Jellyfish Characteristics, History, Behavior and Development

Home | Category: Jellyfish, Sponges, Sea Urchins and Anemones


20120518-jellyfish -Tiny_Jelly.jpg
tiny jelly
Jellyfish are simple invertebrates and members of the 9,000-species phylum Cnidaria (meaning "stringing thread"), which includes creatures such as sea anemones, sea whips, and corals. Like all members of the phylum, the body parts of a jellyfish radiate from a central axis. This “radial symmetry" (symmetry around a central axis),” allows jellyfish to detect and respond to food or danger from any direction.

Most are also coelenterates, which also include sea anemones and coral. All coelenterates, are simply a hollow sac of cells with a mouth at one end surrounded by tentacles. Armed with stinging cells, the tentacles help them to paralyze small swimming animals which are then pushed into its mouth.

There are a number of animals that look like jellyfish but are actually something else. Chief among these are ctenophores (comb jellies). There are also some worms, snails and squid that look like jellyfish. The main difference between Cnidaria and ctenophores is that the former has a sting and the latter doesn't. [Source: Richard Conniff, National Geographic, June 2000; Jack and Anne Rudlow, Smithsonian]

Jellyfish are among the world's oldest creatures. Because they have soft bodies, jellyfish fossils are rare. They oldest, found in Australia, are 650 million years old. Jellyfish "invented" neurons, the cells that build brains and nervous systems, and were among the first to develop stinging poison.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio


Sea anemones, jellyfish and corals belong to the family of colonizing organisms called coelenterates (Greek for cavity) and the 9,000-species phylum Cnidarians (meaning "stringing thread"), a group of tentacled creatures which also includes anemones, jellyfish and corals and hydriods. Most reproduce asexually without mating by producing buds from their own bodies.

sea nettle
All coelenterates are simply a hollow sac or shallow cup of cells with a mouth at one end surrounded by tentacles. Armed with stinging cells, the tentacles help them to paralyze small swimming animals which are then pushed into its mouth. Coelenterates have a primitive gut for digestion and their mouth also serves as an anus.

Coelenterates take in food through their mouthes and ingest it in the stomach, with the indigestible parts being expelled back out the mouth. They are almost exclusively carnivores but have no teeth. Instead they have tentacles lined with whiplike structures called nematocysts that release poison barbs that are strong enough to paralyze prey and allow it to be pulled into the coelenterate’s mouth and gut. Jellyfish have their tentacles pointed downward while anemones and corals have theirs pointed upwards.


Cnidarians are essentially the same as coelenterates — cup-like animals — but looked at in a slightly different way. The tube can either be a medusa, flattened into a bell shape, or a polyp, with the closed end attached to a hard surface. Corals and sea anemones are polyps. Medusas are mostly jellyfish. Some hydriods and jellyfish exist in both medusa and polyp forms in their lifetimes.

Phil Myers wrote in Animal Diversity Web: The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry regard as primitive. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

20120518-jellyfish Chrysaora_quinquecirrha.JPG
Chrysaora quinquecirrha
Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella. Polyps, in contrast, are usually sessile (fixed in one place). They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense. /=\

Reproduction in polyps is by asexual budding (polyps) or sexual formation of eggs and sperm (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming. /=\

If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death. /=\

Oldest Jellyfish Fossils — Over 500 Million Years Old

Jellyfish have been around for at least 500 million years. That means they appeared more than 250 million years before the first dinosaurs. Because jellyfish are soft-bodied and almost all water, jellyfish fossils are incredibly rare. Of those that do exist, some of the oldest-known jellyfish fossils, found in Utah, date to 505 million years ago. These are detailed enough to show clear linkages to modern species of jellyfish. [Source: Orlando Science Center]

Four different types of jellyfish dated back to the Cambrian Period (539 to 485 million years ago) were found by researchers in Utah in 2007. These very old jellyfish showed the same complexity as modern jellyfish, suggesting they developed rapidly into their present form 500 million years ago. Fossils of soft-bodied creatures, such as jellyfish, rarely survive in the fossil record, unlike animals with hard shells or bones. "The fossil record is biased against soft-bodied life forms such as jellyfish, because they leave little behind when they die," said Bruce Lieberman of the University of Kansas, one of the scientists involved in Utah fossil research.

505-Million-Year-Old Jellyfish Fossil from Utah

Live Science reported: These jellyfish left their lasting imprint because they were deposited in fine sediment, rather than coarse sand. The film that the jellyfish left behind shows a clear picture, or "fossil snapshot," of the animals. "You can see a distinct bell-shape, tentacles, muscle scars and possibly even the gonads," said study team member Paulyn Cartwright, also of the University of Kansas. [Source: Andrea Thompson, Live Science, October 31, 2007]

The rich detail of the fossils allowed the team to compare the cnidarian fossils to modern jellyfish. The comparison confirmed that the fossils were, in fact, jellyfish and pushed the earliest known occurrence of definitive jellyfish back from 300 million to 505 million years ago.

The fossils also offer insights into the rapid species diversification that occurred during the Cambrian radiation, which began around 540 million years ago and when most animal groups start to show up in the fossil record, Lieberman said.

540-Million-Year-Old “Jellyfish Graveyard

In 2017, scientists announced the discovery of a 540-million-year-old “jellyfish graveyard in Death Valley, California. The ancient jellyfish were preserved in a slab of sandstone believed to have come a beach. Live Science reported: Scientists identified 13 of these oval specimens on the rocky surface, ranging from 1.2 to 8.3 inches (3 to 21 centimeters) in diameter. The fossils were lighter than the rock surrounding them, and they varied not only in size but also their style of preservation. Some included convex, circular ridges; others held concave rings around a convex interior; and several were fossilized as more pronounced, rounded mounds, the scientists wrote in the study. published online in the July 2017 issue of the journal Geological Magazine.[Source: Mindy Weisberger, Live Science, August 7, 2017]

In one jellyfish specimen, shapes of some of the animal's body parts were still faintly visible. Additional marks in the rock around the fossilized jellyfish hinted at the movements of ancient currents, which may have pushed and distorted the bodies of the stranded jellyfish prior to fossilization. Other marks might have been made by a stranded jellyfish's attempts to move back into the water, according to the study authors.This jellyfish was likely buried in sand after it became stranded; its body collapsed, and the carcass was preserved in the microbe-rich sediment. Jellyfish that wash up on beaches today are frequently eaten by scavenging birds and crustaceans, said Aaron Sappenfield/ of the University of California, Riverside.

But during the Cambrian period, when marine life was bountiful and diverse, there were no large terrestrial scavengers to pick at the jellies' carcasses. If they became stranded, chances were good that their remains would stay in one place long enough to fossilize, he said. However, the jellies' preservation was equally dependent on the gummy, microbe-rich sand that they stranded themselves on, which was also a characteristic of the Cambrian period, Sappenfield said. "A jellyfish lands on the beach — that big, wet sack settles in the sand — and you get this nice impression with really high resolution because of that binding agent," he said.

Jellyfish Characteristics

Basic parts of a jellyfish

Jellyfish are very simple creatures, Only about five percent of the body of a jellyfish is solid matter; the rest is water. They don’t have brains, skeleton, blood, or even hearts, The five percent of their bodies that are solid are the only parts that contain organic material. Around 95 percent watery part is held together with a jellylike material that is more like mucous with rubbery fibers than jelly. Because jellyfish are mostly saltwater the can easily manipulate their buoyancy to move up and own in the water column. It doesn't need skeletons to hold itself up as it would if it lived on land and had to deal with gravity.

Jellyfish are venomous, radially symmetrical (symmetrical around a central axis), ectothermic (use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment) and polymorphic (“many forms”, species in which individuals can be divided into easily recognized groups, based on structure, color, or other similar characteristics). Television naturalist David Attenborough wrote: "Jellyfish are constructed from two layers of cells. The jelly which separates them gives the organism a degree of rigidity needed to withstand the buffeting of the sea...Their cells, unlike those of sponges, are incapable of independent survival.”

Jellyfish are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis. An elementary nervous system, or nerve net, allows jellyfish to smell, detect light, and respond to other stimuli. The simple digestive cavity of a jellyfish acts as both its stomach and intestine, with one opening for both the mouth and the anus. [Source: NOAA]

A typical Cnidaria jellyfish is umbrella-shaped with a number of tentacles dangling below and a mouth and digestive cavity at the center of the base of the tentacles. Spoke-like canals deliver nutrient from the digestive cavity to the rest of the body. All of these things are made from mucous. Jellyfish have the ability to sting with their tentacles. While the severity of stings varies, in humans, most jellyfish stings result only in minor discomfort.

The body of jellyfish are mainly composed of an outer epidermis cup, an inner gastrodermis layer, and tentacles. The dome-shaped body of the sea nettle has eight scalloped, flower-petal shaped lobes from which tentacles extend. Each octant bears around seven to 10 tentacles, all of which are lined with nematocysts (specialized stinging organelles). four long, ribbon-like oral arms extend from the middle of the umbrella. The arms bring food up to the mouth, which is the only opening comprising the digestive system. This opening is lined with thousands of small mouthlet pores. [Source: Nate Lanier and Alexi Weber, Animal Diversity Web (ADW) /=]

Jellyfish Behavior, Senses and Nervous System

Cross section of a flow hat jelly (Olindias formosa)

Jellyfish don't have ears or brains. Only a few species have eyes. Some cells are modified to transmit electrical impulses and are linked into a primitive nervous system. Many that seem to pitch and roll in the water can orient themselves with light sensors along their bell margins. Most jellyfish don’t really need senses as they filter -feed whatever comes their way. They can catch prey and eat in areas of total darkness because they don’t need to see what they catch or eat. Little information is known about communication in jellyfish. Some release and react to chemicals in the water during breeding seasons. Due to limited material on cnidarian nervous systems, how these chemicals are interpreted remains unclear.

Sea Nettle sense using polarized light, touch and chemicals usually detected with smelling or smelling-like senses. They communicate with vision, touch, chemicals usually detected by smelling and pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species). [Source: Stephanie Braccini, Animal Diversity Web (ADW) /=]

Jellyfish nervous systems are usually comprised of a scattered net of cells, while some species display more organized nerve rings. According to Animal Diversity Web:In those species where nerve rings appear to be nonexistent the nerve cells form structures called rhopallia, arranged around the rim of the umbrella. Rhopalliums are typically associated with a pair of sensory pits, a balance organ for orientation, and sometimes pigment-cupped ocelli, or “eye spots.”

Commonly these eye-like structures are found in the medusa stage, even though polyps from all cnidarian classes are defined as light-sensitive. Photoreceptors of jellyfish are classified as the ciliary type, meaning one or more adapted cilia form the photoreceptive structure. Rhabdomeric photoreceptors are found in other invertebrate groups, whereas ciliary types are normally found in vertebrate eyes. Therefore, the photoreceptors of cnidarians may belong to the same evolutionary line as those of vertebrates. Extra ocular photosensitivity is prevalent throughout the cnidarians, with neurons, epithelial cells, and muscle cells facilitating light detection. /=\

Jellyfish Movement

jellyfish locomotion

One of the most observable behaviors of many species of jellyfish is the pulsation of the swimming bell. Pulsations are coordinated by nerve centers along the outside of the bell. Most jellyfish have groups of cells able to contract in length and are considered simple muscles. Ones on the underside of the bodies can contract but not expand, allowing it to swim. Constant movements also facilitate oxygen exchange, which occurs over the jelly's entire body surface.

Regardless of their means of propulsion, most jellyfish just drift with the current. Most large jellyfish spend their time swimming about freely. Some smaller ones spend much of their lives living as attached polyps. Some comb jellies have comb-like paddles comprised of cilia.

Scientists are interested in the jellyfish’s remarkable swimming ability, which, according to AFP, comes from muscles that open its bell-like body and then contract it, thus ejecting water and driving the creature along. This pump-like design has been honed by more than 500 million years of evolution to be as simple and energy-efficient as possible. Scientists have even copied jellyfish movement to come up with new ways to treat human heart disease. Kevin Kit Parker, a professor of bio-engineering at the Harvard School of Engineering and Applied Sciences, even invented a robotic jellyfish with the aim using similar technology to one day help heart patients. "I started looking at marine organisms that pump to survive. Then I saw a jellyfish at the New England Aquarium, and I immediately noted both similarities and differences between how the jellyfish and the human heart pump." A paper on the subject was published in the journal Nature Biotechnology in July 2012. [Source: AFP, July 23 2012]

Jellyfish Feeding, Stingers and Toxins

Most jellyfish are predators that feed on zooplankton, small crustaceans and small fish. Some rely on stealth and the fact they produce no bow wave to sneak up on prey. Most filter food passively and just wait and eat whatever floats their way within their grasp. Many jellyfish have mucous lures that look like larvae. They attract small fish or crustaceans that are zapped with toxins from tentacles and moved to the digestive cavity.

jellyfish mematocyst discharge

According to the University of Hawaii: Cnidarians can capture and digest animals ranging in size from small plankton to rather large organisms, such as small reef fish. After a cnidarian stings and captures its prey, the tentacles work together to move it into the mouth. The prey is swallowed into the gastrovascular cavity and digested. Endoderm cells produce digestive chemicals called enzymes, which break the chunks of food into tiny particles. The cells then engulf the particles and further digest them into nutrient molecules. The cnidarian expels indigestible material through the mouth, which serves the gastrovascular cavity as both entrance and exit. [Source: University of Hawaii]

Jellyfish can quickly convert food into mass. When food supplies are plentiful they can regenerate damaged parts and grow in size and number. When food is scarce they can shrink in size and not reproduce. In many places, jellyfish are their chief predator, consuming more krill and crustaceans than whales and large fish. A single saucer-size sea nettle can consume as many as 18,700 copepods a day. Copepods are flealike seas creatures and one of the most abundant animals on earth.

According to Animal Diversity Web: Moon jellyfish feeds on small zooplankton such as molluscs, crustaceans, fish eggs, and other small jellies. In a gut sampling study, they primarily selected for crustacean prey and their diet is dominated by whatever prey type is abundant, adjusting to the availability of given food types. Plankton is caught on the mucus lining the bell of the jelly. It is moved by ciliary action to the bell margin, where the short fringe of tentacles helps funnel the food into the manubrium and the four horseshoe-shaped stomach pouches at the top center of the bell. Source: Chelsea Macentimetersullan, Animal Diversity Web (ADW)]

All true jellyfish sting with tinging cells called nematocysts, which are there to deter potential predators and help catch prey. The venom, which humans only feel if it penetrates their skin, is injected with millions of microscopic barbs, coiled like springs and fired by touch or chemicals. The toxin immobilizes prey by disrupting the transmission of information between the synapses of the prey's nerve cells. The prey is digested by cells in a primitive stomach. When used as protection from predators, the toxin is automatically discharged when jellyfish bump into something warm — like another sea creature — from poison-containing stingers in mantles, arms or long threadlike tendrils which can grow to one meters long.

Jellyfish Don't Have Brains, But They Do Sleep

In September 2017, scientists reported in a study, published in the journal Current Biology that even though jellyfish don't have brains, they do sleep — making them the first-ever animals with no central nervous system to have been observed sleeping. That finding gives credence to the theory that sleep is an emergent property of neurons — in other words, sleep might be something that nerve cells connected in a network just do, even without complex organization. "The real novelty of what we've shown is that this animal that is almost as far away, evolutionarily, from humans and higher animals as you can go, also seems to have this conserved behavioral state" of sleep, said study co-author Claire Bedbrook, a doctoral student in bioengineering at the California Institute of Technology. [Source: Stephanie Pappas, Livescience, September 22, 2017]

Stephanie Pappas wrote in Live Science: Another Caltech graduate student, Michael Abrams, happened to be cultivating jellyfish at the same time for an entirely unrelated project. He noticed that one genus, Cassiopea, or the upside-down jellyfish, seemed to become less active at night. Cassiopea spends the vast majority of its time sitting upside down on the ocean or tank floor, pulsing its bell about once a second, Abrams told Live Science. This sedentary behavior makes the upside-down jellyfish an easy animal to track behaviorally.

Abrams and Ravi Nath, a Caltech graduate student and a co-author of the jellyfish study, joined forces with Bedbrook to investigate just what the jellies were doing. They knew that to show that the jellyfish were sleeping, they'd have to prove that their behavior met the standard criteria for sleep: decreased activity that is rapidly reversible, unlike a coma or unconsciousness; reduced responsiveness to stimuli compared to a waking state; and homeostatic regulation, meaning there is some sort of innate "drive" toward sleep and that the animal needs sleep to function.

To measure activity, the researchers counted the rate of the bell's pulsation in 23 jellyfish for six straight days and nights. They found that the rate dropped by 32 percent at night, going from about 1,155 pulses per 20 minutes during the day to 781 pulses per 20 minutes at night. When the researchers put a little midnight snack in the water column, the jellies perked up and started pulsing at daytime rates, indicating that this quiescent period was easily reversible.

But were the jellyfish less responsive than usual? To find out, the researchers put the jellyfish into small containers made of PVC pipe with a mesh bottom. They raised the jellies gently up from the bottom of the tank, then rapidly yanked the container downward, leaving the jellyfish suspended in the water.

giant jellyfish off Japan
Next, the researchers tested whether the sleepy behavior in jellyfish was under homeostatic control. Put more simply, the question was: Would jellies act tired the next day if they were deprived of their quiescence at night? To find out, the researchers blew gentle pulses of water at the jellies for 10 seconds every 20 minutes. They found that when they disturbed the jellyfish this way during the last 6 hours of the night, the jellyfish showed a 12 percent decline in pulsing in the first 4 hours of the next day, as if they were having trouble waking up. When the researchers continued the disturbances all night, the jellyfish were 17 percent less active over the entire next day. After a full night without any disturbances, the jellyfish returned to normal activity levels the following day.

The researchers weren't able to look for genes and molecules related to sleep in this study, but they did dose the jellyfish's water with melatonin and the antihistamine pyrilamine, two substances that make humans drowsy. The jellyfish, too, became less active in the presence of these substances, suggesting that the sleep state in the oldest known animals and in humans might have the same biological roots. The next step, Nath said, might be to use electrodes to track the activity of the jellyfish's neurons during the sleep-like state. "We'd love to see whether there are other species of jellyfish that also sleep," Bedbrook added. "We would also like to see whether or not sponges, the next level down, sleep." Sponges don't have nervous systems at all, though they do possess some of the rudimentary genes and proteins found in other animals' nervous systems.

Jellyfish Reproduction

Jellyfish are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They are dioecious (male and female reproductive organs are in separate individuals) rather than hermaphrodites and engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body. There is pre-fertilization provisioning but no parental involvement after spawning.[Source: Stephanie Braccini, Animal Diversity Web (ADW) /=]

Jellyfish employ both sexual and asexual reproduction and engage in internal reproduction in which sperm from the male fertilizes the egg within the female. They engage in seasonal breeding and employ broadcast (group) spawning, the main mode of reproduction in the sea. It involves the release of both eggs and sperm into the water and contact between sperm and egg and fertilization occur externally. There is no parental involvement in the raising of young. Jellyfish are altricial. This means that young are born relatively underdeveloped and are unable to feed or care for themselves or move independently for a period of time after birth. /=\

Many jellyfish species reproduce by budding, and by releasing eggs and sperm in the sea. A typical jellyfish releases sperm and eggs into the water during a specific time or when stressed. The fertilized eggs multiply into "balls of cells" called planulae that settle onto a hard surface like a rock and then grow into a polyp. The polyp reproduces asexually into several creeping polyps which later metamorphose into adult jellyfish (called medusas) in a 12-day period.

In their polyp form, sea nettles reproduces asexually. This is done through a variety of ways: strobilation, cyst production, and by changing polyp position through the use of stolons. Medusae are able to reproduce sexually. Females catch the sperm released into the water from the mouths of the males. The eggs, which are also held in the mouth, become fertilized, and remain attached to the female's oral arms. As the fertilized eggs develop, they grow into planula. These planula have a flattened, bean shape. Once the polyps develop fully into flower-shaped progeny, they are released into the ocean where they settle, and begin asexual reproduction. The polyp buds to produce identical copies of themselves, and eventually detach to be released into the ocean where it will undergo metamorphosis to the medusa stage.

Jellyfish Development

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developmental stages of scyphozoan jellyfish's life cycle:
1) Larva searches for site; 2) Polyp grows;
3) Polyp strobilates and 4) the Medusa grows
Jellyfish development is characterized by metamorphosis, colonialism (living together in groups or in close proximity to each other) and indeterminate growth. (they continue growing throughout their lives). Fertilized eggs do not develop into miniature jellyfish but rather free-swimming creatures, quite different from their parents, that settle on a surface and develop onto a flower-like organism called a polyp (coral is also a polyp). The polyps filter feed using cilia. The polyps eventually bud and produce miniature medusas that break away from a surface and begin swimming freely in the sea. Jellyfish budding can result in the blooming of thousands of new individuals.

The life of jellyfish are dominated by two main cycles, each with a distinct body plan. First, the jellyfish live as a sessile (fixed in one place) polyp, then as a mobile medusa. The polyp stage is characterized by strobilation, in which the segmented polyp asexually produces young medusa. The medusa is the second stage of the life cycle. [Source: Nate Lanier and Alexi Weber, Animal Diversity Web (ADW) /=]

Sea nettles generally have a stalk only millimeters long during the first, (polypoid) stage, Tentacles facilitate feeding. The polyp may either remain sessile, resembling coral and sea anemones, or it may be free-floating. Due to the polyp’s ability to bud asexually, it can either remain solitary or be colonial (living together in groups or in close proximity to each other). Polyp strobilation, or budding, may lead to the appearance of ephyra, which are small, immature jellies. /=\

From ephyra to adult medusa, Sea nettles have six different stages. These stages are categorized by the change in morphological structure. Two stages involve the growth of the ephyra, while the other four stages are for medusa development. The first four stages seen in species growth have been reproduced in the laboratory, while the last two stages have been recorded from nature. Stage I consists of newly-liberated ephyra, from polyps, which average between two to three and a half millimeters wide from lappet-tip to lappet-tip across the tiny medusa. Stage II is characterized by the presence of primary tentacles, and the development of the oral arms. As the medusa enters stage III, the lappets tend to fold under the medusa, thereby reducing its resemblance to its ephyra stage. Stage IV development is noted by the appearance of secondary tentacles between the primary tentacles. Stage V introduces the growth of 16 tertiary tentacles in the medusa. At this point there are 40 tentacles, and 48 lappets. The last stage, stage VI, is when the medusa has grown to a size of seven or more tentacles and eight or more lappets per octant. As previously mentioned, these last two stages have not been successfully reproduced in the lab, but research has shown that tentacle numbers in adult medusa vary, and are not a dependable taxonomic character in this group of Schyphozoa. /=\

Jellyfish as a Model of Efficiency

John Dabiri, an assistant professor of aeronautics and bioengineering at Caltech and MacArthur Award winner, is fascinated by jellyfish. He believes jellyfish propulsion can inform engineering, which in turn can inform efficiency in wave and wind technology. On how jellyfish move Dabiri told the Los Angeles Times, “For a long time, people thought of jellyfish swimming as like jet propulsion — like a rocket that shoots out exhaust and goes the other way. But it's a bit more subtle than that. They create vortex rings, like the smoke rings you might create with a cigar. And those doughnut-shaped swirls of water are an efficient way of propulsion because [the animals] can basically push off of those doughnuts of water...What we wanted to understand was how do they form these swirling currents, and whether then we could build underwater vehicles that could also create these same type of water currents while they propelled themselves. [Source: Lori Kozlowski, Los Angeles Times, November 6, 2010]

These vortex rings show up in other animals. So you could have picked a trout, let's say, or a shark. They have more complicated wake flow patterns: The shape of their fins and the way they move is just more complicated. With a trout or a shark, as it's flapping its tail, it's creating these vortex rings, but they are sort of linked up into more complicated chains. So if you were to do that dye experiment — if you could do it with a shark — it would be messier, and harder to interpret what you were seeing.

When asked what species he studied, Dabiri said, “The moon jelly — that's the most common one. You see them in the aquarium; they are sort of white-colored. They don't sting humans very much, and they're very plentiful, so it's pretty easy to find them. Then there's one called the lion's mane — which has a reddish color with really long tentacles. Those are the two main species we looked at because they are easy to access and the sting isn't horrible.

On the applications of what has been learned about jellyfish motions Dabiri said,”There is another technology out there for wind energy generation — instead of using these large wind turbine structures, they rotate around a vertical axis. They are smaller structures, so they are maybe 30 feet tall instead of 300 feet. We've been interested in how many of these smaller structures could be situated very close together in order to generate as much power as you get from the very large ones. We were able to learn something about this from how fish school.

Image Source: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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