Irukandji Jellyfish Facts and Adaptations (Can They Kill You? Are they spreading?)

We like to snorkel in tropical waters throughout the world and, as biologists, we’ve learned to be cautious when it comes to jellyfish.  Not only are jellyfish stings painful; some species can kill you!  In this article I will explore some of the latest scientific findings on one of the most dangerous groups of jellyfish – the Irukandji.

Irukandji are tiny transparent jellyfish that cause the painful, and potentially life-threatening, Irukandji syndrome. They are less than 1 inch (approximately 2 cm) in diameter with 4 tentacles that can reach up to 3 feet (one meter) in length.  Due to its small size these jellyfish can sting people even inside net enclosures.  Researchers have discovered up to 25 species of Irukandji-type box jellyfish throughout the world. 

Carukia barnesi is commonly referred to as the Irukandji jellyfish, as this was the first species, identified in 1952, found to cause Irukandji syndrome.  Other notable species that also cause this painful condition include Alatina mordens, Malo maxima, Carybdea alata, Carybdea xaymacana, and Carybdea rastonii.

Irukandji-type box jellyfish are about the size of a sugar cube and can be particularly dangerous to swimmers and snorkelers because they are so tiny and almost transparent in ocean water.  

Photo of small vial containing an individual specimen of Malo kingi jellyfish.
Malo kingi in a clear plastic vial. Wikimedia

International visitors to tropical settings, especially Australia and South-east Asia, are often unaware of the dangers posed by the various species of box jellyfish, including the tiny Irukandji.  Hopefully, this article will help travelers understand the potential risks, so they can prepare accordingly.

Newspaper headline "Invasion of the killer jellyfish"
Headline in a British newspaper warning of the threat posed by Irukandji.
Photo of Irukandji jellyfish Carukia barnesi.
Irukandji jellyfish (Carukia barnesi). Photo by Lisa-Ann Gershwin CC BY 4.0

Irukandji adaptations

The key to the Irukandji’s success over hundreds of millions of years is the venom packed nematocyst or stinging cell.  Stinging cells are pressurized structures that fire a tiny, barbed harpoon into predators and prey, injecting a painful and paralyzing venom. Even though the Irukandji is tiny, it is covered with many thousands of stinging cells, as are most species of jellyfish.  Unlike most other jellyfish species, C. barnesi has nematocysts on the bell as well as the tentacles.

Drawing of nematocyst structure and mechanism, from Montgomery et al, 2016.
Nematocyst structure and mechanism, from Montgomery et al, 2016.

Using high-speed cameras, researchers have found that the discharge of the tiny harpoon takes place in about 700 ns (0.0000007 s), which makes it one of the fastest biological events ever measured.

Nematocyst discharge in slow motion

What makes the Irukandji-type box jellyfish especially dangerous is the lethal power of the venom it manufactures in its tissues.

Like many box jellies, Irukandji-type jellyfish are active hunters feeding on fish, even though they are so small.  To successfully kill fish, they need an extremely powerful venom that kills with even in tiny doses.  The venom of the Irukandji is considered to be one of the most powerful toxins in the natural world.

Dr Robert Courtney, James Cook University, is a leading expert on the complicated lifecycle of the Irukandji jellyfish: Carukia barnesi.  While observing Irukandji in seawater tanks in his laboratory, he has uncovered the amazing story of how they are actively fishing.  “We had thought these animals were swimming around haphazardly collecting prey,” he writes, “but the opposite was happening.”

Dr. Courtney has observed the jellyfish extending and twitching their tentacles in the water to attract specific sizes and species of larval fish. “We have a jellyfish that is essentially jigging for fish with its tentacles, mimicking something else, and small fish are seeing this as a food source and getting a face-full of venom,” said Dr Courtney.

Adult Irukandji actively attract larval fish with their tentacles and lure-like nematocyst nodes, and proceed to sting and consume the fish. Watch the video below with Dr. Courtney explaining his groundbreaking research.

Irukandji actively hunt and kill fish

The fish (Acanthochromis sp.) was captured by a twitching tentacle of an adult Carukia barnesi.

The bell size of this specimen is approximately 15 mm in height and the fish is approximately 10 mm in length. Note the envenomation site is on the head region of the fish. Screenshot of video from Courtney et al, 2015.

Irukandji syndrome

Irukandji syndrome is an extremely painful, potentially lethal condition caused by tiny jellyfish within the class Cubozoa (box jellyfish).  Although the initial sting is comparatively mild, usually less painful than a bee sting, after 10 to 40 minutes the ensuing symptoms become much more serious and potentially life-threatening.  These can include severe pain, muscle cramping, vomiting, and breathing difficulties.  In a small number of cases, life threatening hypertension, pulmonary oedema, and cardiac complications may develop.  One notable symptom is a feeling of “impending doom.”  Currently, there is no antivenin available.

Excerpt from Irukandji Syndrome by Erwin L. Kong; Thomas M. Nappe.

Patient education is paramount in the prevention of Irukandji syndrome. Although people native to the regions endemic to Irukandji-causative species are often well aware of the dangers and necessary safety precautions, foreign travelers often fall victim to stings and therefore have a higher incidence of Irukandji syndrome. Travelers need to know that “stinger-resistant” enclosure nets, often used to prevent stings from other jellyfish species, will not prevent stings from species that cause Irukandji syndrome, due to the organisms’ small size. The most effective form of protection while swimming is a full-body lycra “stinger suit.”

Although these suits can decrease the incidence of Irukandji syndrome by decreasing the total body surface area at risk for stings, it will not eliminate their occurrence, as it only requires one sting at an exposed area to cause the full-blown clinical syndrome. As such, the absolute best form of prevention would be to avoid swimming/submersion in waters endemic to suspected species altogether.”

Kong EL, Nappe TM. Irukandji Syndrome

Dr. Lisa-Ann Gershwin, one of the world’s leading authorities on Irukandji jellyfish and author of the must-read book Stung: On jellyfish blooms and the future of the ocean, describes the symptoms like this: “It gives you incredible lower back pain that you would think of as similar to an electric drill drilling into your back. It gives you relentless nausea and vomiting. How does vomiting every minute to two minutes for up to 12 hours sound? Incredible. It gives waves of full body cramps, profuse sweating…the nurses have to wring out the bed sheets every 15 minutes. It gives you very great difficulty in breathing where you just feel like you can’t catch your breath. It gives you this weird muscular restlessness so you can’t stop moving but every time you move it hurts. It gives you a feeling of impending doom. Incredible. Patients believe they’re going to die and they’re so certain of it that they’ll actually beg their doctors to kill them just to get it over with. And all of this from this little tiny jellyfish.”

“It’s difficult to know how many victims the Irukandji have claimed,” writes well-known Australian biologist Tim Flannery, since “many deaths have doubtless been put down to stroke, heart attack or drowning.”

Signs and Symptoms of Irukandji Syndrome

Low back pain
Generalized muscle pain
Chest pain
Abdominal pain
Catecholamine Excess
Sweating, localized or generalized
Anxiety (feeling of impending doom)
Pallor or peripheral cyanosis
Cerebral edema
Cardiopulmonary Decompensation
Acute pulmonary edema
Left ventricular dysfunction
From Gussow, Leon MD The Amazing and Bizarre Discovery of Irukandji Syndrome,
Emergency Medicine News: April 2005 – Volume 27 – Issue 4 – p 44-45

The Irukandji syndrome was named in 1952 by Hugo Flecker, after the Aboriginal tribe living near Cairns, North Queensland, Australia (probably no coincidence that the most dangerous box jellyfish is named Chironex fleckeri).

The jellyfish Carukia barnesi was named after Jack Barnes, the scientist who discovered the species responsible for the condition.  In one of the more fascinating stories of medical discovery, Barnes established the causative relationship by placing an Irukandji jellyfish against his upper arm, along with that of his 9-year-old son and a local lifeguard.  Within minutes all three began to suffer from severe back and abdominal pain, generalized muscle cramping, spasm, nausea and vomiting, and had to be rushed to hospital. 

Fortunately, all three survived the ordeal, thanks to opiate painkillers, and managed to recover completely within 24 hours. I wonder if Barnes’ son ever volunteered for another medical experiment?!

What to do if stung by an Irukandji

Here is the latest medical advice for dealing with an Irukandji sting, as recommended by the Australian Resuscitation Council, the American Heart Association, and the Red Cross.

Disclaimer: This information is not intended nor implied to be a substitute for up-to-date professional medical advice. For the latest first aid recommendations, consult the Australian Resuscitation Council (see Guideline 9.4.5).

As most Irukandji stings will result in severe symptoms, you should alert emergency medical services as soon as possible, and have the patient transported to hospital. Respiratory failure can ensue, prompting the need for CPR, oxygen, non-invasive ventilation, or even endotracheal intubation.

Immediately Douse With Vinegar

It is important for any suspected Irukandji sting to liberally douse the area with vinegar as early as possible to neutralize remaining stinging cells. In tropical Australia, vinegar (4%-6% acetic acid) has traditionally been used as first aid for the treatment of stings of the deadly box jellyfish C. fleckeri, the irukandji C. barnesi and many other species.

What does vinegar do?

Vinegar will not alleviate the pain of jellyfish stings or help with any potential scarring.  Most importantly, though, vinegar will neutralize any undischarged nematocysts to prevent further stings.   Most of the stinging cells remain undischarged on the initial sting and, over time, stinging cells remaining on the skin will continue to inject more venom until vinegar is applied.

If vinegar or another acetic acid solution is not available, nematocysts can be washed away with seawater.  Do not douse the area with fresh water as this may cause undischarged stinging cells to fire off and worsen the symptoms. 

Can dead jellyfish still sting? Yes, read my article to find out why. Can Dead Jellyfish Sting?  How Do Stinging Cells (Nematocysts) Work? Is Vinegar Effective?

Jellyfish stings can be life threatening, so it’s vitally important to provide effective first aid

If someone is stung, you should immediately:

• Call for an Ambulance
• Provide emergency care including CPR if needed
• Treat the sting by liberally pouring vinegar onto the affected area  Note –vinegar will only prevent the stinging cells from firing but will not stop the pain. If vinegar or another acetic acid solution is not available, nematocysts can be washed away with seawater. Care should be used not to expose the nematocysts in the skin to freshwater, as this might lead to a massive discharge of nematocysts
• Seek medical aid and transport to hospital
If possible, remove any tentacles from the skin using forceps (do not use bare fingers)

Where are Irukandji-type jellyfish found?

Irukandji syndrome is usually associated with jellyfish found in coastal waters of tropical Australia, where hospitalizations range from 50-100 annually and two fatalities have been attributed to this condition.  It may surprise many people that this syndrome has been reported throughout the world, in places as widespread as Hawaii, the Caribbean, Asia, Europe, North America, and South America.

Without doubt, Irukandji jellyfish, and therefore Irukandji syndrome, can occur anywhere in the tropical and temperate seas of the world.  There is growing evidence that Irukandji stings are just as common overseas as in tropical Australia, if not more so, but have been recognised longer and more well studied in Australia, thus skewing the records.

Dr. Lisa-ann Gershwin PhD, Australian Marine Stinger Advisory Services

Here’s more of what Dr. Lisa-ann Gershwin has to say:

“And by the way, you might be thinking, I’m not going to Australia, they’ve got these jellyfish there. Well, guess what? They’re in North America also and they’re in Europe and they’re in South America and they’re in India and they’re in South East Asia and they’re in Hawaii, they’re all over the place, they’re in the Florida Keys. We get Irukandjis, believe it or not, from North Wales in the UK, to Melbourne in southern Victoria and Cape Town in South Africa. So pretty much in all the usable oceans of the world we get Irukandjis. Just because you haven’t heard of them doesn’t mean they’re not there.”

Irukandji, along with box jellyfish, are seasonally common throughout North Queensland and the Great Barrier Reef area, posing the threat of serious injury and even death. Venomous jellyfish cause hundreds of hospitalizations annually. The financial impact on tourism can be significant – the cost of cancelled tourism bookings following two fatalities on the Great Barrier Reef in 2002 was more than $65 million.  According to the Queensland tourism industry, Irukandji are the number two reason for Australians taking their tourism dollars overseas.

In recent years, a scientific research project was initiated to develop real-time forecasting models in major stinger hotspots in Queensland, based on environmental conditions and water quality.  Unfortunately, the project was cancelled due to lack of funding before it was completed.

Number of recorded Irukandji stings in North Queensland 1985-2016.  The dark blue spots indicate locations where more than 100 stings have occurred during that period.
Photo of beach with sign indicating beach closed due to marine stingers.
Beach closed due to marine stingers, possibly Irukandji. Photo by Lisa-ann Gershwin CC4.0
Photo of beach in Australia with stinger nets.
Stinger nets are effective for box jellyfish, but not Irukandji. Photo Lisa-ann Gershwin CC4.0

Stinger-resistant swimming enclosures have supported the growth of tourism in North Queensland by offering protection against the deadly box jellyfish. However, the much smaller Irukandji can pass through the enclosure net and beaches must be closed when conditions require.

The Gear We Love to Travel With

We love to travel in search of exceptional wildlife viewing opportunities and for life-enhancing cultural experiences.

Here’s some of the gear we love to travel with for recording our adventures in safety and comfort:

How can you protect yourself against Irukandji stings?

The best protection is to avoid swimming in the ocean altogether.  Seems a bit extreme though, doesn’t it?!  Given that we face risks every day of our lives while driving in traffic, riding a bicycle, crossing a busy street, or hopping on a jet plane, a better solution might entail realistic risk assessment and mitigation of risks, where possible.

Stinger suits

One of the best protective measures against Irukandji in tropical waters is a full-body suit or, as commonly referred to in Australia, a “stinger suit.” 

Before the 1980s, surf life savers experimented with wearing two pairs of nylon pantyhose for jellyfish protection.  The bottoms were worn as usual, while the other pair were pulled over the arms with a hole cut out of the crotch for the head and neck.  (I’m still trying to find a photo of a buff surf lifesaver wearing two pairs of pantyhose!) In the early 1980s, a full-body Lycra suit was developed through collaboration between James Cook University, Townsville General Hospital, and Surf Life Saving Australia.

Whenever we snorkel or swim in tropical seas, we wear lycra tops with long sleeves and lycra bottoms with full-length pants.

We wear lycra suits whenever snorkeling or swimming in the ocean – anywhere in the world. Lycra suits can be easily rolled up to fit inside your suitcase and take up minimal space. We wouldn’t travel without one!!

You can also purchase a more stylish one-piece lycra suit that looks more like a wet suit.

A one-piece suit with a hood provides the best protection, having been proven to minimize the risks of jellyfish stings.  We often go snorkeling without a hood, but there is a chance we could receive a sting on exposed skin around our neck or face. No stinger suit will provide absolute protection.  However, a well-designed one-piece stinger suit with a hood can go a long way towards mitigating the risks.  That said, I would not enter the ocean in North Queensland during the November to May stinger season even if I was wearing a stinger suit.  The risk is simply not acceptable to me personally.

Photo of people wearing lycra stinger suits.
Early stinger suits were not flattering regardless of body type!

In Australia, Surf Life Saving recommends a full body Lycra suit for normal recreational activities and requires two Lycra suits or a neoprene wet suit, plus booties and gloves, for researchers monitoring for Irukandji on high-risk days.  James Cook University requires a minimum 0.5-mm neoprene wetsuit, plus booties, gloves, and a hood, for sampling in marine stinger-infested waters.

Here’s a recent find! You can get these full-body suits on Amazon for a reasonable price and they will even protect your feet, hands and head!! And they come in various colors. (Some of these colors could even scare a blind jellyfish!)

According to Dr. Gershwin one of the common questions when wearing a standard stinger suit is, ‘‘what about my hands, feet, and face?’’  Researchers in Queensland maintain an Irukandji Sting Database which provides data on actual sting sites (body, legs, etc).  In 2005–2006, for example, of 101 recorded Irukandji stings, 66 have sting site data available; 46 (69%) were stung on a part of the body that would have been covered by a standard Lycra suit; and an additional three would have been prevented with foot protection such as booties. 

Face protection masks are available and researchers have also determined that a layer of petroleum jelly can provide a measure of protection for the face.


Courtney, R., 2019. Understanding Irukandji Jellyfish Life-Cycle and Ecology – The Key To Avoiding Stings.

Courtney R., Sachlikidis N., Jones R., Seymour, J., 2015. Prey capture ecology of the cubozoan Carukia barnesi. PLoS One,

Flecker, H. Fatal stings to North Queensland bathers. Med J Aust. 1952 Jan 12;1(2):35-8. PMID: 14909885

Gershwin, L. Interview on ABC radio by Bob McDonald. The Science Show with Robyn Williams

Gershwin, Dr. Lisa-Ann, Stung! On jellyfish blooms and the future of the ocean

Gershwin, L. and Dabinett, K., 2009. Comparison of eight types of protective clothing against Irukandji jellyfish stings. Journal of Coastal Research, 25(1), 117–130. West Palm Beach (Florida), ISSN 0749-0208.

Gussow, Leon MD The Amazing and Bizarre Discovery of Irukandji Syndrome, Emergency Medicine News: April 2005 – Volume 27 – Issue 4 – p 44-45

Kong EL, Nappe TM. Irukandji Syndrome. [Updated 2022]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from:

Montgomery, L., Seys, J., Mees, J., 2016. To Pee, or Not to Pee: A Review on Envenomation and Treatment in European Jellyfish Species.  Mar. Drugs 2016, 14(7), 127;

Figure 1 Light microscopic view of the Irukandji jellyfish ( Carukia barnesi) (10 mm diameter at base). Photograph taken by Lisa-Ann Gershwin.