Related to the deadly sea wasp, the Irukandji box jellyfish (Carukia barnesi) is very small, about the size of a walnut and almost invisible. The tentacles can be up to three feet long and cause potentially life threatening symptoms known as Irukandji Syndrome. There have been fatalities caused by the Irukandji. The initial sting is not painful, but the venom is a direct cardiac toxin that sets in thirty minutes later causing high blood pressure and fatal strokes. It is found in the coastal waters of Australia and the name comes from an Aboriginal tribe that lived in the region. It can be identified by the 'string of pearls nematocysts (stinging cells) on its tentacles.

The Irukandji (Carukia barnesi) box jelly can be very dangerous.
The Irukandji box jelly