˚₊‧ ଳ ‧₊˚ Jellyfish˚₊‧ ଳ˚ ⋅

I absolutely adore jellyfish. Even more than I love cats (and that's saying a lot!) They are the most ethereal creatures - like whispers of the ocean, glowing softly in the depths of darkness. Their translucent bodies drift along the currents, as if they're dancing in the water. The way they move is mesmerising, and I could observe them for hours. Watching them is like watching a living piece of art moving in slow motion; they're delicate yet graceful.

Jellyfish are simple yet complex at the same time. Despite having no centralised brain, a heart, or any bones, they've thrived for over 500 million years. Their nervous system is a simple network of nerves, yet they can sense light, touch, and even hunt and avoid predators. They're truly mysterious. Because of their simple anatomy, they've fascinated scientists with their mystique.

Beyond their biology, I find them calming. They remind me to flow with life’s tides, to shine even in the shadows, and to carry light wherever I go.

That's why I've decided to make a shrine dedicated to them. This is going to be a really text-heavy shrine so be warned!

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Typical characteristics of jellyfish
Anatomy
How jellyfish stings work
True jellyfish species
Creatures that AREN'T true jellyfish
Fun facts

Typical characteristics of jellyfish

So what even are jellyfish? Well, they're definitely not fish! They’re invertebrates (animals without backbones) that belong to the phylum Cnidaria, just like corals, sea anemones, and hydras. True jellyfish belong to the class Scyphozoa. Their name comes from the Greek root cnid- meaning “nettle,” because they have specialised stinging cells called cnidocytes, which contain tiny harpoon-like structures called nematocysts to inject venom into prey.

Typical characteristics of true jellyfish include:

Radial symmetry - their body parts are arranged around a central axis symmetrically, like an orange slice!

Jelly - most of their mass is a soft, jelly-like substance called mesoglea. The mesoglea serves as an internal skeleton to support the body and is elastic to allow flexibility. It's basically the "jelly" of a jellyfish

No brain, heart, or bones - instead of a central brain, jellyfish have a network of nerves called a nerve net, which allows them to sense their environment.

Dominant medusa stage: The medusa (bell-shaped, free-swimming form) is the conspicuous and dominant life stage in their life cycle

Oral arms- they possess four to eight prominent oral arms that extend from the mouth and are used to manipulate and transport captured prey

Simple sensory structures- their specialised sensory clusters (rhopalia) along the bell margin contain simple light-sensing ocelli (eyespots)

Asexual and sexual reproduction - depends on the life stage. Medusae are adults and reproduce sexually by releasing sperm and eggs into the water, where fertilisation occurs externally. Once fertilised, the egg develops into a larva called the planula and then forms a polyp. Polyps are stationary and fixed to a surface, so they reproduce asexually by budding to release new medusae.

Tentacles with cnidocytes - used to inject venom in prey

Bell-shaped medusa - the free-swimming, umbrella-like form we usually picture when we think of jellyfish.

Single opening - a single opening called the gastrovascular opening that serves as both mouth and anus...yes you read that right...

Anatomy

True jellyfish (Scyphozoa) are made of almost pure water - up to 98%. They have no bones, head, or organs, only a soft, transparent body held together by the mesoglea, a jelly-like substance that acts as a hydrostatic skeleton, giving them shape and buoyancy.

The bell, or umbrella, forms the main body, layered between the outer epidermis and inner gastrodermis. Along its edge are soft lappets that ripple as the jellyfish drifts, while delicate rhopalia hang between them - tiny sensory organs that detect light, movement, and balance.

From the centre extends the manubrium, ending in a single mouth that also serves as the anus. Four oral arms trail from it, lined with stinging nematocysts used to catch prey. Inside, the gastrovascular cavity digests food and distributes nutrients through branching canals.

They move by pulsing their bells - slow, rhythmic contractions that let them float with unhurried grace.

Nervous & Sensory System

Though brainless, jellyfish aren’t mindless. A loose nerve net woven through the skin coordinates their every motion, allowing them to sense and react to touch, light, and current.

Each rhopalium along the bell rim contains a statocyst for balance and one or more ocelli for detecting light. Most species see only light and shadow, but their relatives, the box jellyfish (Cubozoa), have 24 eyes that can perceive colour and simple shapes - granting them a near 360° awareness of the world around them.

Respiration & Circulation

Without hearts or lungs, jellyfish breathe through their skin. Oxygen diffuses directly into their tissues as the bell pulses water across the surface.

Inside, tiny cilia-lined canals move nutrients throughout the body - a simple, elegant system shaped perfectly for life adrift.

Feeding & Digestion

Tentacles and oral arms are armed with nematocysts, microscopic stinging cells that paralyse prey on contact. The captured food is drawn to the mouth and digested in the gastrovascular cavity.

Enzymes break it down, while cilia spread the nutrients through the canals. One opening serves as both mouth and anus - an ancient simplicity that endures.

Reproduction

Most species are either male or female (gonochoristic), with gonads nestled in the stomach walls. Gametes are released through the mouth into open water, where fertilisation occurs.

Some species brood their young along the oral arms, sheltering the eggs until they hatch. Others release them to drift freely among the plankton.

Growth & Life Cycle

A jellyfish’s life unfolds in shifting forms. After fertilisation, the egg becomes a free-swimming planula that settles onto a solid surface and grows into a tiny polyp (scyphistoma).

Through strobilation, the polyp divides into stacked discs, each breaking free as an ephyra - a miniature jellyfish. These juveniles mature into the graceful medusa, the drifting adult form we know.

How jellyfish stings work

I've never been stung by a jellyfish before, so I often wonder what it's like.

I've already mentioned cnidocytes before, but let's look at how they work. Cnidocytes are found all over the tentacles of a jellyfish. Inside these specialised cells are nematocysts, a tiny capsule that contains a venomous harpoon that's often barbed. Part (a) of the diagram shows when it's not triggered. It will remain coiled inside a fluid. The fluid consists of neurotoxins that paralyse or kill prey, accompanied by enzymes to cause tissue damage and aid in the spread of the venom. It also contains tiny amines that contribute to the inflammation experienced. The fluid is stored under immense hydrostatic pressure, sometimes exceeding 2,000 pounds per square inch. This stored energy is the "spring" for the harpoon.

Part (b) of the diagram shows the mechanism being triggered. This occurs due to a combination of stimuli. There's physical contact involved when prey brushes against a hair-like trigger on the surface of the cell called the cnidocil. But there's also chemical recognition. The nematocyst often also needs to sense specific chemicals on the target's surface (e.g., proteins or sugars) to prevent it from wasting its single shot on inanimate objects like debris.

Once triggered, the cell rapidly and dramatically changes the osmotic pressure inside the capsule, causing a sudden rush of water into the nematocyst. This massive increase in internal pressure forces the lid of the capsule to pop open, and the coiled tube is forcefully and explosively ejected. The harpoon-like thread rapidly uncoils and everts (turns inside-out) as it shoots out, accelerating at speeds claimed to exceed 5 million times the force of gravity!

Soon, the harpoon will penetrate the skin of the prey. The venom, which is a cocktail of potent neurotoxic, cardiotoxic, or hemolytic proteins, is then rapidly delivered through the hollow tube and into the victim's tissue.

This entire process is carried out in as little as 700 nanoseconds, making it one of the fastest biological processes known. If you don't think that's cool as hell then idk what is!!

For minor stings, the pain is immediate and localised. Redness, irritation and swelling are observed at the site of the sting, with the itching lasting for up to a week sometimes. Welts and an imprint of the tentacles can also be found. The initial sharp pain subsides over an hour and may turn into a feeling of numbness or tingling. These stings are, of course, uncomfortable, but you won't face any long-term consequences as long as you are treated immediately.

On the other hand, severe stings may cause intense pain, blistering, large areas of swelling, or deep skin damage. In some cases, symptoms like nausea, vomiting, difficulty breathing, muscle cramps, or chest pain are described because the venom spreads through the bloodstream. Rarely, stings from highly venomous jellies can lead to dangerous complications such as heart problems, shock, or even death without immediate medical treatment.

True jellyfish species

The following are true jellyfish. Meaning they belong to the Scyphozoa class and have the stinging cells (nematocysts)

Moon Jellyfish

Common Name: Moon Jellyfish

Scientific Name: Aurelia aurita

Habitat: North, Black, Baltic and Caspian Seas, Northeast Atlantic, Greenland, northeastern USA and Canada, Northwest Pacific and South America

Water type: Temperate or tropical shallow waters near the coastline

Depth: Near the surface

Size range: 25-40 cm (10-16 in) in diameter

Lifespan: 6 months to 1 year

Sting: Harmless to humans, very mild stinging

Diet: Plankton

Distinguishing features:

Gonads - those four clover-shaped markings you see on top of a moon jelly bell, are actually their reproductive organs!
Short tentacles - they have hundreds of short, fine tentacles that line the edge of the bell. They're used to capture plankton and bring it into their bodies for digestion. Harmless to humans

Lion's mane jellyfish

Common Name: Lion's mane jellyfish

Scientific Name: Cyanea capillata

Habitat: Arctic, northern Atlantic, and northern Pacific Oceans

Water type: Cold, boreal waters

Depth: No more than 20 m (66 ft)

Size range: 1 m (3 feet) in bell diameter. Tentacles can grow up to 30.5 m (100 feet)

Lifespan: 1 year

Sting: Venomous; causes local inflammation and temporary pain. Not fatal but medical attention may be required

Diet: Fish, zooplankton, and smaller jellyfish.

Distinguishing features:

Largest jellyfish species in the world
Colouration - ranges from pale yellow/orange in smaller, juvenile individuals to deep crimson or dark reddish-brown in larger, mature ones.
Eight lobes - the bell is divided into 8 distinct lobes (radial symmetry).
Long, hair-like tentacles - hundreds to thousands of very fine tentacles grouped in clusters, giving its shaggy appearance.
Frilly bell edge - the bell margin has numerous folds that look ruffled, almost like a lion’s mane.

Cauliflower jellyfish

Common Name: Cauliflower jellyfish

Scientific Name: Cephea cephea

Habitat: Indo-West Pacific, eastern Atlantic and the Red Sea

Water type: Tropical and sub-tropical waters

Depth: Over 914 m (3000 feet) below the surface

Size range: Up to 60 cm (23.6 in) in diameter

Lifespan: 3 to 6 months

Sting: Harmless to humans, in fact they're considered a delicacy and are eaten by people in China and Japan. But they're very venomous to their prey

Diet: Algae, shrimp, plankton

Distinguishing features:

Bell - the top of the bell has wart-like lumps that give it a bumpy, cauliflower-like appearance.
Colouration - often bluish, purplish, or brownish tones.
Short oral arms - thick, stubby arms beneath the bell used to help move food toward the mouth.
Symbiotic hitchhiking - sometimes small fish or crustaceans live among their tentacles for protection.

White-spotted jellyfish

Common Name: White-spotted jellyfish

Scientific Name: Phyllorhiza punctata

Habitat: Western Pacific from Australia to Japan

Water type: Warm, temperate

Depth: Up to 20 m (65.6 feet)

Size range: Up to 50 cm (20 in) in bell diameter

Lifespan: 1 year

Sting: Mild, not a threat to humans

Diet: Zooplankton, fish eggs or larvae and small fish. They were unintentionally introduced to other regions and became an invasive species as they multiplied. They pose a threat to the fish industry as they consume fish eggs.

Distinguishing features:

White spots - unlike most jellyfish, their bell is covered in distinct white spots, making them easy to identify.
Frilly oral arms - they lack the long stinging tentacles common in many jellies, instead having thick, curtain-like oral arms.

Cannonball jellyfish

Common Name: Cannonball jellyfish

Scientific Name: Stomolophus meleagris

Habitat: Eastern Pacific, the Western Atlantic and the Western Pacific Ocean

Water type: Tropical and sub-tropical waters

Depth: up to 85 m (280 feet)

Size range: Up to 25 cm (10 in) in bell diameter, 16 cm (5 in) in height

Lifespan: 3 to 6 months

Sting: Mild, not a threat to humans

Distinguishing features:

Shape - it has a nearly spherical, dome-shaped bell hence the name, cannonball
Short oral arms - short, thick oral arms instead of long, trailing tentacles

Upside-down jellyfish

Common Name: Upside-down jellyfish

Scientific Name: Cassiopea xamachana

Habitat: Pacific, Atlantic oceans and Mediterranean sea

Water type: Shallow tropical and subtropical waters

Depth: Less than 2 m (6.5 feet)

Size range: 10–12 cm (3.9 to 4.7 in) bell width, 30.5 cm (12.01 in) in length

Lifespan: 16 months

Sting: Mild, irritating and itchy.

Diet: Zooplankton and food provided by zooxanthellae

Distinguishing features:

Symbiotic relationship - these jellyfish have a symbiotic relationship with single-celled protists called zooxanthellae. These little guys are found in the mesogloea (the "jelly" of the jellyfish) which also gives the jellyfish a bluish-grey or bluish-green colour. So the protists provide the jellyfish nutrients via photosynthesis in exchange for shelter, like how coral!
Upside down appearance - well I mean duh, it's in the name. Their bells are on the seafloor and their tentacles reach out. This maximises the sunlight absorption, thus increasing the rate of photosynthesis in the protists.
Mobility - mostly sedentary, their bell is stuck to the seabed, so they don't swim around like other jellyfish.
Sting - they can actually sting, without you touching them! First, they release a mucus into the water. This mucus contains cassiosomes, structures that are a bunch of stinging cells around a piece of jelly. They move by using their own cilia and can survive for a few days, to hunt for prey. Once they do capture prey, they sting them and the dead prey falls into the jellyfish's mouth. However, if you're not prey and just a regular human swimming by, getting into contact with cassiosomes in the water can still trigger a sting, leaving you with irritation or a painful rash.

Mauve Stinger Jellyfish

Common Name: Mauve Stinger Jellyfish

Scientific Name: Pelagia noctiluca

Habitat: Atlantic Ocean and the Mediterranean Sea

Water type: Tropical and warm temperate waters

Depth: Surface to 150 m (492 ft) (more common), recorded to 1,400 m (4593 ft)

Size range: ell diameter of 3–12 cm (1.2–4.7 in)

Lifespan: 2 to 6 months

Sting: Painful but generally not dangerous

Diet: Planktonic organisms, fish eggs, invasive comb jellies, and even members of their own species.

Distinguishing features:

Bioluminescence - it can produce visible flashes of light when disturbed, though it is brief and gradually fades.
Stinging cells on both bell and tentacles - most jellyfish only have cnidocytes on their tentacles only but these jellies have them on their bell too.
No polyp stage - most jellyfish alternate between immobile polyp and free-swimming medusa stages. However, the Mauve Stinger Jellyfish has a completely pelagic (open ocean) life cycle.
Impact - reported to have formed massive swarms (blooms) that can clog fishing nets, destroy fish farms, and shut down tourism. This is why they are one of the most studied jellyfish species.

Atolla jellyfish

Common Name: Atolla jellyfish

Scientific Name: Atolla wyvillei

Habitat: Around the globe, deep in the ocean.

Water type: Deep waters, in the midnight zone

Depth: 1000 to 4000 m (3300 to 13100 ft)

Size range: 20–174 mm (0.79–6.85 in) in bell diameter, 3.6 m (12 ft) in length

Lifespan: Unknown

Sting: Painful, but not serious

Diet: Crustaceans and other floating nutrients

Distinguishing features:

One extra-long tentacle - most jellyfish have tentacles of equal length, but the Atolla jellyfish has a single, much longer hypertrophied tentacle than the others, to catch prey
Crown-like bell shape - the bell has a ridged "crown" look to it, which is why they fall under the Coronate family of true jellyfish.
A beautiful red - the atolla jellies have a deep red colour to them, which is perfect for camouflage in the deep sea, as red light doesn't penetrate in those depths
Burglar alarm defence - this jellyfish is bioluminescent, common in deep-sea animals. When attacked by a predator, it produces a blue flashing in propagations (circular waves) to attract a larger predator that could prey on the predator attacking the jellyfish. Interestingly, this unique adaptation inspired marine biologist Edith Widder to create the E-Jelly. A device that mimics the Atolla's alarm to lure in rare and elusive deep-sea creatures for documentation. One notable instance was when her device was used to lure and capture the video evidence of a giant squid in 2012.

Giant Phantom Jelly

Common Name: Giant Phantom Jelly

Scientific Name: Stygiomedusa gigantea

Habitat: All around the world with the exception of the Arctic Ocean

Water type: Deep waters, in the midnight and twilight zone

Depth: 6,665 m (21,867 ft)

Size range: Bell can grow up to 1 m (3.3 ft) in diameter and 10 m (33 ft) in length

Lifespan: Unknown

Sting: None!

Diet: Plankton and small fish

Distinguishing features:

Monotypic genus - the only species in its genus (Stygiomedusa) no other jellyfish shares its genus classification, so its genetic lineage is entirely unique.
Size - one of the largest invertebrate predators in the deep sea.
No stinging tentacles - they have no stinging cells at all, instead, it uses its four massive, paddle-like arms to trap and engulf prey
Arm structure - the four arms are broad and kite-shaped, growing in a V formation from the bell’s base and tapering outward. They're flexible and can stretch to 4-5 times their resting length to envelop prey.
Symbiotic relationship - the large umbrella-shaped bell of S. gigantea provides food and shelter for the T. pelagica, the first-ever documented symbiotic relationship from an ophidiiform fish. In return, the fish helps the jellyfish by removing parasites from its body. This relationship is important for T. pelagica, since shelter is extremely scarce in the deep ocean where both animals live. Further studies revealed that the two species will reassociate even after being separated. T. pelagica is able to locate and return to the giant phantom jellyfish using neuromasts—special sensory organs that detect low-frequency water movements emitted by the jelly’s bell.

Atlantic sea nettle

Common Name: Atlantic sea nettle

Scientific Name: Chrysaora quinquecirrha

Habitat: Atlantic coasts of the United States

Water type: Sub-tropical waters

Depth: Near the surface

Size range: 40 cm (16 in) in bell diameter and a length of 5 cm (2 in)

Lifespan: 1 year

Sting: Painful, but moderate

Diet: Zooplankton, ctenophores, other jellies, and crustaceans

Distinguishing features:

Confusion between species - For years, the Chrysaora quinquecirrha was mistaken for other members of the Chrysaora genus. It is semi-transparent with small white spots and reddish-brown stripes on its bell.

Northern sea nettle

Common Name: Northern sea nettle

Scientific Name: Chrysaora melanaster

Habitat: Native to the northern Pacific Ocean and Arctic Ocean

Water type: Temperate open ocean waters

Depth: 100 m (328 ft)

Size range: Bell diameter of 60 cm (2 ft)

Lifespan: Unknown

Sting: Mild

Diet: Copepods, larvaceans, small fish, large zooplankton, and other jellies

Distinguishing features:

Thriving - Due to climate change, the top layer of the ocean is warmer and doesn't mix with the cool layers below. So the surface water becomes warmer and food-rich, thus this species started thriving and multiplying.

Bay nettle

Common Name: Bay nettle

Scientific Name: Chrysaora chesapeakei

Habitat: Estuaries of the eastern coast of the U.S.A., especially Chesapeake Bay, but can also be found in the Gulf of Mexico

Water type: Coastal waters

Depth: 7 m (23 ft)

Size range: 10 cm (3.9 in) in bell diameter

Lifespan: 1 year

Sting: Moderate

Diet: Plankton, small crustaceans and ctenophores

Distinguishing features:

Confusion between species - As mentioned before, this species was mistaken for the Chrysaora quinquecirrha. We know now that they are different, as the C. chesapeakei has a smaller bell size and fewer tentacles.
Ecological impact - A species of ctenophore has negatively affected many of the economically important fishing industries in the Chesapeake Bay. However, since this jellyfish feeds on ctenophores, the predatory impact of the species subsides.

Pacific sea nettle

Common Name: Pacific sea nettle

Scientific Name: Chrysaora fuscescens

Habitat: Pacific Ocean

Water type: Temperate to cooler waters

Depth: No less than 100 m (328 ft)

Size range: Less than 50 cm (1.6 ft) wide in bell diameter and 15 feet (4.6 m) in tentacle length.

Lifespan: 6 months

Sting: Moderate

Diet: Zooplankton, crustaceans, salps, pelagic snails, small fish including their eggs and larvae, and other jellyfish

Distinguishing features:

Colour - They have a golden-brown bell with a reddish tint
Captivity - Popular cnidarian species in aquariums due to their bright colours, long tentacles but also because of their low maintenance.
Hitchhikers - Often carry small hitchhikers like fish or crabs inside its bell.

Japanese sea nettle

Common Name: Japanese sea nettle

Scientific Name: Chrysaora pacifica

Habitat: Pacific Ocean

Water type: Temperate

Depth: No less than 100 m (328 ft)

Size range: Bell with a diameter of 15–21 cm (5.9–8.3 in)

Lifespan: 6 months

Sting: Strong

Diet: Zooplankton, including small crustaceans and other jellyfish

Distinguishing features:

Accidental discovery - In 2009, a Japanese scientist was looking for a different jellyfish species when he came across the polyps of this species, attached to dead crabs and shells.
Confusion between species - Often confused with Chrysaora melanaster which is much larger. This jelly on the other hand, has distinguishing red stripes on its bell and is smaller in size.

Black sea nettle

Common Name: Pacific sea nettle

Scientific Name: Chrysaora achlyos

Habitat: Pacific Ocean

Water type: Temperate

Depth: No less than 100 m (328 ft)

Size range: Bell diameter potentially up to 1 m (3 ft) and arms extending to 5 m (16 ft)

Lifespan: 6 months

Sting: Moderate

Diet: Zooplankton and other jellyfish

Distinguishing features:

Size - It was actually the largest invertebrate species formally described in the 20th century.
Rarity - Often disappears for years at a time, then suddenly appears in massive swarms. Sightings often coincide with red tide events, suggesting a link to blooms of their zooplankton prey.

False jellyfish species

The following are species that resemble jellyfish but are not true jellyfish as they aren't classified as Scyphozoa and don't exhibit all the characteristics of a true jellyfish.

Portuguese man o' war

Common Name: Portuguese man o' war

Scientific Name: Physalia physalis

Habitat: Atlantic, Indian, and Pacific oceans

Water type: Saltwater

Depth: Surface

Size range: 9–30 cm (3.5–12 in) width, tentacles reach to 10–30 m (33–98 ft)

Lifespan: Around 1 year

Sting: Very painful, not deadly but medical attention is needed

Diet: Small fish, fish larvae, and planktonic animals

Distinguishing features:

Colonial organism - The Portuguese man o' war is not one creature, but a floating colony of many tiny animals called zooids. They are not individual cells, as they have their own cells and tissues. They came from the evolution of individual hydrozoans that lived together in colonies, fused permanently over time and began specialising in different roles. These zooids are genetically identical and are permanently fused together, but they serve different purposes: feeding, stinging, reproduction, and forming the gas-filled float, for example. None of the zooids can survive alone, so the whole colony functions as one "super-organism."
Gas-filled float - The colourful balloon-like structure you see at the surface is called the pneumatophore. It’s filled mostly with nitrogen and carbon dioxide, plus a little carbon monoxide. It's a "sail" that catches the wind, allowing the colony to drift across the ocean. Some individuals are "right-handed" and some "left-handed" which means their sail leans one way or the other. This helps spread the population by making them drift in slightly different directions during storms.
Long tentacles - Hanging below the float are long fishing-line-like tentacles that can reach 10–30 m (33–98 ft). They still contain nematocysts like true jellyfish and use them to capture prey.
Blue-purple colouration - They have striking blue, purple, or pink hues that help protect the organism from UV light and camouflage it among the blue water. These colours are also a warning to predators of their powerful sting.

Why it's NOT a true jellyfish: Because it is a siphonophore, a colony of organisms not a single organism.

Immortal jellyfish

Common Name: Immortal jellyfish

Scientific Name: Turritopsis dohrnii

Habitat: Mediterranean Sea, but now found globally due to ship ballast water transport.

Water type: Saltwater

Depth: Shallow waters to the intertidal zone, spanning about 220 m (720 feet)

Size range: Bell diameter: about 4–5 mm (0.18 inches)

Lifespan: Biologically immortal, meaning it can revert to its juvenile stage and repeat its life cycle indefinitely (unless eaten, diseased, or killed)

Sting: Very mild

Diet: Zooplankton, tiny crustaceans, fish eggs

Distinguishing features:

Immortality - The most obvious aspect of this jelly is its "immortality". It's able to reverse its life cycle whenever it is injured, stressed, or old, transforming its adult medusa body back into a polyp stage instead of dying. This process is called transdifferentiation, where mature cells change into different types of cells. The polyp can then grow into a new adult jellyfish, allowing the organism to repeat its life cycle again and again.
Transparent body - The immortal jellyfish has a nearly invisible, transparent bell, making it difficult to see in the water. It has a red stomach in the centre, which is often the most visible part of its body.
Global spread - Because of its tiny size and survival strategies, it has spread around the world through human activities like ballast water in ships, making it one of the most successful invasive marine species.

Why it's NOT a true jellyfish: Immortal Jellyfish are Hydrozoans in its life cycle, the polyp stage is the dominant, long-lived stage, whereas in Scyphozoa, the medusa (jellyfish) stage is dominant.

Crystal jellyfish

Common Name: Crystal jellyfish

Scientific Name: Aequorea victoria

Habitat: West coast of North America

Water type: Coastal waters

Depth: 200 m (656 ft)

Size range: Bell diameter 3–25 cm (1–10 in)

Lifespan: 6 months to 1 year

Sting: Very mild, harmless to humans

Diet: Zooplankton, small crustaceans, fish larvae, and microscopic organisms

Distinguishing features:

Source of Green Fluorescent Protein (GFP) - Crystal jellyfish produce light through a ring of light organs around the bell margin. When calcium enters their cells, the protein aequorin emits blue light, which then activates Green Fluorescent Protein (GFP) to shift the colour to green. This two-protein system is extremely rare and is a protein now used worldwide to label genes, track cancer cells, visualise neurons, and study embryo development. No other jellyfish species has had such a massive impact on biotechnology and medicine.
Hundreds of thin tentacles - Unlike jellyfish with a few long tentacles, this jelly has dozens to hundreds of very thin tentacles arranged evenly around the bell, forming a delicate feeding net for trapping plankton.

Why it's NOT a true jellyfish: Crystal jellyfish are Hydrozoans in their life cycle, the polyp stage is the dominant, long-lived stage, whereas in Scyphozoa, the medusa (jellyfish) stage is dominant.

Bloody-Belly Comb Jelly

Common Name: Bloody-Belly Comb Jelly

Scientific Name: Lampocteis cruentiventer

Habitat: North Pacific Ocean

Water type: Marine

Depth: 200 m to over 1,000 m (656 ft to over 3280 ft)

Size range: 5–10 cm long (2-4 inches)

Lifespan: Unknown

Sting: None, completely harmless

Diet: Small crustaceans, copepods, larval fish, and other zooplankton

Distinguishing features:

Blood-red colour - The deep red colouration of its gut and internal tissues gives it the “bloody belly” appearance. In deep waters, red light does not penetrate, so red tissue appears black and invisible, hiding the glow of bioluminescent prey inside its stomach from predators. This is a rare and highly specialised form of internal camouflage.
Delicate, egg-shaped body form - Its body is laterally compressed and fragile, adapted for slow, energy-efficient drifting in deep water where food is scarce.
Like all ctenophores, it swims using eight rows of fused cilia (comb plates) that beat in waves, producing shimmering rainbow patterns. This is not bioluminescence but light diffraction, and it is the primary mode of locomotion - unlike true jellyfish, which pulse their bells.

Why it's NOT a true jellyfish: It belongs to Ctenophora (a separate phylum for comb jellies); they do not have nematocysts, so they do not sting at all and instead use colloblasts, which are sticky adhesive cells to trap prey. Instead of pulsing a bell, it moves using rows of beating cilia, making it the largest animal to swim using cilia. They also have bilateral symmetry, while jellyfish have a radial symmetry. Despite the “jelly” name, comb jellies are not closely related to true jellyfish and may represent one of the earliest branches of animal evolution.

Sea gooseberry

Common Name: Sea gooseberry

Scientific Name: Pleurobrachia pileus

Habitat: Atlantic Ocean and the North Sea

Water type: Temperate waters

Depth: Between 80 and 150 m (260 and 490 ft)

Size range: 2.5 cm (1 in) in length

Lifespan: 4 to 6 months

Sting: No sting

Diet: Zooplankton

Distinguishing features:

Colloblasts - they have adhesive cells called colloblasts that trap zooplankton instead of stinging them with cnidocytes

Why it's NOT a true jellyfish: This critter belongs to the phylum Ctenophora. They have comb rows instead of a bell head and have sticky cells instead of stinging ones.

Australian box jelly

Common Name: Australian box jelly

Scientific Name: Australian box jelly

Habitat: Waters from northern Australia and Papua New Guinea to Indonesia, East Timor, Cambodia, Malaysia, Singapore, the Philippines and Vietnam

Water type: Coastal

Depth: Less than 5 meters (16 ft)

Size range: Bell reaches 16 cm (6.3 in) in diameter though it can grow up to 35 cm (14 in). Tentacles 150 mm (6 in) long and about 5 mm (0.20 in) in diameter; when they hunt, the tentacles are thinner and extend to about 3 m (9.8 ft) long

Lifespan: Less than a year, the longest record was 9 months in captivity

Sting: Instant and very intense, it has a fast-acting venom that travels around the body. Survivable if treated immediately

Diet: Prawn and small fish

Distinguishing features:

Pale blue, transparent colour - difficult to see in the water, making it more dangerous for swimmers
More tentacles than other species - 15 tentacles per corner, so around 60 in total, much more than other box jellyfish species
Most venomous jellyfish - Its venom reaches the nervous system, heart, and skin cells in a short span of time, leading to an intense burning and excruciating pain. Itchy red welts form on the skin which may lead to skin necrosis (tissue death). It may lead to cardiovascular collapse and respiratory failure, where the circulatory and respiratory systems fail, causing a lack of oxygen supplied to vital organs. Most people die within 3 to 5 minutes after contact. Sometimes, the intense muscle spasms cause drowning or shock before the swimmer can even reach the shore.

Why it's NOT a true jellyfish: The Australian box jellyfish is called a jellyfish, but it is actually a box jellyfish (class Cubozoa), not a true jellyfish (class Scyphozoa). It has a box-shaped body and four sides. It has advanced sensory organs that can detect light and movement, while true jellyfish detect light. Box jellyfish can actively swim and steer using muscular contractions, while true jellyfish mostly drift passively with ocean currents.

Fire jelly

Common Name: Fire jelly

Scientific Name: Morbakka virulenta

Habitat: Waters near the islands of Japan

Water type: Temperate waters

Depth: 30 m (98 ft)

Size range: Bell width up to 200 mm (7.8 in) and bell height up to 250 mm (9.8 in)

Lifespan: Unknown

Sting: A fiery sting, hence why it's called Hikurage in Japanese which means "fire jellyfish". Unknown if it causes Irukandji syndrome

Diet: Japanese anchovy

Distinguishing features:

Rabbit-ear shaped horns - near the rhopalia, the sensory organs that have receptors, are rhopaliar horns, which resemble rabbit ears
Nematocysts on bell - the bell surface is covered with nematocyst warts
Velarial canals - its internal canal pattern helps scientists distinguish it from other species in the same family

Why it's NOT a true jellyfish: A box jellyfish (class Cubozoa), not a true jellyfish (class Scyphozoa). It has a box-shaped body and four sides. It has advanced sensory organs that can detect light and movement, while true jellyfish detect light. Box jellyfish can actively swim and steer using muscular contractions, while true jellyfish mostly drift passively with ocean currents.

Common Name:

Scientific Name:

Habitat:

Water type:

Depth:

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Lifespan:

Sting:

Diet:

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Why it's NOT a true jellyfish:

˚₊‧ ଳ ‧₊˚ Jellyfish˚₊‧ ଳ˚ ⋅

content

Typical characteristics of jellyfish

Anatomy

Anatomy

Nervous & Sensory System

Respiration & Circulation

Feeding & Digestion

Reproduction

Growth & Life Cycle

How jellyfish stings work

True jellyfish species

Moon Jellyfish

Lion's mane jellyfish

Cauliflower jellyfish

White-spotted jellyfish

Cannonball jellyfish

Upside-down jellyfish

Mauve Stinger Jellyfish

Atolla jellyfish

Giant Phantom Jelly

Atlantic sea nettle

Northern sea nettle

Bay nettle

Pacific sea nettle

Japanese sea nettle

Black sea nettle

False jellyfish species

Portuguese man o' war

Immortal jellyfish

Crystal jellyfish

Bloody-Belly Comb Jelly

Sea gooseberry

Australian box jelly

Fire jelly

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