Irukandji – The Smallest Jellyfish In The World

By D.C. Demetre •  Updated: 07/25/22 •  4 min read
Irukandji jellyfish dome close up

3× enlargement of the jellyfish’s bell, which is shaped like a gumdrop and is about 1.5 cm long in scale. Credit: GondwanaGirl CC-BY

The world’s smallest jellyfish measures about one cubic centimetre across. It is also one of the most poisonous jellyfish. The Irukandji Jellyfish is a box jellyfish species found off Australia in shallow waters near shore.

The Irukandji Jellyfish (Malo kingi), has four tentacles that are up to 10 centimetres long. This jellyfish can cause Irukandji Syndrome.

Irukandji’s scientific name commemorates research scientist Robert King. In 2002 King and his partner travelled from Ohio to Opal Reef in Australia for snorkelling.

While there he was stung by a box jellyfish, which sent him to a hospital suffering from toxin-induced hypertension. He died in hospital after hypertension led to bleeding in multiple brain areas.

A number of toxin-containing barbs were recovered from King’s body and examined. CSIRO researcher and Australian jellyfish expert Lisa-Anne Gershwin determined the barbs, or nematocysts, matched those from specimens of a new species she had discovered in 1999. Gershwin named this new species M. kingi in 2007.

How Fast Can Irukandji Jellyfish Kill You?

There are two box jellyfish species that are known to cause the syndrome – Carukia barnesi and Malo kingi – however, there may be others, including Malo maxima, Malo filipina and Malo bella.

What Is irukandji Syndrome

Caused by one of the world’s worst jellyfish stings, the syndrome is a group of symptoms that sets in 5-10 minutes after the sting, which according to writer Robert Drewe, is “100 times as potent as that of a cobra and 1,000 times stronger than a tarantula’s”.

Common symptoms are excruciating muscle cramps in the arms and legs, severe pain in the back and kidneys, a burning sensation on the skin and face, headaches, nausea, restlessness, sweating, vomiting, an increase in heart rate and blood pressure, and psychological effects like a feeling of impending doom.

Treatment for the syndrome involves addressing the symptoms, rather than systemic treatment. To control inflammation and hypertension, anti-hypertensives and antihistamines are administered. To manage pain, intravenous opioids are used, for example, morphine or fentanyl.

What Makes the World’s Smallest Jellyfish So Special?

The body of this jellyfish is 96 percent water, and they have no brain central nervous system control. Yet they can actively hunt prey – small quick-swimming – fish by luring them with the stinging structures along their tentacles.

There is not yet much known about the life cycle and venom of Irukandji jellyfish. They are very small and fragile, and their transparent body makes them very difficult to see in the water, so collecting specimens is difficult.

The first of this kind of jellyfish, Carukia barnesi, was identified in 1964 by toxicologist Jack Barnes. In order to verify it was the cause of Irukandji syndrome, he captured the tiny jellyfish and allowed it to sting him, his nine-year-old son and a young lifeguard. They all became seriously ill but thankfully survived.

It seems to be only the mature Irukandji that is highly venomous, and even in adults the toxicity of stings is quite variable, according to Lisa-Ann Gershwin, discussing Barnes:

“His specimens had been in captivity for hours prior to the stinging experiment; the degree to which the condition of the animal affects its sting ability is undocumented, but I have often observed cubozoans stinging each other and the sides of the collecting container. It is not difficult to imagine that they could have expended much of their stinging ability without sufficient time to regenerate it.”

She suffered the stings herself;

“I was personally stung quite extensively across the palms of both hands by this species in June 2003 at Port Douglas, without systemic effects. However, both hands blistered badly and several layers of skin completely peeled about one week after the sting event. The specimen proved to be immature when examined, and was subsequently used for DNA analysis,”

she wrote.

Awareness campaigns are needed in order to teach the public about the dangers of the world’s smallest jellyfish. They need to be aware of how dangerous they can be and how to protect themselves against them. Irukandji syndrome has the potential to be preventable by avoiding areas with jellyfish.


  1. Crew, Becky, The Smallest and Deadliest Kingslayer in the World, October 7, 2013, Scientific American blog
  2. Drewe, Robert (1 November 2015). The Beach: An Australian Passion. National Library of Australia. ISBN 9780642278807
  3. Gershwin, Lisa-Ann (2007). Malo kingi: A new species of Irukandji jellyfish (Cnidaria: Cubozoa: Carybdeida), lethal to humans, from Queensland, Australia. Zootaxa. 1659 (1659): 55–68.
  4. Irukandji jellyfish actively hunt prey, researchers find. ABC Far North Qld. 3 June 2015.

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