, develop into a multiple-polyp cluster, connected to each other by strands of tissue called stolons, they are said to be "colonial." A few polyps never proliferate and are referred to as "solitary" species.^[16]

Anatomy

Jellyfish do not have specialized digestive The Human gastrointestinal tract or digestive system is the system by which ingested food is acted upon by physical and chemical means to provide the body with nutrients it can absorb and to excrete waste products; in mammals the system includes the alimentary canal extending from the mouth to the anus, and the hormones and enzymes assisting in, osmoregulatory Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The, central nervous The central nervous system is the part of the nervous system that coordinates the activity of all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish. It contains the majority of the nervous system and consists of the brain and the spinal cord, as well as, respiratory The respiratory system's function is to allow oxygen exchange through all parts of the body. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on organism. In humans and other mammals, for example, the anatomical features of, or circulatory The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis systems. They digest using the gastrodermal The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians lining of the gastrovascular cavity Gastro vascular cavity, as the name indicates, functions in both digestion and the distribution of nutrients to all parts of the body. Organisms belonging to two major phyla, the Cnidaria and the Platyhelminthes, possess gastrovascular cavities. The radially symmetrical cnidarians have a sac-like body in two distinct layers, the epidermis and, where nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion Diffusion describes the spread of particles through random motion from regions of higher concentration to regions of lower concentration. The time dependence of the statistical distribution in space is given by the diffusion equation. The concept of diffusion is tied to that of mass transfer driven by a concentration gradient, but diffusion can. They have limited control over movement, but can use their hydrostatic skeleton A hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding muscles are used to change an organism's shape and produce movement, such as burrowing or swimming to accomplish movement through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are passive much of the time.^{[citation needed]} Jellyfish are composed of more than 90% water; most of their umbrella mass is a gelatinous material — the jelly — called mesoglea Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phylum Cnidaria. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or exoskeletons, or similar means of support. Because mesoglea is a gel with which is surrounded by two layers of epithelial cells which form the umbrella (top surface) and subumbrella (bottom surface) of the bell, or body.

Jellyfish do not have a brain or central nervous system The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain, spinal cord, and retina, but rather have a loose network of nerves, located in the epidermis In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell. Epithelium may possess only one layer of these cells, in which case it is referred to as simple squamous epithelium; or it may possess multiple layers, referred to then as stratified squamous, which is called a "nerve net A nerve net is a type of simple nervous system that is found in members of the cnidaria and echinodermata phyla . Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may then detect food and other chemicals in a rudimentary way". 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 Rhopalia are small sensory structures of scyphozoan and cubozoan jellyfish; in Aurelia they lie in marginal indententions around the bell and are flanked by rhopalial lappets. Specialized structures to sense light (ocelli) and perceive gravity (statoliths) are usually present. They also control the pace of swimming-muscle contraction, located at the rim of the jellyfish body, to other nerve cells. Some jellyfish also have ocelli A simple eye refers to a type of eye design or optical arrangement that contains a single lens. "Simple" in this case does not refer to the number of cells present in the eye or the visual acuity of the eye. Indeed, the optical properties and anatomy of simple eyes is often incredibly complex. Human eyes and camera lenses are classed as &: light-sensitive organs that do not form images but which can detect light, and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented.

Jellyfish blooms

The presence of ocean blooms is usually seasonal, responding to prey availability and increasing with temperature and sunshine. Ocean currents tend to congregate jellyfish into large swarms Swarm describes a behaviour of an aggregate of animals of similar size and body orientation, often moving en masse or migrating in the same direction. Swarming is a general term that can be applied to any animal that swarms. The term is applied particularly to insects, but can also be applied to birds, fish, various microorganisms such as bacteria, or "blooms", consisting of hundreds or thousands of individuals. In addition to sometimes being concentrated by ocean currents, blooms can result from unusually high populations in some years. Bloom formation is a complex process that depends on ocean currents An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis force, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun. Depth contours, shoreline configurations and interaction with other currents, nutrients A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. Nutrients are the substances that enrich the body. They build and repair tissues, give heat and energy, and regulate body processes. Methods for nutrient intake vary, with animals and, temperature, predation In ecology, predation describes a biological interaction where a predator feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption. The other main, and oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly concentrations. Jellyfish are better able to survive in oxygen-poor water than competitors, and thus can thrive on plankton without competition. Jellyfish may also benefit from saltier waters, as saltier waters contain more iodine, which is necessary for polyps to turn into jellyfish. Rising sea temperatures caused by climate change may also contribute to jellyfish blooms, because many species of jellyfish are better able to survive in warmer waters.^[17] Jellyfish are likely to stay in blooms that are quite large and can reach up to 100,000 in each.

There is very little data about changes in global jellyfish populations over time, besides "impressions" in the public memory. Scientists have little quantitative data of historic or current jellyfish populations.^[14] Recent speculation about increases in jellyfish populations are based on no "before" data.

The global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food resources and foraging methods. A shorthand definition formerly occupied by now overfished Ultimately overfishing may lead to resource depletion in cases of subsidised fishing, low biological growth rates and critical low biomass levels . For example, overfishing of sharks has led to the upset of entire marine ecosystems creatures, but this hypothesis lacks supporting data.^[14] Jellyfish researcher Marsh Youngbluth further clarifies that "jellyfish feed on the same kinds of prey as adult and young fish, so if fish are removed from the equation, jellyfish are likely to move in."^[18]

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

Increased nutrients, ascribed to agricultural runoff Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, snowmelt, or other sources flows over the land. This is a major component of the hydrologic cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made, have also been cited as an antecedent to the proliferation of jellyfish. Monty Graham, of the Dauphin Island Sea Lab in Alabama, says that "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 Eutrophication is an increase in the concentration of nutrient content to an extent that increases the primary productivity of the waterbody. In other terms, it is the "bloom" or great increase of phytoplankton in a water body. Negative environmental effects include particularly anoxia, or loss of oxygen in the water with severe, 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."^[18]

By sampling sea life in a heavily fished region off the Namibian Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March 1990 following the Namibian War of Independence. Its coast, total jellyfish biomass Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms, plants or animals. The mass can be expressed as the has overtaken that of fish, following intense fishing in the area in the last few decades.^[19]

Areas which have been seriously affected by jellyfish blooms include the northern Gulf of Mexico The Gulf of Mexico is the eleventh largest body of water in the world. Considered a smaller part of the Atlantic Ocean, it is an ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and, about which, Graham states, "Moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf."^[18]

Detrimental effects

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

Other severe implications are destroying fish nets, poisoning or crushing captured fish, consuming fish eggs and young fish.^[20]

Clogging also causes many problems including stoppage of nuclear power plants and desalination plants, as well as clogging engines of ships^[20] and even overturning of boats by one of the largest species, the Nomura's jellyfish.

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