This story appears in the October 2018 issue of National Geographic magazine.

Moon jellies, which are found in shallow bays around the world, look like small, not entirely friendly ghosts.

They have translucent bells fringed with pale tentacles, and as they pulse along, it almost seems as if the water itself has come alive. At the National Aquarium in Baltimore, when visitors are invited to touch moon jellies, their first reaction is usually fear. Assured the jellies won’t hurt them, the visitors roll up their sleeves and hesitantly reach into the tank.

“They’re squishy!” I hear one boy squeal.

“They’re cool!” a girl exclaims.

“I think they’re just mesmerizing,” Jennie Janssen, the assistant curator who oversees the care of the aquarium’s jellyfish, tells me. “They don’t have a brain, and yet they’re able to survive—to thrive—generation after generation.”

View Images Flower hat jellies exemplify the paradox of the medusas, or bell-shaped jellyfish: They’re both delicate and menacing. Sitting on the seafloor, waving colorful tentacles, they lure fish, sting them, and eat them. Olindias formosus, 3.9 inches across ALL JELLIES IN THIS ARTICLE WERE PHOTOGRAPHED IN A KREISEL TANK—A CYLINDRICAL TANK WITH FLAT FRONT AND BACK—AT KAMO AQUARIUM, TSURUOKA, JAPAN, UNLESS STATED OTHERWISE.

View Images Humans eat cannonball jellyfish mostly in Asia, in such dishes as “rubber band salad.” Endangered leatherback turtles devour them too. Like other jellies, cannonballs swim by contracting their bell, which in their case is relatively muscular. These specimens, however, are aging: The bells are a bit caved in. Fishing boats have started targeting cannonball jellies off the Americas as well. Stomolophus meleagris, 2.8 inches across

Jellyfish 101 How much do you really know about jellyfish?

Scary, squishy, cool, brainless, mesmerizing—jellyfish are all of these and a whole lot more. Anatomically they’re relatively simple animals; they lack not just brains, but also blood and bones, and possess only rudimentary sense organs. Despite their name, jellyfish aren’t, of course, fish. In fact they aren’t any one thing.

Many of the creatures lumped together as jellyfish are no more closely related than, say, horseflies are to horses. Not only do they occupy disparate branches of the animal family tree, but they also live in different habitats; some like the ocean surface, others the depths, and a few prefer freshwater. What unites them is that they’ve converged on a similarly successful strategy for floating through life: Their bodies are gelatinous.

View Images True jellies in youth are boring stick-to-the-rock polyps, like their relatives the corals. They reproduce asexually by strobilation, spinning off tiny snowflake-like clones. The clones float off and grow into the tentacled medusas we know and sometimes hate—which proceed to have sex afloat and rain future larval polyps onto the seafloor. Overall then, not so boring. The polyps here are moon jellies, a common type. Aurelia coerulea

Snowflake-like clone is about 0.1 inches across.

View Images Aurelia aurita. Moon jellies swirl in a tank at Japan’s Kamo Aquarium. Popular with professional and home aquarists, moon jellies have nematocysts, or stinging cells, but they’re so small they don’t hurt humans.

View Images Moon jellies, found all over the world, are named for their otherworldly, translucent bells. The fringe of hairlike cilia sweeps food—mostly plankton—toward their mouths. The jellies change color depending on what they eat. Aurelia sp.

Biggest in photo is 6.7 inches across.

Not surprisingly, given their diverse evolutionary history, jellies exhibit a fantastic range of shapes, sizes, and behaviors. When it comes to reproduction, they’re some of the most versatile creatures on the planet. Jellyfish can produce offspring both sexually and asexually; depending on the species, they may be able to create copies of themselves by dividing in two, or laying down little pods of cells, or spinning off tiny snowflake-shaped clones in a process known as strobilation. Most astonishing of all, some jellies seem able to reproduce from beyond the grave.

The so-called immortal jellyfish resembles a tiny, hairy thimble and lives in the Mediterranean Sea and also off Japan. Members of the species can reverse the aging process so that instead of expiring, they reconstitute themselves as juveniles. The juvenile then starts the jellyfish’s life cycle all over again. It’s as if a frog, say, were to revert to a tadpole or a butterfly to a caterpillar. Scientists call the near-miraculous process transdifferentiation.

JELLYFISH INVASION? Some scientists are concerned that climate change and overfishing are changing the oceans in favor of jellyfish. But it’s unclear whether observed increases in jellies reflect actual ones or just better reporting. Limited knowledge Scientists often must rely on sporadic obser- vations during blooms—a natural part of many jellies’ life cycles—making them less confident in their findings. Observed jellyfish population High confidence Low confidence INCREASING POPULATIONS PACIFIC OCEAN ATLANTIC OCEAN INDIAN OCEAN STABLE POPULATIONS DECREASING POPULATIONS SOREN WALLJASPER, NGM STAFF; LAWSON PARKER SOURCE: LUCAS BROTZ AND OTHERS, HYDROBIOLOGIA, 2012 JELLYFISH INVASION? Some scientists are concerned that climate change and overfishing are changing the oceans in favor of jellyfish. But it’s unclear whether observed increases in jellies reflect actual ones or just better reporting. SWEDEN SWEDEN Black Sea SPAIN ASIA ITALY EUROPE NORTH AMERICA SPAIN ITALY JAPAN PACIFIC Mediterranean Sea ATLANTIC AFRICA OCEAN EQUATOR SOUTH AMERICA OCEAN INDIAN OCEAN INDIAN OCEAN Limited knowledge Scientists often must rely on sporadic observations during blooms—a natural part of many jellies’ life cycles—making them less confident in their findings. AUSTRALIA Observed Jellyfish Populations Increase Decrease Stable High confidence Low confidence SOREN WALLJASPER, NGM STAFF; LAWSON PARKER SOURCE: LUCAS BROTZ and others, HYDROBIOLOGIA, 2012

Boneless Beauties Being 95 percent water and gelatinous is a good strategy on an ocean planet, which is why jellyfish have survived for hundreds of millions of years. The term covers thousands of species in two barely related categories: the comb jellies and the medusozoans, such as the Atlantic bay nettle, featured at bottom. It’s a familiar menace to swimmers in the Chesapeake Bay. FAMILY TREE ANTHOZOA (sea anemones, corals, sea pens) ENDOCNIDOZOA (parasites) Cnidaria Medusozoa Staurozoa (stalked jellyfish) Cubozoa (box jellies) Scyphozoa (true jellies) Hydrozoa (most species remain in polyp form) Ctenophora (comb jellies) Some 2,000 species are known to have a medusa (swimming) phase of their life cycle. A sample of their diversity is shown here. Lion’s mane jellyfish Cyanea capillata The largest of this cold-water species live in Arctic waters. 120 ft long 120 ft long 3 ft Nomura’s jellyfish Nemopilema nomurai Swarms of these gigantic jellies have massed in Japanese waters. 440 lb 6 ft Sea wasp Chironex fleckeri This box jelly’s venom can kill a human in less than five minutes. 1 ft Sea fur Obelia sp. One of the smallest jellies of all is typically found in shallow waters. 0.25 in Lucernaria janetae These stalked jellies live in the depths, attached to rocks. 4 in Atlantic bay nettle Chrysaora chesapeakei This jelly thrives in brackish waters along the U.S. East Coast and Mexico. 7 in Shown below HOW THEY FEED Atlantic bay nettles only swim to find food. Currents help bring them in contact with their prey: zooplankton, crustaceans, and even other jellies. Prey 1 Water vortex Bell Relax rim Contract rim Mesoglea (jellylike substance) 5 Ammonia Stomach 24 TENTACLES Epidermis Nutrients Gonads 2 3 Gastrodermis Waste Mouth 4 ORAL ARMS 4 Stomach pouch 1 Swimming Like other jellies, the Atlantic bay nettle hunts not by sight but by direct contact, spreading its oral arms and tentacles wide. Vortices help draw in prey. Rowers Atlantic bay nettles and many other large medusozoans contract the rims of their bells, generating vortices as they swim with a rowing motion. Jetters Many smaller medusozoans contract the entire bell, expelling a jet of water that propels them forward. Relax bell Contract bell Water in Water out 2 Stinging Millions of stinging cells (A) line the tentacles and arms. Some have coiled, barbed tubules that, when triggered, (B) uncoil with enough force to pierce shells or flesh to (C) inject venom. Stinging cell Trigger PREY Venom Tubule A C B TENTACLE OR ARM Small prey Zooplankton Big prey Mnemiopsis comb jelly 3 Ingesting Once the prey is caught, hairlike cilia on the oral arms help convey it toward the mouth. If it fits, it’s swallowed in minutes; larger prey can take hours to ingest. Cilia The oral arms can begin dissolving and digesting larger prey as they move it toward the jelly’s mouth. 4 Digesting A central cavity acts as both stomach and intestine to churn and break down food. Sixteen stomach pouches transport nutrients throughout the bell. 5 Removing waste Waste (mostly ammonia) is secreted through tissue or excreted from the single orifice that serves the jelly as both mouth and anus. Ammonia Stomach Nutrients Waste Mouth Stomach pouch THE MAKING OF A MEDUSA Gonads look the same in both sexes, producing sperm in males and eggs in females. Mature medusas release sperm or up to 40,000 eggs a day into the water. Egg Sperm Fertilized eggs grow into larvae that attach to a hard surface. Larvae The larvae become stationary polyps, waiting days or years to reproduce. Polyps grow and reproduce asexually, budding off clones. The clones develop rapidly into adult medusas FERNANDO G. BAPTISTA, EVE CONANT, Katherine Appel, and Alison Harford, NGM STAFF; LAWSON PARKER SOURCES: KEITH BAYHA, REBECCA HELM, AND ALLEN COLLINS, SMITHSONIAN NATIONAL MUSEUM OF NATURAL HISTORY; JENNIE JANSSEN, NATIONAL AQUARIUM Boneless Beauties Being 95 percent water and gelatinous is a good strategy on an ocean planet, which is why jellyfish have survived for hundreds of millions of years. The term covers thousands of species in two barely related categories: the comb jellies and the medusozoans, such as the Atlantic bay nettle, featured at bottom. It’s a familiar menace to swimmers in the Chesapeake Bay. FAMILY TREE ANTHOZOA (sea anemones, corals, sea pens) ENDOCNIDOZOA (parasites) Cnidaria Medusozoa Staurozoa (stalked jellyfish) Cubozoa (box jellies) Scyphozoa (true jellies) Hydrozoa (most species remain in polyp form) Ctenophora (comb jellies) Some 2,000 species are known to have a medusa (swimming) phase of their life cycle. A sample of their diversity is shown here. Lion’s mane jellyfish Cyanea capillata 120 ft long 3 ft The largest of this cold-water species live in Arctic waters. 440 lb Nomura’s jellyfish Nemopilema nomurai Swarms of these gigantic jellies have massed in Japanese waters. 6 ft Sea wasp Chironex fleckeri 1 ft This box jelly’s venom can kill a human in less than five minutes. Sea fur Obelia sp. 0.25 in One of the smallest jellies of all is typically found in shallow waters. Lucernaria janetae These stalked jellies live in the depths, attached to rocks. 4 in Atlantic bay nettle Chrysaora chesapeakei 7 in This jelly thrives in brackish waters along the U.S. East Coast and Mexico. Shown below 1 HOW THEY FEED Swimming Like other jellies, the Atlantic bay nettle hunts not by sight but by direct contact, spreading its oral arms and tentacles wide. Vortices help draw in prey. Atlantic bay nettles only swim to find food. Currents help bring them in contact with their prey: zooplankton, crustaceans, and even other jellies. Rowers Atlantic bay nettles and many other large medusozoans contract the rims of their bells, generating vortices as they swim with a rowing motion. Atlantic bay nettle Prey Water vortex Bell Contract rim Relax rim Relax bell Contract bell Water in Water out Jetters Many smaller medusozoans contract the entire bell, expelling a jet of water that propels them forward. 2 3 4 5 Stinging Ingesting Digesting Removing waste Millions of stinging cells (A) line the tentacles and arms. Some have coiled, barbed tubules that, when triggered, (B) uncoil with enough force to pierce shells or flesh to (C) inject venom. Once the prey is caught, hairlike cilia on the oral arms help convey it toward the mouth. If it fits, it’s swallowed in minutes; larger prey can take hours to ingest. A central cavity acts as both stomach and intestine to churn and break down food. Sixteen stomach pouches transport nutrients throughout the bell. Waste (mostly ammonia) is secreted through tissue or excreted from the single orifice that serves the jelly as both mouth and anus. Stinging cell Ammonia Trigger PREY Epidermis Gonads Venom Nutrients Mesoglea (jellylike substance) Tubule 24 TENTACLES 4 ORAL ARMS Gastrodermis A C B Cilia Stomach Mouth TENTACLE OR ARM Nutrients Waste Small prey Zooplankton Sight and signaling The oral arms can begin dissolving and digesting larger prey as they move it toward the jelly’s mouth. Rhopalia, sensory structures that sense light and gravity, trade data with nerves on the bell. This exchange drives jelly behavior. Big prey Mnemiopsis comb jelly Stomach pouch THE MAKING OF A MEDUSA Larvae Sperm Egg Gonads look the same in both sexes, producing sperm in males and eggs in females. Mature medusas release sperm or up to 40,000 eggs a day into the water. Fertilized eggs grow into larvae that attach to a hard surface. The larvae become stationary polyps, waiting days or years to reproduce. Polyps grow and reproduce asexually, budding off clones. The clones develop rapidly into adult medusas. FERNANDO G. BAPTISTA, EVE CONANT, Katherine Appel, and Alison Harford, NGM STAFF; LAWSON PARKER SOURCES: KEITH BAYHA, REBECCA HELM, AND ALLEN COLLINS, SMITHSONIAN NATIONAL MUSEUM OF NATURAL HISTORY; JENNIE JANSSEN, NATIONAL AQUARIUM

Moon jellies and their cousins, which include lion’s mane jellies and sea nettles, are known as true jellies. They belong to the class Scyphozoa, in the phylum Cnidaria, which also includes corals. (A phylum is such a broad taxonomic category that humans, fish, snakes, frogs, and all other animals with a backbone belong to the same one—the chordates—as do salps, which are sometimes lumped with jellies.) As adults, true jellies are shaped like upside-down saucers or billowing parachutes. They propel themselves through the water by contracting the muscles of their bells, and their tentacles are equipped with stinging cells that shoot out tiny barbed tubes to harpoon floating prey. To reel it into their mouths, they use streamer-like appendages known as oral arms. In some species the oral arms have mouths of their own.

Jellyfish like the dreaded Portuguese man-of-war are also related to corals, but they’re part of a different subgroup, the siphonophores, which practice an unusual form of collective living. What looks like a single man-of-war is technically a colony that developed from the same embryo. Instead of simply growing larger, the embryo sprouts new “bodies,” which take on different functions. Some develop into tentacles, for example; others become reproductive organs.

View Images Comb jellies may be the most ancient living animal. They have a nervous system and—this shocked specialists—two tiny anuses. They aren’t related to true jellies, which poop out of their mouths. Comb jellies’ eight rows of comblike cilia act as paddles. In the 1980s a rapidly multiplying species decimated Black Sea fisheries. Beroe abyssicola, 4.7 inches long

“In the human life cycle, our body, when we’re born, has all the pieces that are going to be there as an adult,” observes Casey Dunn, a professor of evolutionary biology at Yale University. “The really cool thing about siphonophores is they’ve gone about things in a very different way.”

Then there are the ctenophores (pronounced TEH-nuh-fores), which are such oddballs they’ve been placed in a phylum of their own. Also known as comb jellies, for the comblike rows of tiny paddles they use to swim, they tend to be small, delicate, and hard to study. They come in an array of weird body types: Some are flat and ribbonlike; others look more like pockets or little crowns. Most use an adhesive to nab their prey. “They have what’s like exploding glue packets embedded in their tentacles,” explains Steve Haddock, a senior scientist at the Monterey Bay Aquarium Research Institute.

View Images Cassiopea andromeda. Aptly named upside-down jellyfish lie with their bells on the seafloor in shallow waters and their arms waving upward. In effect they’re farmers. Tiny photosynthesizing organisms housed in the jellies’ tissues give them their earthy colors and also much of their food. The biggest here is 1.7 inches across. PHOTOGRAPH BY DAVID LIITTSCHWAGER AND ZACH KOBRINSKY

PHOTOGRAPHED AT COUNCIL ON INTERNATIONAL EDUCATIONAL EXCHANGE RESEARCH STATION BONAIRE PHOTOGRAPHED AT COUNCIL ON INTERNATIONAL EDUCATIONAL EXCHANGE RESEARCH STATION BONAIRE

View Images Spotted jellies, which drift in South Pacific bays and lagoons, swim up during the day so that tiny plantlike organisms that live inside them and nourish them can catch the sun. The jellies don’t live off those symbionts alone, however. Their feathery arms are lined with stinging cells and mini-mouths that gobble animal plankton. Mastigias papua

Biggest in photo is just over 3 inches across.

In recent decades jellyfish populations in some parts of the world have boomed. In the 1980s a comb jelly that’s known formally as Mnemiopsis leidyi and informally as the sea walnut showed up in the Black Sea. A native of the western Atlantic, it presumably had been transported in a ship’s ballast water and then been discharged. In the Black Sea it reproduced so prolifically that by 1989 it had reached densities of up to 11 per cubic foot of water. Fish couldn’t compete with the jellies for food—sea walnuts eat as much as 10 times their body weight a day—and many fish became food for the jellies. Local fisheries collapsed.

In other parts of the world, swarms of jellyfish have menaced swimmers and clogged fishing nets. In 2006, beaches in Italy and Spain were closed because of a bloom of jellyfish known as mauve stingers. In 2013 a Swedish nuclear plant temporarily shut down because moon jellies were blocking its intake pipes.

Situations like these led to a spate of reports that jellyfish were taking over the seas. One website warned of the “attack of the blob.” Another predicted “goomageddon.”

But scientists say the situation is more complicated than such headlines suggest. Jellyfish populations fluctuate naturally, and people tend to notice only the boom part of the cycle.

View Images Hydrozoans are in a class apart from true jellies. Some species never grow up; they remain branching colonies of polyps. Nemalecium lighti

0.5 inches tall PHOTOGRAPHED AT COUNCIL ON INTERNATIONAL EDUCATIONAL EXCHANGE RESEARCH STATION BONAIRE

View Images Other hydrozoans form medusas and resemble true jellies. Vallentinia gabriellae

0.4 inches across (bell) PHOTOGRAPHED AT SMITHSONIAN MARINE STATION, FORT PIERCE, FLORIDA

View Images And one hydrozoan, the immortal jellyfish (Turritopsis dohrnii), seen here in a water drop, has a life cycle that skips death: It reverts from a medusa back to a polyp. How, no one knows. 0.03 inches across (bell) PHOTOGRAPHED AT SMITHSONIAN MARINE STATION, FORT PIERCE, FLORIDA

“A big jellyfish bloom makes the headlines, while a lack of a jellyfish bloom isn’t even worth reporting,” says Lucas Brotz, a marine zoologist at the University of British Columbia. While some jellyfish species seem to thrive on human disturbance—off the coast of Namibia, for example, overfishing may have tipped the ecosystem into a new state dominated by compass and crystal jellyfish—other more finicky species appear to be declining. Researchers in a couple parts of the world have reported a drop in the number of jellyfish species they are encountering.

Meanwhile, if people are having more unpleasant encounters with jellyfish, is it because they’re taking over the seas or because we are?

“Anytime we have an adverse encounter with jellyfish, it’s because humans have invaded the oceans,” Haddock says. “We’re the ones who are encroaching into their habitat.” Jellyfish are only doing what they’ve been doing generation after generation for hundreds of millions of years—just pulsing along, silently, brainlessly, and, seen in the right light, gorgeously.

View Images Worldwide more than 825,000 tons of jellyfish are harvested each year for human consumption. Jellies like this one, of the genus Rhopilema, make up nearly a third of the total. The sturdy arms are packed with nematocysts, cells that blast spiral barbed tubes laced with venom at victims—fishermen, for example. But eating these jellies dried or cooked is safe. Rhopilema sp., 2 inches across