Jellyfish

Jellyfish, also called sea jellies or simply jellies, are the medusa stage of certain soft, jelly-like members of the subphylum Medusozoa. This group is a major part of the phylum Cnidaria. Most jellyfish are free-swimming marine animals, though a few are attached to the seabed by stalks instead of moving around. Each jellyfish has an umbrella-shaped main body made of mesoglea, called the bell, and a group of long, trailing tentacles on its underside.

The bell pulses in and out to move the jellyfish through open water. The tentacles have stinging cells. Jellyfish use them to catch food or to protect themselves from predators. Jellyfish have a complex life cycle. The medusa is usually the stage that reproduces, and it makes planula larvae. These larvae drift away and settle onto the seabed, where they grow into a stationary polyp stage. The polyp may make copies of itself before the animal becomes an adult that can reproduce.

Jellyfish live all over the world, from the surface of the water to the deep sea. Scyphozoans, the "true jellyfish," live only in the sea. Some hydrozoans that look similar live in fresh water. Large, often colorful, jellyfish are common in coastal waters around the world. Jellyfish have existed for at least 500 million years, and possibly 700 million years or more. This makes them one of the oldest groups of animals with many organs.

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

The stinging cells that jellyfish use to catch prey can hurt people. Many swimmers are stung each year. The effects can range from mild discomfort to serious injury. When conditions are right, jellyfish can gather in huge groups called swarms. These swarms can cause problems for fishing. They may fill fishing nets, and they sometimes block the cooling systems of power and desalination plants that take their water from the sea.

Names

The name jellyfish has been used since 1796. Long ago, it was used for jellyfish and similar animals, including the comb jellies (ctenophores), which belong to a different group. The words jellies or sea jellies are newer. Public aquariums started using them so people would not use the word "fish," which today means an animal with a backbone. Still, shellfish, cuttlefish and starfish do not have backbones either.

In science writing, "jelly" and "jellyfish" have been used in the same way. Many sources call only scyphozoans "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, but staurozoans are the exceptions because they are stalked. The Merriam-Webster dictionary describes a jellyfish as a free-swimming marine coelenterate. Jellyfish is a common name, so it does not match biological groups exactly. Some experts 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 experts include.

The subphylum Medusozoa includes all cnidarians that have a medusa stage in their life cycle. The medusa is the sexual stage. It follows a planula larva and a polyp. The four main classes are Scyphozoa (large jellyfish), Cubozoa (box jellyfish), Hydrozoa (small jellyfish), and Staurozoa (stalked jellyfish). Medusozoa does not include Anthozoa, which has corals and sea anemones. Some Hydrozoa, such as Hydra, never form medusae, so they are not considered jellyfish. Jellyfish have no hard parts, so fossils are rare. The oldest clear fossil of a free-swimming medusa is Burgessomedusa, from the middle Cambrian period, found in the 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 period in Wisconsin.

Anatomy

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

Food is digested in the gastrovascular cavity. Food enters 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 what is around them. Many jellyfish have light-sensitive ocelli. Box jellyfish have the most advanced eyes in 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. Their tentacles and mouth parts usually reach beyond the bell. The smallest are the creeping jellyfish in the genera Staurocladia and Eleutheria. Their bells are from 0.5 millimetres to a few millimetres across, and many 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 once thought to be 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 dangerous. 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 plenty of food. In most species, light helps set the time, so 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, which means they have both. In most cases, sperm and eggs are released into the water. The eggs are fertilized and grow into a planula larva, which is covered with cilia. Tiny hairlike structures that help it move. When it is ready, 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 stay in one place, or it may float or attach itself to small bits of plankton. Polyps can reproduce without a partner by budding, which means making new 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. These segments break off as ephyrae. They grow into the adult medusa, which is the stage most people call a jellyfish. Scientists still know little about the life histories of many jellyfish. An unusual species, Turritopsis dohrnii, was once called Turritopsis nutricula. It might be effectively immortal because it can change from a medusa back into a polyp. So far, this change has only been seen in the laboratory.

Scientists have found that jellyfish are the most energy-efficient swimmers of all animals. This was studied using the moon jelly Aurelia aurita. They swim by expanding and contracting their bell-shaped bodies to push water behind them. Between these movements, they create two vortex rings. The second ring gives them a free boost forward. Most of the jelly-like tissue of the bell is inactive, so it uses no energy while swimming.

Ecology

Diet

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

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 balls use tiny hairs called cilia to move through the water and stinging cells to stun prey. They seem able to digest food.

Predation

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

Symbiosis

Some small fish are not harmed by 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. It also has a relationship with the longnose spider crab, which lives inside the bell, shares the jellyfish's food and nibbles its tissues.

Blooms

Jellyfish form large groups, called blooms, under certain conditions. These conditions include ocean currents, nutrients, sunshine, temperature, season, available prey, fewer predators and oxygen levels. Currents can gather jellyfish together. Jellyfish can also sense marine currents and swim against them to gather in blooms. Jellyfish survive better than their competitors in water that is rich in nutrients but low in oxygen. This means 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 these 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 ocean life is disturbed. They reproduce quickly and prey on many species, while few species prey on them. They 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. This is because jellyfish eat plankton that includes fish eggs and larvae.

Scientists have noticed that jellyfish blooms are becoming more frequent. From 2013 to 2020, the Mediterranean Science Commission tracked how often these blooms happened in coastal waters from Morocco to the Black Sea. Blooms appeared at a fairly high rate nearly all year. They were most common from March to July and often came again in autumn. Different species cause blooms in different places. Pelagia noctiluca and Velella velella are common in the western Mediterranean. Rhizostoma pulmo and Rhopilema nomadica are common in the eastern Mediterranean. Aurelia aurita and Mnemiopsis leidyi are common in the Black Sea.

Some jellyfish populations have clearly grown in recent decades. These are invasive species that arrived from other habitats. Examples include the Black Sea, Caspian Sea, Baltic Sea, central and eastern Mediterranean, Hawaii, and warm parts of the western Atlantic. These warm areas include the Caribbean, Gulf of Mexico and Brazil.

Blooms can change a whole ocean community. Some jellyfish eat zooplankton, while others feed on the plants and tiny organisms that make their own food. Fewer zooplankton and ichthyoplankton can affect many levels of the food chain. Dense jellyfish populations can outcompete other predators and reduce the number of young fish. Blooms also use up large amounts of nutrients, which leaves less for other living things. They can also create low-oxygen areas.

Blooms affect people's work too. Jellyfish can outcompete fish in fisheries that have already been overfished. 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. This is when gelatinous zooplankton fall to the seafloor and feed benthic organisms there. In temperate and subpolar regions, jelly-falls usually happen right after a bloom.

Habitats

Most jellyfish live in the sea, but a few hydromedusae live in freshwater. The best known is the cosmopolitan hydrozoan jellyfish Craspedacusta sowerbii. It 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. 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, 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 they 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 melting mountain snow. They dive until they find enough salt.

Parasites

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

Relation to humans

People have eaten jellyfish for a long time in some parts of the world. Fisheries have started catching the American cannonball jellyfish, Stomolophus meleagris, along the southern Atlantic coast of the United States and in the Gulf of Mexico. Many are sent to Asia. Jellyfish are also used to make collagen, which scientists are studying as a possible help for rheumatoid arthritis. Aquaculture and fishing for other kinds of fish often lose catches because of 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 special food. They are dried so they do not spoil. Only a few kinds, mostly from the Rhizostomeae group, are caught 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 they are safe to eat.

Preparing jellyfish for eating can take 20 to 40 days. First, the mucous membranes are removed. Then the jellyfish is treated with table salt and alum and pressed. This makes it drier and gives it a crisp texture. In China, jellyfish are soaked overnight to remove the salt. Then they are eaten cooked or raw. In Japan, prepared jellyfish are cut into strips and served with vinegar. Jellyfish are also useful in science. Long ago, Pliny the Elder wrote in his Natural History that the slime of a jellyfish glowed when rubbed on a walking stick. In 1961, Osamu Shimomura removed green fluorescent protein (GFP) and aequorin from Aequorea victoria while studying photoproteins, which make bioluminescence. Other scientists, including Douglas Prasher, Martin Chalfie, and Roger Tsien, then turned GFP into a useful tool. 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, it is joined to a gene so 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. The tanks are often blue, with light from the side. They are usually round so the animals do not get stuck in corners. Jellyfish depend on water currents, so these tanks have carefully controlled water flow. Jellyfish are also becoming popular in home aquariums.

Jellyfish have special stinging cells called nematocysts. A touch on a tentacle can make many of them release venom, but only some kinds of jellyfish have venom that harms people. 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. However, some box jellyfish, such as the sea wasp and Irukandji jellyfish, can be dangerous. Stings can cause anaphylaxis, a serious kind of shock. Jellyfish sometimes cause deaths, including in the Philippines. For first aid, vinegar may help box jellyfish stings but not those of the Portuguese man o' war. Removing any tentacles and gently scraping the skin can help. Hydrocortisone cream can reduce 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 groups can fill and tear fishing nets and clog cooling equipment. This has stopped 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.