Jellyfish are classified under the phylum Cnidaria, a group that also includes corals, sea anemones, and hydroids. Within this phylum, most jellyfish belong to the class Scyphozoa, which includes the so-called “true jellyfish.” Other jellyfish-like creatures fall under classes such as Cubozoa (box jellyfish) and Hydrozoa, but all share a similar body structure and possess specialized stinging cells called cnidocytes.
Jellyfish are some of the oldest living animals on Earth, with fossils dating back over 500 million years. They have survived multiple mass extinctions and remain highly successful despite their simple anatomy. Unlike most animals, jellyfish lack bones, a heart, or even a brain. Instead, they rely on a simple nerve net to detect changes in their environment and control their movements.
A jellyfish’s body is typically composed of a gelatinous bell and trailing tentacles. The bell contracts rhythmically to help the jellyfish move through water, while the tentacles are equipped with stinging cells used to capture prey. These stings can range from harmless to extremely painful or even lethal, depending on the species.
Jellyfish are found in all of the world’s oceans, from surface waters to deep-sea trenches. Some species prefer warm, coastal environments, while others thrive in the cold, dark depths. Certain species, such as the moon jellyfish, are commonly seen near beaches, while deep-sea species like Deepstaria enigmatica are rarely observed.
Their diet consists primarily of plankton, small fish, and other jellyfish. They use their stinging tentacles to immobilize prey, which is then transported to the mouth by oral arms. Some jellyfish, like the upside-down jellyfish, also rely on symbiotic algae living in their tissues, which provide nutrients through photosynthesis.
Jellyfish play important roles in marine ecosystems. They serve as food for sea turtles, sunfish, and other marine animals. At the same time, jellyfish blooms—large swarms that can appear suddenly—can disrupt fisheries, clog power plant intakes, and upset ecological balances. These blooms are sometimes linked to overfishing and climate change.
In recent years, jellyfish have become subjects of scientific interest and public fascination. Species like the Turritopsis dohrnii, known for its ability to revert to an earlier life stage, challenge our understanding of aging. Others, like the crystal jellyfish, have contributed fluorescent proteins used in medical research. Despite their simplicity, jellyfish continue to surprise scientists with their resilience and biological complexity.
Species of Jellyfish
Moon Jellyfish (Aurelia aurita)
Moon jellyfish are among the most recognizable jellyfish due to their translucent, saucer-shaped bell with four horseshoe-shaped gonads visible through the top. They are typically found in coastal waters around the world and are harmless to humans, with only mild stings. They feed mainly on plankton and are often seen drifting gently with the current.
Lion’s Mane Jellyfish (Cyanea capillata)
This is the largest known species of jellyfish, with tentacles that can reach over 100 feet long. It has a reddish or yellow bell with long, trailing stingers. Found in cold northern waters of the Atlantic and Arctic, the Lion’s Mane delivers a painful sting but is not usually deadly. It plays a role in marine food webs by consuming small fish and zooplankton.
Box Jellyfish (Chironex fleckeri)
Box jellyfish, particularly the species Chironex fleckeri, are infamous for their extremely potent venom. They have a box-like, transparent body and long, ribbon-like tentacles. Native to the Indo-Pacific region, their sting can cause cardiac arrest in humans within minutes. Despite their danger, they are an important part of tropical marine ecosystems.
Purple Striped Jellyfish (Chrysaora colorata)
Also called the purple-striped jelly, this species has a pale bell marked with vivid purple bands and long tentacles. It is found in the Pacific Ocean, especially along the California coast. Though beautiful, its sting can irritate human skin. It preys on zooplankton, small fish, and other jellyfish.
Cannonball Jellyfish (Stomolophus meleagris)
Named for its round, compact bell that resembles a cannonball, this jellyfish is common in the Gulf of Mexico and southeastern U.S. waters. It’s relatively harmless to humans and often harvested for food in Asia. Cannonball jellyfish play a role in marine food chains and are sometimes prey for sea turtles.
Crystal Jellyfish (Aequorea victoria)
This small, bioluminescent jellyfish is completely transparent and is notable for its green fluorescent protein (GFP), widely used in genetic and biomedical research. Native to the Pacific Northwest, it produces a soft glow and poses no danger to humans. Its discovery has had a significant impact on science, especially in cell biology.
Atlantic Sea Nettle (Chrysaora quinquecirrha)
The Atlantic sea nettle has a creamy white or pinkish bell with long, flowing tentacles and can be found in estuaries along the eastern U.S. coast. Its sting can be painful but is rarely dangerous. It often appears in large blooms and feeds on small fish, crustaceans, and other jellyfish.
Mauve Stinger (Pelagia noctiluca)
This jellyfish is known for its vibrant purple hue and its ability to glow in the dark (bioluminescence). It’s found mostly in the Mediterranean and Atlantic oceans and is capable of delivering a painful sting. Mauve stingers often cause beach closures when they appear in large numbers during jellyfish blooms.
Fried Egg Jellyfish (Cotylorhiza tuberculata)
With a bell resembling a fried egg — yellow center with translucent edges — this Mediterranean jellyfish is often considered harmless to humans. It has a short bell and stubby tentacles with colorful appendages. It feeds on plankton and small organisms, and its mild sting is barely noticeable.
Blue Blubber Jellyfish (Catostylus mosaicus)
This jellyfish has a thick, blue or white bell with short, frilly oral arms. Common in Australian coastal waters, the blue blubber jellyfish thrives in estuaries and bays. Its sting can cause minor irritation, and it often forms massive swarms. It plays an important role in local marine ecosystems.
Nomura’s Jellyfish (Nemopilema nomurai)
One of the largest jellyfish species, Nomura’s jellyfish can weigh over 400 pounds and have bells up to 6.5 feet across. Native to the seas around China, Korea, and Japan, these jellyfish occasionally appear in massive blooms that affect fishing industries. Their sting is potent and can cause serious injury.
Upside-Down Jellyfish (Cassiopea spp.)
Unlike most jellyfish, this species rests upside-down on the ocean floor with its bell downward and tentacles upward. Found in shallow tropical waters, it hosts symbiotic algae in its tissues that photosynthesize and provide nutrients. It has a weak sting and is often found in mangrove swamps or lagoons.
Flower Hat Jellyfish (Olindias formosa)
This stunning jellyfish has a translucent bell adorned with vibrant, fluorescent tentacles that coil and pulse with color. It’s found mainly off the coasts of Japan, Brazil, and Argentina. Although small, its sting can be painful to humans. The flower hat jellyfish is a rare and mesmerizing species often displayed in public aquariums.
Comb Jelly (Ctenophora phylum)
Though not true jellyfish, comb jellies are often mistaken for them. They lack stinging cells and instead capture prey with sticky cells. Their shimmering, rainbow-like rows of cilia help them move and refract light beautifully. Found in oceans worldwide, comb jellies are harmless and fascinating for their primitive but effective biology.
Australian Spotted Jellyfish (Phyllorhiza punctata)
This jellyfish has a translucent, dome-shaped bell covered in white spots and trailing oral arms. Native to the western Pacific, it has spread to the Gulf of Mexico and Caribbean as an invasive species. Its sting is mild, but its feeding habits can disrupt local ecosystems by consuming large amounts of plankton.
Japanese Sea Nettle (Chrysaora pacifica)
This beautiful jellyfish is found in the waters of the western Pacific, especially around Japan. It has a bell with pink, red, or purplish radial stripes and long, trailing tentacles. While closely related to the Atlantic sea nettle, its sting is more potent and can cause burning pain. It’s known to form large blooms in Japanese coastal waters during summer.
Papuan Jellyfish (Mastigias papua)
Also called the spotted jelly or lagoon jelly, this species is found in Indo-Pacific lagoons. It features a dome-shaped bell covered in white spots and pulsing arms that are used for locomotion rather than stinging. It has a weak sting and contains symbiotic algae, which provide nutrients through photosynthesis, similar to coral. They are popular in aquariums for their gentle movement and colorful appearance.
Black Sea Nettle (Chrysaora achlyos)
One of the largest jellyfish in the Pacific Ocean, the Black Sea Nettle can grow to over 3 feet in diameter. It has a deep purple to black bell and long, stinging tentacles. It’s rare and usually appears only sporadically off the coast of California. Though not often seen, its size and dark coloration make it striking when it does appear.
Pink Meanie (Drymonema larsoni)
Discovered relatively recently in the Gulf of Mexico, this jellyfish is large and aggressive. It preys on other jellyfish, especially moon jellies, consuming them in large numbers. It has a pinkish bell and numerous thick, long oral arms. Though it looks menacing, it poses little threat to humans but plays a major role in controlling jellyfish populations.
Purple People Eater (Pelagia noctiluca variant)
This is a nickname often given to especially large or brightly colored specimens of the Mauve Stinger species. It is sometimes used in popular media or by beachgoers in areas where swarms occur. While the name is dramatic, this jellyfish’s sting is more irritating than dangerous, though painful enough to warrant beach closures when in bloom.
Deepstaria Jelly (Deepstaria enigmatica)
An eerie jellyfish found in deep ocean waters, Deepstaria enigmatica has a wide, translucent, bag-like bell and lacks visible tentacles. It can expand and contract its body to trap prey, making it unique among jellyfish. Little is known about its biology due to its deep-sea habitat, but its ghostly appearance has made it a subject of deep-sea exploration footage.
Firework jellyfish (Halitrephes Maasi)
This deep-sea jellyfish is sometimes called the “firework jellyfish” due to its spectacular bioluminescent display. It has a bell lined with radiating canals that glow in vivid colors when illuminated. It is found in deep tropical waters and is rarely seen by humans except through remote-operated vehicles (ROVs). It’s a visual marvel of the deep ocean.
Oh Boy” jellyfish (Tamoya Ohboya)
Nicknamed the “Oh Boy” jellyfish for the reaction it inspires when found, this species of box jellyfish is found in the Caribbean. It is rarely seen and poorly understood but is known to be venomous. It has a bell with vibrant stripes and long, trailing tentacles. Its sting has caused severe reactions in a few documented cases.
Atolla Jellyfish (Atolla wyvillei)
Also known as the crown jellyfish, Atolla jellyfish are deep-sea dwellers known for their bioluminescent alarm response. When attacked, they emit bright flashes of light to attract larger predators that might scare off their attacker. They are small, with trailing tentacles and a frilled edge, and inhabit oceans at great depths worldwide.
Deep red jellyfish (Crossota Norvegica)
This deep-sea hydrozoan jellyfish lives in Arctic and cold North Atlantic waters. Unlike many jellyfish, it lives its entire life in the medusa stage (free-swimming) and does not settle as a polyp. It is small, often red in color, and bioluminescent. It contributes to deep-sea food chains and is one of the few polar jellyfish species.
Immortal Jellyfish (Turritopsis dohrnii )
This fascinating jellyfish is known for its ability to revert its cells back to an earlier life stage, effectively avoiding death by aging. Native to the Mediterranean but now found globally, it’s tiny (about the size of a fingernail) and transparent. While it does not truly live forever in practice, its biological process of rejuvenation has fascinated scientists studying aging and regeneration.
Ghost Jellyfish (Cyanea nozakii)
Native to the waters of East Asia, particularly around China, Korea, and Japan, the Ghost Jellyfish has a large, bell-shaped body that appears milky white or translucent, giving it a ghostly appearance. It can grow quite large and is sometimes confused with the Lion’s Mane jellyfish. Though it can sting, the venom is mild compared to its relatives. It is often found in bays and coastal waters, especially during plankton blooms.
Crowned Jellyfish (Cephea cephea)
This species has a firm, semi-transparent bell with a crown-like shape and is found in the Indo-Pacific and Red Sea. It has a pinkish or purplish hue and thick oral arms. Though large and dramatic in appearance, its sting is mild. In some parts of Asia, it is harvested for food.
Barrel Jellyfish (Rhizostoma pulmo)
Common in the northeast Atlantic and Mediterranean, this large jellyfish has a thick, barrel-shaped bell and a bluish or white coloration. It can grow to the size of a basketball and has eight thick oral arms instead of long tentacles. It delivers a mild sting and is commonly seen near the coast during the warmer months.
Flame Jellyfish (Rhopilema esculentum)
This lesser-known relative of the cannonball jellyfish is often referred to as the Flame Jellyfish because of its reddish-orange hues and rippling appearance. It’s commonly found in the warm coastal waters of the Pacific, especially near Taiwan and the Philippines. Though its sting is mild, it can still irritate sensitive skin. It is sometimes harvested for jellyfish dishes in East Asian cuisine.