Jellyfish are aquatic animals with a soft, jellylike body and no bones. They are not fish are related to corals and sea anemones. Many jellyfish have tentacles that they use to sting their prey. A typical jellyfish is shaped like a bell or a parachute. The term 'Jellyfish' refers to two groups of creatures that look similar but are unrelated. The larger group includes the bell-shaped beings that most people envision when they think of jellyfish: the so-called “true jellies” and their kin. The other group consists of comb jellies—ovoid, ghostly creatures that swim by beating their hairlike cilia and attack their prey with gluey appendages instead of stinging tentacles. (Many other gelatinous animals are often referred to as jellyfish, including the Portuguese man-of-war, a colony of stinging animals known as a siphonophore.)
What are Jellyfish?
Jellyfish (also called 'jellies') drift through the ocean's water column around the world with their pulsating bells and long, trailing tentacles. The oldest ancestors of modern day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago. That makes jellyfish three-times as old as the first dinosaurs! They have two major cell layers: the external epidermis and the internal gastrodermis. The gastrodermis lines the all-purpose gut and an opening where food enters and reproductive cells are released and taken in. Jellies have no need for a stomach, intestine, or lungs: nutrients and oxygen slip in and out of their cell walls through the gastrodermis or even their bodies' outer cells. The outer cells that make up the epidermis contain a loose network of nerves called the "nerve net." This is the most basic nervous system known in a multicellular animal. Between these layers is a gelatinous material called mesoglea, which makes up most of their bodies. Jellyfish are 95 percent water and so, rightly, mesoglea is mostly water! It also contains some structural proteins, muscle cells, and nerve cells, forming a kind of internal skeleton.
Jellies were here even before dinosaurs
Jellyfish have a complex life cycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms: medusa and polyps. Polyps can reproduce asexually by budding, while medusae spawn eggs and sperm to reproduce sexually. An adult jellyfish is called a medusa, which is the familiar umbrella-shaped form that we see in the water. Medusa jellyfish reproduce sexually by spawning—the mass release of eggs and sperm into the open ocean—with entire populations sometimes spawning all together. Male and female jellyfish release the sperm and eggs from their mouths. In most species, fertilization takes place in the water; in others, the sperm swim up into the female's mouth and fertilize the eggs within. The fertilized eggs then develop into planulae, which are ciliated free-swimming larvae shaped a bit like a miniature flattened pear. After several days of development, the planulae attach to a firm surface and transform into flower-like polyps. The polyps have a mouth and tentacles that are used to feed on zooplankton. Polyps reproduce asexually by budding — when a polyp divides roughly in half to produce a new genetically identical polyp — or they can produce or transform into medusae, depending on the type of jellyfish. Hydrozoan polyps bud medusae from their sides; cubozoan polyps each transform into a medusa. In schyphozoans, a process called strobilation takes place (shown in video and in diagram). During strobilation, a polyp splits into 10-15 plate-like segments stacked atop one another in a tower called a strobila. After a segment separates from the strobila, it is called an ephyra, a juvenile jellyfish. Ephyrae mature into the medusa form.
What is the lifecycle of Jellyfish?
Jellyfish vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide! The largest jellies are the Lion’s Mane Jellyfish (Cyanea capillata), which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long. Jellies don't have brains; rather, they have a network of neurons ("nerve net") that allows jellies to sense their environments, such as changes in water chemistry indicating food or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light. Additionally, some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals and movement. One group of jellyfish, the cubozoan jellyfish, have complex eyes with lenses, corneas and retinas in their rhopalia. Although they respond to visual stimuli, scientists don’t know how the jellyfish interpret the images created by their eyes since they don’t have a brain with which to process them. Their nerve ring, a ring-shaped concentration of nerves found in jellyfish, seems to be involved, however.
Jellyfish have no brains and no hearts
All jellyfish are Cnidaria, an animal phylum that contains jellies, sea anemones, and corals, among others. There are more than 10,000 species of Cnidaria, and less than 4,000 of these are Medusazoa—those animals we think of as jellyfish. Those 4,000 jellyfish can be divided into four different groups: Scyphozoa (true jellyfish), Cubozoa (box jellyfish), Staurozoa (stalked jellyfish) and Hydrozoa (small jellyfish)
What are Jellyfish?
SCYPHOZOA are the most familiar jellyfish, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason. Scyphozoa spend most of their lives in the medusa body form, and there are at least 200 species.
HYDROZOA are jellyfish look-alikes but not in the same group as the “true jellyfish.” The swimming medusa stages of this group are often small and inconspicuous, whereas the bottom-dwelling polyps, or hydroids, usually take the form of large colonies. In the water column, the colonial siphonophores may be quite spectacular. These include the notorious Portuguese Man-o-Wars and many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets. Colonial siphonophores are composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota, 12,140 feet (3,700 meters) deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3,700 species of Hydrozoa. CUBOZOA are the box jellyfish, named for their box-like bells. Some cubozoans, such as the sea wasp (Chironex fleckeri), produce some of the most potent venom known. Cubozoan jellyfish also have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some even engage in elaborate (for a jellyfish) courtship behavior! There are at least 36 species. In 2011, Allen Collins, a jellyfish expert at the Smithsonian, discovered a new species, which was named Tamoya ohboya in a public naming contest. STAUROZOA are the stalked jellyfishes, which don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage. are the stalked jellyfishes, which don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.
Types of Jellyfish
Jellyfish and ctenophores are carnivorous, and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish and even other jellyfish and comb jellies. Some jellyfish sit upside down on the bottom and have symbiotic algae (zooxanthellae) in their tissues, which photosynthesize, and so get much of their energy the way plants do. While their nematocysts and colloblasts do help them defend themselves, plenty of animals manage to catch and eat jellies: more than 150 animal species are known to eat jellies, including fish, sea turtles, crustaceans, and even other jellyfish. Jellies are the favorite food of the ocean sunfish (Mola mola) and endangered leatherback turtle (Dermochelys coriacea), which will migrate thousands of miles for the gelatinous delicacy. Young jellyfish are small enough to be part of the general zooplankton population and are eaten by many animals. Humans also eat jellyfish: people have fished for jellies for at least 1700 years off the coast of China. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia. Eating jellyfish may become more common around the world as we overfish more preferable fish species.
What do Jellyfish eat?
Jellyfish and ctenophores both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin—a process that takes only 700 nanoseconds. A small number of jellyfish are very toxic to humans, such as the box jellyfish (Chironex fleckeri) and Irukandji jellyfish (Carukia barnesi), which can cause severe reactions and even death in some people. Many comb jellies have colloblasts lining their tentacles, which work like nematocysts but release glue instead of venom. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth. One species of ctenophore (Haeckelia rubra) recycles nematocysts from hydrozoan jellyfish it consumes and uses these to stun and kill prey.
Some Jellyfish have tentacles
Many jellyfish and comb jellies are able to produce light—an ability known as bioluminescence. They have proteins in some tissues that undergo a chemical reaction to produce blue or green light in response to stimuli such as touch. No one's quite sure why jellies bioluminesce, but it seems to be mainly a defense tactic. A bright enough flash could be enough to startle a predator—or to attract an even bigger predator to make the jelly's predator into prey. Jellies have also adapted their body color to camouflage in the darkness. Most are nearly colorless and transparent, so they can be difficult for predators to see. However, some deep sea jellyfish and comb jellies are a bright red or orange color. Why would they be red instead of black to blend in with the dark water? Red cannot be seen in dark water (deeper than 200 meters), so there's no greater protection from black than red. But red is preferred to black because pigment is easier for animals to produce. Some deep sea jellies just have dark red guts, possibly serving to mask luminescent prey from other larger predators with eyes.
Some Jellyfish glow in the dark
Most jellyfish are short lived. Medusa or adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. Polyps can live and reproduce asexually for several years, or even decades. One jellyfish species is almost immortal. Turritopsis nutricula, a small hydrozoan, can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation. This is the only animal known to do so.
Life span of the Jellyfish
In comparison to the jellyfish, comb jellies have a very simple lifecycle. Most species are hermaphroditic and able to release both eggs and sperm into the water, which drift with the waves until they find other gametes. Because most species have both male and female gametes, it's thought that they can self-fertilize as well. This method may not seem very efficient, since it's likely that most of the gametes never find a match. But ctenophores make up for this by releasing them every day. If they run out of food while producing so many eggs and sperm, they can shrink and hunker down until they run into more food and can start reproducing again. Once eggs and sperm find each other, the embryo develops into a larva that looks just like a small adult ctenophore—and, from there, all it has to do is grow up. One species (Mertensia ovum) can reproduce even when it is still larva, and scientists think other species are also able to reproduce at a young age. This means that comb jelly populations can grow very fast under certain conditions.
Jellyfish have special structures which enable them to swim and orient. These are called gravity receptors and they resemble microscopic fingers. These structures have calcium crystals at their tips called statoliths, which move when the animals and the gravity receptors move. These sensitive structures provide positional information to the animal based on the direction of gravity and whether the jellyfish are tilted up or down. In 1991, NASA made history by sending 2478 jellyfish polyps to space. It was part of an experiment called “The Effects of Microgravity-Induced Weightlessness on Aurelia Ephyra Differentiation and Statolith Synthesis.” The creatures were kept in flasks and bags that contained artificial seawater, which astronauts then injected with chemicals that encouraged them to reproduce. By the end of the experiment there were approximately 60,000 jellyfish in the Earth’s orbit
Jellyfish went to space
The main feature of a true jellyfish is the umbrella-shaped bell. This is a hollow structure consisting of a mass of transparent jelly-like matter known as mesoglea, which forms the hydrostatic skeleton of the animal. 95% or more of the mesogloea (the tissue that functions as a hydro-static skeleton) consists of water, but it also contains collagen and other fibrous proteins, as well as wandering amoebocytes which can engulf debris and bacteria. The mesogloea is bordered by the epidermis on the outside and the gastrodermis on the inside. The edge of the bell is often divided into rounded lobes known as lappets, which allow the bell to flex. In the gaps or niches between the lappets are dangling rudimentary sense organs known as rhopalia, and the margin of the bell often bears tentacles. the centre, with the mouth, which also functions as the anus, at its tip. There are often four oral arms connected to the manubrium, streaming away into the water below. The mouth opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. This is subdivided by four thick septa into a central stomach and four gastric pockets. The four pairs of gonads are attached to the septa, and close to them four septal funnels open to the exterior, perhaps supplying good oxygenation to the gonads. Near the free edges of the septa, gastric filaments extend into the gastric cavity; these are armed with nematocysts and enzyme-producing cells and play a role in subduing and digesting the prey. In some scyphozoans, the gastric cavity is joined to radial canals which branch extensively and may join a marginal ring canal. Cilia in these canals circulate the fluid in a regular direction. The box jellyfish is largely similar in structure. It has a squarish, box-like bell from each of the four lower corners of which hang a short pedalium or stalk which bears one or more long, slender tentacles. The rim of the bell is folded inwards to form a shelf known as a velarium which restricts the bell's aperture and creates a powerful jet when the bell pulsates, allowing box jellyfish to swim faster than true jellyfish. Hydrozoans are also similar, usually with just four tentacles at the edge of the bell, although many hydrozoans are colonial and may not have a free-living medusal stage. In some species, a non-detachable bud known as a gonophore is formed that contains a gonad but is missing many other medusal features such as tentacles and rhopalia. Stalked jellyfish are attached to a solid surface by a basal disk, and resemble a polyp, the oral end of which has partially developed into a medusa with tentacle-bearing lobes and a central manubrium with four-sided mouth. Most jellyfish do not have specialized systems for osmoregulation, respiration and circulation, and do not have a central nervous system. Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around the mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift. The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation. A loose network of nerves called a "nerve net" is located in the epidermis. Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species. A jellyfish detects various stimuli, and transmits impulses both throughout the nerve net and around a circular nerve ring, to other nerve cells. The rhopalial ganglia contain pacemaker neurones which control swimming rate and direction. In many species of jellyfish, the rhopalia include ocelli, light-sensitive organs able to tell light from dark. These are generally pigment spot ocelli, which have some of their cells pigmented. The rhopalia are suspended on stalks with heavy crystals at one end, acting like gyroscopes to orient the eyes skyward. Certain jellyfish look upward at the mangrove canopy while making a daily migration from mangrove swamps into the open lagoon, where they feed, and back again. Box jellyfish have more advanced vision than the other groups. Each individual has 24 eyes, two of which are capable of seeing color, and four parallel information processing areas that act in competition, supposedly making them one of the few kinds of animal to have a 360-degree view of its environment. Jellyfish range from about one millimeter in bell height and diameter, to nearly 2 metres (6.6 ft) in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension. The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria, which have bell disks from 0.5 millimetres (0.02 in) to a few millimeters in diameter, with short tentacles that extend out beyond this, which these jellyfish use to move across the surface of seaweed or the bottoms of rocky pools. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by fission (splitting in half). Other very small jellyfish, which have bells about one millimeter, are the hydromedusae of many species that have just been released from their parent polyps; some of these live only a few minutes before shedding their gametes in the plankton and then dying, while others will grow in the plankton for weeks or months. The hydromedusae Cladonema radiatum and Cladonema californicum are also very small, living for months, yet never growing beyond a few mm in bell height and diameter. The lion's mane jellyfish, Cyanea capillata, was long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles that may extend up to 36.5 metres (120 ft) long (though most are nowhere near that large). They have a moderately painful, but rarely fatal, sting. The increasingly common giant Nomura's jellyfish, Nemopilema nomurai, found in some, but not all years in the waters of Japan, Korea and China in summer and autumn is another candidate for "largest jellyfish", in terms of diameter and weight, since the largest Nomura's jellyfish in late autumn can reach 2 metres (6 ft 7 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight, with average specimens frequently reaching 0.9 metres (2 ft 11 in) in bell diameter and about 150 kilograms (330 lb) in weight. The large bell mass of the giant Nomura's jellyfish can dwarf a diver and is nearly always much greater than the Lion's Mane, whose bell diameter can reach 1 metre (3 ft 3 in). The rarely encountered deep-sea jellyfish Stygiomedusa gigantea is another candidate for "largest jellyfish", with its thick, massive bell up to 100 centimetres (39 in) wide, and four thick, "strap-like" oral arms extending up to 6 metres (20 ft) in length, very different from the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane. Jellyfish have a complex life cycle which includes both sexual and asexual phases, with the medusa being the sexual stage in most instances. Sperm fertilize eggs, which develop into larval planulae, become polyps, bud into ephyrae and then transform into adult medusae. In some species certain stages may be skipped. Upon reaching adult size, jellyfish spawn regularly if there is a sufficient supply of food. In most species, spawning is controlled by light, with all individuals spawning at about the same time of day, in many instances this is at dawn or dusk. Jellyfish are usually either male or female (with occasional hermaphrodites). In most cases, adults release sperm and eggs into the surrounding water, where the unprotected eggs are fertilized and develop into larvae. In a few species, the sperm swim into the female's mouth, fertilizing the eggs within her body, where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing planula larvae. The planula is a small larva covered with cilia. When sufficiently developed, it settles onto a firm surface and develops into a polyp. The polyp generally consists of a small stalk topped by a mouth that is ringed by upward-facing tentacles. The polyps resemble those of closely related anthozoans, such as sea anemones and corals. The jellyfish polyp may be sessile, living on the bottom, boat hulls or other substrates, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish or other invertebrates. Polyps may be solitary or colonial. Most polyps are only millimetres in diameter and feed continuously. The polyp stage may last for years. After an interval and stimulated by seasonal or hormonal changes, the polyp may begin reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. Budding produces more scyphistomae and also ephyrae. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. In a process known as strobilation, the polyp's tentacles are reabsorbed and the body starts to narrow, forming transverse constrictions, in several places near the upper extremity of the polyp. These deepen as the constriction sites migrate down the body, and separate segments known as ephyra detach. These are free-swimming precursors of the adult medusa stage, which is the life stage that is typically identified as a jellyfish. The ephyrae, usually only a millimeter or two across initially, swim away from the polyp and grow. Limnomedusae polyps can asexually produce a creeping frustule larval form, which crawls away before developing into another polyp. A few species can produce new medusae by budding directly from the medusan stage. Some hydromedusae reproduce by fission. Little is known of the life histories of many jellyfish as the places on the seabed where the benthic forms of those species live have not been found. However, an asexually reproducing strobila form can sometimes live for several years, producing new medusae (ephyra larvae) each year. An unusual species, Turritopsis dohrnii, formerly classified as Turritopsis nutricula, might be effectively immortal because of its ability under certain circumstances to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other ocean organism. So far this reversal has been observed only in the laboratory. Using the moon jelly Aurelia aurita as an example, jellyfish have been shown to be the most energy efficient swimmers of all animals. They move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion to create two vortex rings. Muscles are used for the contraction of the body, which sheds the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. By doing so, the second vortex ring rolls under it and begins to spin faster. This sucks in water which refills the bell and is pushed up against the centre of the body, giving it a secondary and "free" boost forward. The mechanism, called passive energy recapture, only works at low speeds and relatively small body sizes, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (the amount of food and oxygen consumed, versus energy spent in movement) than other animals in similar studies. One reason for this is that most of the gelatinous tissue of the bell is inactive, using no energy during swimming. Jellyfish medusae are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting food and voiding undigested waste through the mouth. They hunt passively using their tentacles as drift nets, or sink through the water with their tentacles spread widely; the tentacles, which contain nematocysts to stun or kill the prey, may then flex to help bring it to the mouth. Their swimming technique also helps them to capture prey; when their body expands it sucks in water which brings more potential prey within reach of the tentacles.
Anatomy of a Jellyfish