Upside-Down Jellyfish: Facts, Habitat & More

Spot the Jelly That Lives Life Upside-Down

Get up close to one of nature’s most unique jellyfish. Visit SEA LIFE Sydney to see the captivating Upside-Down Jellyfish, along with Moon Jellyfish and Jelly Blubbers.

Scientific Name

Cassiopea species (commonly Cassiopea andromeda or Cassiopea xamachana)

Habitat

Shallow tropical waters such as mangroves, lagoons, and estuaries in northern Australia, as well as Southeast Asia, the Caribbean, and the Indo-Pacific

Size

Typically, 10-20cm wide

Lifespan

Roughly 6-12 months in both the wild and aquarium environments

Diet

Microscopic plankton, small aquatic organisms, and nutrients from symbiotic algae (zooxanthellae)

Sting

Produces venomous mucus (“stinging water”) that can irritate human skin

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Upside-down jellyfish, also known as the mangrove jellyfish or Cassiopea, are tropical marine jellies that live on the sea floor, bell down, arms up. This quirky behaviour helps expose their symbiotic algae to the sun so they can generate food through photosynthesis.

💡Did you know? They release venom-filled mucus into the water - nicknamed 'stinging snot' - to trap prey and deter predators.

Hidden in plain sight

They might be easy to spot in our aquarium tanks, but out in the wild, Upside-Down Jellyfish (Cassiopea) are masters of disguise.

They rest upside-down on the seafloor

With their bell pressed flat against the sand and their frilly oral arms facing up, they resemble a swaying sea flower more than a jellyfish.

Camouflaged by colour

Thanks to symbiotic algae living in their tissues, they take on earthy shades of green, brown, or blue - helping them blend into shallow, sunlit lagoons.

No central mouth? No problem

Instead of one big mouth, these jellies have dozens of tiny ones along their oral arms - perfect for feeding while staying still.

Where you’ll find them in the wild

Upside-down jellyfish love warm, sunny waters, and their favourite hangouts might surprise you.

Found in shallow tropical zones
They thrive in mangroves, lagoons, and estuaries across northern Australia, especially in Queensland and Western Australia.
☀️ They need sunlight for their algae to photosynthesise, so you’ll usually find them in calm, shallow water.

Historical range: These tropical jellies have long inhabited northern Australia (e.g., far north QLD and WA), but not temperate regions.

However, that changed over 10 years ago…

First southern sightings (NSW lakes): In 2013-2014, scientists from the Australian Museum confirmed their presence in two coastal NSW lakes: Wallis Lake and Lake Illawarra, marking a southern range expansion of 600-900 km.
Temperate records in Lake Macquarie (2017): They were spotted again in temperate waters off NSW, showing they are establishing beyond tropical zones.

Other than Australia - they are also found in:

Southeast Asia (Indonesia, Philippines)
The Caribbean
Florida & Gulf of Mexico
Red Sea
Parts of the Indo-Pacific

💡 Did you know? Upside-down jellyfish are edging toward invasive status in Australia. Scientists are monitoring their southward spread as shifting marine ecosystems, driven by climate change, allow them to expand beyond their tropical range.

How they move and feed

Capable swimmers, but choose to stay put

Upside-down jellyfish can swim by pulsing their bell, but they rarely do. They actively choose to remain still on the seafloor to maximise sunlight exposure for their symbiotic algae. Movement typically only occurs when disturbed.

Farm energy from the sun

Living inside their tissues are special algae (zooxanthellae) that photosynthesise, creating sugars and nutrients the jellyfish absorb, similar to how coral reefs feed.

Catch plankton with mucus nets

Alongside solar power, Upside-Down Jellyfish catch plankton, fish larvae, and tiny sea critters using sticky mucus webs. Stinging cells on their oral arms help immobilise prey.

💡The gooey mucus released by Upside-Down Jellyfish is laced with stinging cells that paralyse microscopic prey on contact. Inside the slime are special structures that deliver venom directly through the water

Anatomy 101

No brain, heart, or bones

Upside-down jellyfish don’t need organs to function. Instead, they rely on a simple nerve net to sense the world and absorb oxygen directly through their skin.

Yes, they sleep

Despite having no brain, they enter a sleep-like state at night - one of the few invertebrates known to do so.

Their colour comes from algae

Their soft shades of green, brown, or blue come from symbiotic algae living in their tissues.

💡 In 2017, scientists discovered that Upside-Down Jellyfish exhibit sleep-like behaviour, despite having no brain. This challenged what we thought we knew about how sleep works!

Reproduction and Life Cycle

From drifting larvae to upside-down loungers, these jellies follow a remarkable transformation:

Fertilised eggs become larvae

Adult jellies (medusae) release sperm and eggs into the water, which develop into tiny free-swimming larvae (planulae).

Larvae settle and form polyps

Once the larvae find a suitable surface, they attach and transform into stalk-like polyps on the seafloor.

Budding baby jellies

These polyps reproduce asexually by budding, releasing small jellies called ephyrae - especially during warmer months.

Growing into medusae

The ephyrae grow into the familiar upside-down jellyfish form, resting on the seabed with their oral arms raised toward the sun - and the cycle begins again.

Who eats them?

Turtle treats

Loggerhead and Leatherback Turtles munch on Upside-Down Jellyfish, undeterred by their mild sting.

Crabs with jellyfish shields

Some clever crabs have been seen hitching a ride with these jellies, holding them like living shields for protection from predators.

Where to see upside-down Jellyfish

Visit the Jellyfish Zone at SEA LIFE Sydney Aquarium to witness the upside-down jellies up close

Plan your visit

Come meet the jellyfish that prefers lying down!

From their upside-down lifestyle to their sun-powered feeding, these curious creatures are full of surprises.

You can also see other jellyfish at SEA LIFE Sydney!

Check out their fact pages below

Yes! These quirky jellies are part of the aquarium’s jellyfish displays - you can get an up-close look at how they rest upside-down and pulse gently in the water.

They do produce stinging mucus in the wild, but at SEA LIFE Sydney, they’re safely behind glass and pose no danger to guests.

That’s how they live! They rest upside-down to help the algae in their arms absorb sunlight and produce food through photosynthesis.

They can swim, but most of the time at SEA LIFE Sydney, you’ll see them relaxing. It’s completely normal; they conserve energy unless disturbed.

No - like most of the jellyfish at SEA LIFE Sydney, they are for viewing only. But you can capture some mesmerising photos of them in action.

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