Irukandji jellyfish are tiny and extremely venomous jellyfish that are found the world over, and which cause symptoms collectively known as Irukandji syndrome. Its size is roughly no larger than a centimetre cubed.
Irukandji are usually found near the coast, attracted by the warmer water, but blooms have been seen as far as five kilometres offshore.
Its venom is very powerful, 100 times as potent as that of a cobra, and 1000 times as potent as that of a tarantula.
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.
Irukandji syndrome is produced by a very small amount of venom and includes severe pains at various parts of the body (typically 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, high heart rate and blood pressure, and psychological phenomena (such as the feeling of impending death).
So far, there are two known species of Irukandji:
Carukia barnesi and the recently discovered Malo kingi. The sting itself is only moderately irritating; the severe syndrome is delayed for 5 to 120 minutes (30 minutes on average). The symptoms may last from hours to several days, and victims usually require hospitalization. As with box jellyfish, vinegar will deactivate unfired nematocysts on the skin, but has no effect on the venom already in the body.
Treatment is symptomatic, with antihistamines and anti-hypertensive drugs used to control inflammation and hypertension and intravenous opiates, such as morphine and fentanyl, to control the pain.