Jellyfish

Jellyfish, also known as sea jellies or simply jellies, are the medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Most jellyfish are free-swimming marine animals, though a few are anchored to the seabed by stalks rather than being motile. Each jellyfish has an umbrella-shaped main body made of mesoglea, called the bell, and a group of trailing tentacles on its underside.

The bell pulses in and out to move the jellyfish through open water. The tentacles are armed with stinging cells and may be used to catch prey or to defend against predators. Jellyfish have a complex life cycle. The medusa is usually the sexual phase, and it produces planula larvae. These larvae spread widely and settle into a sedentary polyp phase, which may include asexual budding before the animal reaches sexual maturity.

Jellyfish are found all over the world, from surface waters to the deep sea. Scyphozoans, the "true jellyfish," live only in the sea, but some hydrozoans that look similar live in fresh water. Large, often colorful, jellyfish are common in coastal zones around the world. Jellyfish have existed for at least 500 million years, and possibly 700 million years or more, which makes them the oldest multi-organ animal group.

Some people in certain cultures eat jellyfish. They are considered a delicacy in some Asian countries, where species in the Rhizostomeae order are pressed and salted to remove extra water. Australian researchers have described them as a "perfect food" because they are sustainable and rich in protein, though they are relatively low in food energy. Jellyfish are also used in cell and molecular biology research. The green fluorescent protein, which some species use for bioluminescence, has been adapted as a fluorescent reporter for inserted genes and has had a large impact on fluorescence microscopy.

The stinging cells that jellyfish use to subdue their prey can injure people. Thousands of swimmers around the world are stung every year, and the effects range from mild discomfort to serious injury or even death. When conditions are favorable, jellyfish can form huge swarms. These swarms may damage fishing gear by filling fishing nets, and they sometimes clog the cooling systems of power and desalination plants that take their water from the sea.

Names

The name jellyfish has been in use since 1796. Traditionally, it has been applied to medusae and all similar animals, including the comb jellies (ctenophores), which belong to a different group. The words jellies or sea jellies are more recent. Public aquaria introduced them to avoid the word "fish," which today means an animal with a backbone, even though shellfish, cuttlefish and starfish are not backboned animals either.

In scientific writing, "jelly" and "jellyfish" have been used interchangeably. Many sources refer to only scyphozoans as "true jellyfish." A group of jellyfish is called a "smack" or a "smuck."

Mapping to taxonomic groups

The word jellyfish mostly refers to medusae, a life-cycle stage in the Medusozoa. The American evolutionary biologist Paulyn Cartwright explains that most medusozoan cnidarians have a pelagic, predatory jellyfish stage, while staurozoans are the exceptions because they are stalked. The Merriam-Webster dictionary describes a jellyfish as a free-swimming marine coelenterate. Because jellyfish is a common name, it does not match biological groups exactly. Some authorities call comb jellies and certain salps jellyfish, while others say neither is a jellyfish. Jellyfish are not a clade, since they include most of the Medusozoa, except some of the Hydrozoa. A phylogenetic tree shows which medusozoan groups authorities include.

The subphylum Medusozoa includes all cnidarians with a medusa stage in their life cycle. The medusa is the sexual stage, following a planula larva and a polyp. The four major classes are Scyphozoa (large jellyfish), Cubozoa (box jellyfish), Hydrozoa (small jellyfish), and Staurozoa (stalked jellyfish). Medusozoa excludes Anthozoa, which includes corals and sea anemones. Some Hydrozoa, such as Hydra, never form medusae and are not considered jellyfish. Because jellyfish have no hard parts, fossils are rare. The oldest clear fossil of a free-swimming medusa is Burgessomedusa, from the mid-Cambrian Burgess Shale of Canada. It may belong to a stem group of box jellyfish. Other fossils include Rhizostomites lithographicus from the Kimmeridgian period in Solnhofen, Germany, and a conulariid from the Middle Devonian of Wisconsin.

Anatomy

A true jellyfish has an umbrella-shaped bell filled with a clear, jelly-like material called mesoglea. The edge of the bell is often divided into rounded parts called lappets, which help it bend. Small sense organs called rhopalia hang between the lappets, and tentacles often grow along the edge of the bell. Stinging cells called nematocysts are found mostly on the tentacles.

Food is digested in the gastrovascular cavity, which is reached through the mouth on the underside of the bell. Most jellyfish do not have a brain or a central nervous system. Instead, they have a loose nerve net that senses their surroundings. Many jellyfish have light-sensitive ocelli, and box jellyfish have the most advanced eyes of the group. Each box jellyfish has 24 eyes, and two of them can see colour.

Largest and smallest

Jellyfish range from about one millimetre to nearly 2 metres (6½ ft) across the bell, and their tentacles and mouth parts usually reach beyond the bell. The smallest are the creeping jellyfish in the genera Staurocladia and Eleutheria, with bells from 0.5 millimetres to a few millimetres across. Many of them cannot be seen without a hand lens or microscope. They can reproduce by splitting in half, through fission. Two other tiny jellyfish, Cladonema radiatum and Cladonema californicum, live for months but never grow beyond a few millimetres.

The lion's mane jellyfish, Cyanea capillata, was long cited as the largest jellyfish. Its thread-like tentacles may reach 36.5 m (119 ft 9 in), though most are nowhere near that long. Its sting is moderately painful but rarely fatal. The giant Nomura's jellyfish, Nemopilema nomurai, appears in some years in the waters of Japan, Korea and China in summer and autumn. The largest ones can reach 2 m (6 ft 7 in) across the bell and about 200 kg (440 lb). The deep-sea Stygiomedusa gigantea has a thick bell up to 100 cm wide and four strap-like oral arms up to 6 m long. Desmonema glaciale, which lives in the Antarctic region, can reach several metres. Purple-striped jelly (Chrysaora colorata) can grow up to 15 feet long.

Life history and behavior

See also: Biological life cycle and Developmental biology

Jellyfish have a complex life cycle with both sexual and asexual stages. Adults spawn regularly when there is enough food, and in most species light controls the timing so that they spawn at about the same time of day, often at dawn or dusk. Most jellyfish are either male or female, though some are hermaphrodites. In most cases, sperm and eggs are released into the water, where the eggs are fertilized and grow into a planula larva covered with cilia. When it is developed enough, the planula settles on a firm surface and grows into a polyp. Polyps look like those of related anthozoans, such as sea anemones and corals. A polyp may be sessile, or it may float or attach to small bits of plankton. Polyps can reproduce asexually by budding, producing more polyps and small swimming young called ephyrae. Budding can happen from tentacle bulbs or the gonads. In a process called strobilation, the polyp's body narrows and splits into segments that break off as ephyrae. These grow into the adult medusa, which is the stage most people call a jellyfish. Little is known about the life histories of many jellyfish. An unusual species, Turritopsis dohrnii, formerly classified as Turritopsis nutricula, might be effectively immortal because it can change from a medusa back into a polyp. So far, this change has been observed only in the laboratory.

Jellyfish are shown to be the most energy-efficient swimmers of all animals, as studied using the moon jelly Aurelia aurita. They swim by expanding and contracting their bell-shaped bodies to push water behind them. They pause between these phases to create two vortex rings, and the second ring gives them a free boost forward. Most of the gelatinous tissue of the bell is inactive, so it uses no energy while swimming.

Ecology

Diet

Jellyfish, like other cnidarians, mostly eat other animals or live as parasites. They eat prey such as plankton, crustaceans, small fish, fish eggs and larvae, and other jellyfish, taking food in through the mouth. Their tentacles have stinging cells that stun or kill prey, and the tentacles can then bend to bring the food to the mouth. When the bell expands, it sucks in water that brings more prey within reach.

Some species, such as Aglaura hemistoma, eat both zooplankton and phytoplankton (tiny plants), including dinoflagellates. Others, like the spotted jellyfish (Mastigias papua), have mutualistic algae (Zooxanthellae) living in their bodies. The spotted jellyfish gets some food from photosynthesis and some from captured zooplankton. The upside-down jellyfish (Cassiopea andromeda) also works with microalgae, but it catches tiny animals too. It releases small balls of living cells made of mesoglea. These use cilia to move through the water and stinging cells to stun prey, and they seem able to digest food.

Predation

Other jellyfish are among the most common and important jellyfish predators, and sea anemones may eat jellyfish that drift into their area. Other predators include tunas, sharks, swordfish, sea turtles and penguins. Foxes, other land mammals and birds eat jellyfish that wash up on the beach. Few animals prey on jellyfish, so they are considered top predators in the food chain. When jellyfish become dominant, for example after overfishing removes predators of jellyfish larvae, there may be no obvious way to restore the old balance. Jellyfish eat fish eggs and young fish and compete with fish for food, which can stop fish populations from recovering.

Symbiosis

Some small fish are immune to jellyfish stings and live among the tentacles. They are safe from predators and share the food the jellyfish catches. The cannonball jellyfish has a close relationship with ten fish species and with the longnose spider crab, which lives inside the bell, shares the jellyfish's food and nibbles its tissues.

Blooms

Jellyfish form large masses, or blooms, under certain conditions, including ocean currents, nutrients, sunshine, temperature, season, prey availability, fewer predators and oxygen levels. Currents can gather jellyfish together, and jellyfish can detect marine currents and swim against them to gather in blooms. Jellyfish survive better than competitors in nutrient-rich, low-oxygen water, so they can feed on plankton without competition. Saltier water contains more iodine, which polyps need to turn into jellyfish. Warmer seas caused by climate change may also help blooms, since many jellyfish can survive in warm water.

Extra nutrients from runoff from farms and cities cause eutrophication and algal blooms. When the plants die, they can create dead zones, which are hypoxic (low in oxygen). These zones kill fish and other animals but not jellyfish, which allows jellyfish to bloom. Jellyfish populations may be growing around the world because of land runoff and overfishing of their natural predators. Jellyfish also benefit when marine ecosystems are disturbed. They reproduce quickly, prey on many species while few species prey on them, and feed by touch rather than sight, so they can feed at night and in murky water. Once jellyfish dominate, fish stocks may struggle to recover, because jellyfish eat plankton that includes fish eggs and larvae.

As suspected at the turn of this century, jellyfish blooms are increasing in frequency. From 2013 to 2020, the Mediterranean Science Commission tracked how often these outbreaks happened in coastal waters from Morocco to the Black Sea. Blooms appeared at a fairly high rate nearly all year, with peaks from March to July and often again in autumn. Different species cause blooms in different places. Pelagia noctiluca and Velella velella dominate in the western Mediterranean, Rhizostoma pulmo and Rhopilema nomadica in the eastern Mediterranean, and Aurelia aurita and Mnemiopsis leidyi in the Black Sea.

Some jellyfish populations that have clearly increased in recent decades are invasive species that 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.

Blooms can change the whole community. Some carnivorous jellyfish eat zooplankton, while others graze on primary producers. Fewer zooplankton and ichthyoplankton can affect many levels of the food chain. Dense jellyfish populations can outcompete other predators and reduce fish young. Blooms also use up large amounts of nutrients, which limits what is left for other organisms, and they can create low-oxygen areas.

Blooms affect people's work too. Jellyfish can outcompete fish in over-fished fisheries. Catching jellyfish can damage fishing gear, which costs money to repair. Power plants have been shut down when jellyfish blocked their cooling water. Blooms can also harm tourism by increasing stings and sometimes closing beaches.

Jellyfish are part of jelly-falls, when gelatinous zooplankton fall to the seafloor and feed benthic organisms there. In temperate and subpolar regions, jelly-falls usually follow right after a bloom.

Habitats

Most jellyfish are marine animals, but a few hydromedusae live in freshwater. The best known is the cosmopolitan hydrozoan jellyfish Craspedacusta sowerbii, which is less than an inch (2.5 cm) across, colorless and does not sting. Some jellyfish populations have become limited to coastal saltwater lakes, such as Jellyfish Lake in Palau. This marine lake is home to millions of golden jellyfish (Mastigias spp.) that swim across the lake every day.

Most jellyfish float well above the ocean floor as part of the plankton, but a few species stay close to the bottom for much of their lives and can be considered benthic. The upside-down jellyfish in the genus Cassiopea usually lie on the bottom of shallow lagoons. All stauromedusae are attached to seaweed or rocks or other firm material on the bottom.

Some species adapt to tidal changes. In Roscoe Bay, jellyfish ride the current as the tide goes out until they reach a gravel bar, then sink below the current. They wait in still water until the tide rises and carries them back into the bay. They also avoid fresh water from mountain snowmelt, diving until they find enough salt.

Parasites

Jellyfish are hosts to many parasites. They act as intermediate hosts for endoparasitic helminths, which pass to fish when fish eat them. Some digenean trematodes, especially species in the family Lepocreadiidae, use jellyfish as a second intermediate host. Fish become infected when they eat infected jellyfish.

Relation to humans

Jellyfish have long been eaten in some parts of the world. Fisheries have begun harvesting the American cannonball jellyfish, Stomolophus meleagris, along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asia. Jellyfish are also harvested for their collagen, which is being studied for uses such as treating rheumatoid arthritis. Aquaculture and fisheries for other species often suffer losses due to jellyfish. Aristotle wrote in Parts of Animals that jellyfish were eaten in winter in a fish stew. In some countries, including China, Japan, and Korea, jellyfish are a delicacy. They are dried to prevent spoiling. Only about 12 species, mostly from the Rhizostomeae group, are harvested for food, mainly in southeast Asia. Rhopilema esculentum in China and the cannonball jellyfish in the United States are popular because their bodies are large and firm and their toxins are harmless to humans.

Jellyfish are processed over 20 to 40 days. After the mucous membranes are removed, the jellyfish is treated with table salt and alum and pressed, which makes it drier and gives it a crisp texture. In China, jellyfish are soaked overnight to remove the salt, then eaten cooked or raw. In Japan, cured jellyfish are cut into strips and served with vinegar. Jellyfish are also used in science. Pliny the Elder reported in his Natural History that the slime of a jellyfish produced light when rubbed on a walking stick. In 1961, Osamu Shimomura extracted green fluorescent protein (GFP) and aequorin from Aequorea victoria while studying photoproteins that cause bioluminescence. Scientists including Douglas Prasher, Martin Chalfie, and Roger Tsien then developed GFP into a useful tool, and in 2008 the three men shared the Nobel Prize in Chemistry for this work. GFP is widely used as a fluorescent tag, using genetic engineering to join it to a gene so that scientists can see where that gene is active. The DNA may be placed in a cell or, using IVF techniques, in an animal.

Further information: Bioluminescence and Green fluorescent protein

Jellyfish are shown in many public aquariums, such as the Monterey Bay Aquarium. Tanks are often blue, with side lighting, and are usually circular so the animals do not get trapped in corners. Jellyfish depend on currents, so these tanks have carefully controlled water flows. Jellyfish are also becoming popular in home aquariums.

Jellyfish have nematocysts, specialized stinging cells. A touch on a tentacle can trigger millions of them to inject venom, but only some species' venom harms humans. Researchers writing in Communications Biology found that Cassiopea xamachana releases tiny balls of cells that sting anything in their path. Most stings cause only mild discomfort, but some box jellyfish, such as the sea wasp and Irukandji jellyfish, can be deadly. Stings can cause anaphylaxis, a dangerous form of shock. Jellyfish kill 20 to 40 people a year in the Philippines alone. For treatment, vinegar may help box jellyfish stings but not those of the Portuguese man o' war. Clearing away tentacles and scraping the skin can help, and hydrocortisone cream reduces pain and swelling. Antihistamines may ease itching. In Elba Island and Corsica, people use dittrichia viscosa leaves to treat stings. Urinating on a sting does not ease pain and may make it worse. Jellyfish can also cause problems for people and machines. Large numbers can fill and split fishing nets and clog cooling equipment, disabling power stations in several countries. Jellyfish caused a blackout in the Philippines in 1999 and damaged the Diablo Canyon Power Plant in California in 2008. They can also stop desalination plants and ships' engines.