Irukandji jellyfish (// IRR-ə-KAN-jee) are small and extremely venomous jellyfish that inhabit marine waters of Australia and which are able to fire their stingers into their victim, causing symptoms collectively known as Irukandji syndrome. Their size is roughly a cubic centimetre (1 cm3). There are three known species of Irukandji: Carukia barnesi, Malo kingi, and the recently discovered Malo maxima.
The symptoms of Irukandji syndrome were first documented by Hugo Flecker in 1952. They were named after the Irukandji people whose country stretches along the coastal strip north of Cairns, Queensland. The first of these jellyfish, Carukia barnesi, was identified in 1964 by Jack Barnes; in order to prove it was the cause of Irukandji syndrome, he captured the tiny jelly and allowed it to sting him while his son and a lifeguard observed the effects. 
Irukandji jellyfish were at one time thought to be in the northern waters of Australia only. Since then, according to a National Geographic documentary on jellyfish, the species has been found in waters as far north as the British Isles, Japan, and the Florida coast of the United States.
Irukandji jellyfish are very small with a bell about 5 millimetres (0.20 in) to 25 millimetres (0.98 in) wide (or wider) and four long tentacles, which range in length from just a few centimeters up to 1 metre (3.3 ft) in length. The stingers (nematocysts) are in clumps, appearing as rings of small red dots around the bell and along the tentacles.
Very little is known about the life cycle and venom of Irukandji jellyfish. This is partly because they are too small and fragile requiring special handling and containment. Its venom is very powerful. Researchers conjecture that its venom possesses such potency to enable it to quickly stun its prey, which consists of small and fast fish. Judging from statistics, it is believed that the Irukandji syndrome may be produced by several species of jellyfish, but only Carukia barnesi and Malo kingi have so far been proven to cause the condition.
The average jellyfish has stingers only on its tentacles but the Irukandji also has stingers on its bell. Biologists have yet to discover the purpose of this unique characteristic. The hypothesis is that the feature enables the jellyfish to be more likely to catch its prey of small fish.
Irukandji jellyfish differ from other box jellyfish species in that they have the ability to fire stingers from the tips and inject venom. Currently, it is not known whether this is for some special purpose.
Irukandji syndrome is produced by a small amount of venom and induces excruciating muscle cramps in the arms and legs, severe pain in the back and kidneys, a burning sensation of the skin and face, headaches, nausea, restlessness, sweating, vomiting, an increase in heart rate and blood pressure, and psychological phenomena such as the feeling of impending doom. The syndrome is in part caused by release of catecholamines. The venom contains a sodium channel modulator. The sting is moderately irritating; the severe syndrome is delayed for 5–120 minutes (30 minutes on average). The symptoms last from hours to weeks, and victims usually require hospitalisation. Flushing with vinegar, once believed to be useful at neutralizing the tentacle stinging apparatus of box jellyfish, has been shown to amplify rather than palliate the effects of a box jellyfish sting. Treatment is symptomatic, with antihistamines and anti-hypertensive drugs used to control inflammation and hypertension; intravenous opiates, such as morphine and fentanyl, are used to control the pain. Magnesium sulfate has been used to reduce pain and hypertension in Irukandji syndrome, although it has had no effect in other cases.
Irukandji are usually found near the coast, attracted by the warmer water, but blooms have been seen as far as five kilometres offshore. When properly treated, a single sting is normally not fatal, but two people in Australia are believed to have died from Irukandji stings in 2002, greatly increasing public awareness of Irukandji syndrome. It is unknown how many other deaths from Irukandji syndrome have been wrongly attributed to other causes. It is also unknown which jellyfish species can cause Irukandji syndrome apart from Carukia barnesi and Malo kingi.
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