Jellyfish: Facts, Habitat & More

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What is a jellyfish?

Despite the name, jellyfish aren't fish at all. They're gelatinous marine animals from the phylum Cnidaria, the same group that includes sea anemones and corals.
They have:

A soft bell-shaped body
Flowing tentacles and oral arms
No brain, no heart, no bones

💡Did you know jellyfish are 95% water?

Why are jellyfish called fish if they’re not fish?

The term "jellyfish" dates back to the 18th century when early observers thought their gelatinous bodies resembled aquatic fish. In reality, jellyfish are cnidarians, a group of soft-bodied animals that don’t have backbones, gills, or fins like normal fish

These animals are more accurately called sea jellies, a term increasingly used by aquariums and marine educators to reduce confusion.

💡Why aren't jellyfish really fish?

They have no bones, no gills, and no backbone - three things every real fish needs.

How many jellyfish species are there?

There are more than 2,000 known species of jellyfish worldwide. Scientists believe thousands more remain undiscovered, especially in the deep sea.

While about 25% of the ocean has been mapped by sonar, an estimate only 5% of the ocean has been explored by humans, meaning there could be thousands, if not hundreds of thousands, of species of creatures we couldn't even dream of.

Types of jellyfish

Not all jellyfish are created equal; in fact, they don’t even all look like jellies. Scientists group jellyfish into a few main types based on their biology, movement, and structure:

1. Scyphozoa - True jellyfish

These are the classic jellies you see pulsing through the water. They include species like the Moon Jellyfish and Jelly Blubber. They have:

A bell-shaped, gelatinous body
Long tentacles with stinging cells (nematocysts)
A dominant medusa (free-swimming) life stage

2. Cubozoa - Box jellyfish

Known for their cube-like shape and powerful stings, box jellies include species like the infamous Chironex fleckeri. Found in tropical Australian waters, some can cause life-threatening stings.

Transparent, cube-shaped bell
Up to 15 tentacles per corner
Complex eyes and fast movement

3. Hydrozoa - Colonial jellies

This group includes creatures like the Portuguese Man o’ War (also known as the bluebottle), which looks like a jellyfish but is actually a floating colony of specialised polyps.

Often colonial, made up of multiple interconnected organisms
Belong to the same phylum as jellyfish (Cnidaria), but are not considered “true” jellyfish
Some are bioluminescent

4. Ctenophora - Comb jellies (not true jellyfish)

Comb jellies aren’t actually jellyfish, but they’re often confused for them. They use rows of cilia (tiny hairs) to move and can produce rainbows of light through refraction, not bioluminescence.

No stinging cells (harmless to humans)
Move using rows of cilia
Often glow or shimmer

They belong to a group called siphonophores - a subset of Hydrozoa. Rather than being a single creature, they’re a colony of specialised zooids that work together like one animal.

They use their ‘sail’ to move around on the water's surface.

💡 Did you know? Bluebottles aren’t true jellyfish.

Jellyfish have existed for at least 500 million years, and scientists suggest they are possibly up to 700 million years old
Found in every ocean, from surface waters to deep-sea trenches
Their fossils are older than dinosaurs or trees

How jellyfish move and live

Jellyfish swim by pulsing their bell
Most drift with the ocean currents
They absorb oxygen through their skin - no lungs or gills
Some jellyfish can glow in the dark through bioluminescence

What do jellyfish eat?

Jellyfish feed on zooplankton, fish eggs, larvae, tiny crustaceans, and even other jellyfish.

Predators and role in the ecosystem

They are prey for turtles, sunfish, some fish, and even other jellies
Blooms feed seabed organisms when they die
Can outcompete fish for food in large numbers

Do jellyfish eat each other?

Yes, some jellyfish are cannibalistic. Larger jellyfish species will often eat smaller jellies, especially when food is scarce. This includes both other species and even their own kind in some cases.

Size matters: Bigger jellies like Chrysaora (sea nettles) or Nomura’s jellyfish can easily trap and digest smaller jellies with their long tentacles.
Opportunistic feeders: Jellyfish aren’t picky. If a smaller jelly drifts into their stinging range, it's fair game.
Scyphozoans & Cubozoans: These "true" and "box" jellyfish are among those more likely to prey on other jellies.
Moon jellyfish (Aurelia aurita) sometimes eat ephyrae (baby jellies), though it’s not their primary food source.

💡Why have Moon Jellyfish survived so long?

Some jellyfish even absorb the stinging cells (nematocysts) from other jellyfish they eat - and repurpose them for their own defence!

SEA LIFE Sydney Jellyfish Feeding Thumbnail

Watch the video to find out!

Jellyfish stings, and how they hunt

Jellyfish use stinging cells (nematocysts) to catch prey
They feed on plankton, larvae, and small fish
Species like the Upside-Down Jellyfish can release stinging mucus into the water

Do jellyfish hunt in groups?

Jellyfish don’t hunt in coordinated packs like wolves, but some species gather in dense swarms (called blooms) when conditions are right, like warm water, abundant food, and ocean currents.

While not true teamwork, these swarms can act like a net:

Surrounding schools of fish
Concentrating around rich plankton zones
Increasing feeding success through sheer number

💡Some blooms can stretch for kilometres and contain millions of jellyfish.

Can jellyfish outcompete other sea life?

Yes, especially during blooms.

Jellyfish are opportunistic feeders and can consume massive amounts of zooplankton, fish eggs, and larvae.
In some regions, they’ve even disrupted fisheries by reducing food for juvenile fish and clogging fishing nets.

This impact is amplified when human-driven changes like overfishing or warmer oceans reduce jellyfish predators and competitors.

When jellies are in abundance, it provides these animals with an easy buffet of soft, energy-rich prey.

Predators like sea turtles, ocean sunfish (Mola mola), and some species of fish do take advantage, especially loggerhead turtles and leatherback turtles, as they are known to feed almost exclusively on jellyfish.

But it's not always good news for the ecosystem.

Jellyfish blooms are often linked to unstable environments - warming seas, pollution, and reduced biodiversity.
So while predators might enjoy the temporary feast, their populations don’t always increase to match the bloom size.
Also, some jellies in blooms are too small, too venomous, or not palatable to predators, so not all blooms are beneficial.

What happens after a bloom collapses?

Dead jellyfish sink to the seafloor and start to decompose. In massive numbers, this can:

Deplete oxygen from the water
Create dead zones where other marine life can’t survive
Trigger bacterial blooms that disrupt ecosystems even further

So while jellyfish blooms might look stunning and even benefit some predators short-term, their collapse can have serious long-term impacts on ocean health.

Jellyfish life cycle: from polyp to medusa

Asexual polyp phase → buds off baby jellyfish
Sexual medusa phase → releases eggs and sperm
Immortal jellyfish species (Turritopsis dohrnii) can reverse aging (Scientific American)

Are jellyfish immortal?

One species, Turritopsis dohrnii, often called the “immortal jellyfish,” has an incredible ability: when injured, sick, or stressed, it can reverse its ageing process, turning its adult body back into a tiny polyp - the first stage of its life cycle. However, it doesn’t always guarantee survival.

If the jellyfish is too damaged, infected beyond recovery, or in a hostile environment, it might not survive the transition back to the polyp stage. And like all jellyfish, it's still vulnerable to being eaten or affected by pollution, climate change, or parasites during any life stage.

In short

Yes, it can "rewind" to escape death
But it’s not immune to dying from sickness or external threats

What's the most venomous jellyfish in the world?

Meet the world's 2 most dangerous jellyfish - both found in Australian waters

There are approximately 50 known species of box jellyfish (class Cubozoa) worldwide. While most are venomous to some degree, two species commonly found in Australian waters are widely recognized as the two most dangerous jellyfish in the world:

There are around 50 known species of box jellyfish (class Cubozoa) worldwide, ranging from palm-sized Irukandji to large species like Chironex fleckeri.

Known as the world’s most venomous marine animal. Its sting can cause cardiac arrest within minutes.

Found in northern Australia, particularly Queensland, the Northern Territory, and Western Australia.

Also found in: Papua New Guinea, Malaysia, Thailand, and the Philippines.

Tiny but deadly. Causes Irukandji Syndrome: intense pain, nausea, vomiting, and sometimes heart issues.

Found along Australia’s northern coasts and as far south as Fraser Island.
Also reported in: Thailand, the Caribbean, Hawaii, and Malaysia.

💡While they’re not exclusive to Australian waters, Australia is one of the most closely monitored regions for jellyfish stings, making it a hotspot for public awareness and jellyfish research.

Do jellyfish have blood?

No, jellyfish have no blood, no heart, and no circulatory system. They absorb oxygen directly through their skin.

Do jellyfish have a brain?

Nope. Instead of a brain, jellyfish use a nerve net - a simple system that helps them detect touch, light, and food.

Do jellyfish have eyes?

Yes - but not all jellyfish see the same way.

Most jellyfish have simple light-sensing organs called ocelli. These help them detect light, dark, and movement, and are usually located on small sensory structures called rhopalia. They don’t see detailed shapes, but they can sense changes in light and respond by swimming or drifting in a different direction.

Some jellyfish, like box jellyfish, have surprisingly complex eyes, including lens eyes with corneas, retinas, and even the ability to detect shapes and navigate around obstacles. In fact, box jellies have 24 eyes grouped into different types, giving them a limited but functional kind of vision.

These visual abilities help jellyfish orient themselves, avoid objects, detect predators or prey, and even follow light (a behaviour called phototaxis).

Where can I see jellyfish?

Jellyfish are in every ocean worldwide

At SEA LIFE Sydney, you can see three mesmerising species:

Moon Jellyfish
Jelly Blubber
Upside-Down Jellyfish

Ready to dive into the dazzling world of jellyfish?

From glowing Moon Jellies to the upside-down oddballs, meet some of the ocean’s most mesmerising creatures up close at SEA LIFE Sydney Aquarium in our Jellyfish zone!