You would not want to get too close to a box jellyfish (Chironex fleckeri). It is thought to be one of the most venomous creatures to exist in the oceans and it can kill a person in only a few minutes.
Fortunately for most of us, it is mainly confined to waters to the north of Australia and eastern Indonesia, but it is plentiful enough at certain times of the year (November to April in northern Australia) to make it necessary to close beaches to the public.
It is only a small creature – up to 25 cms (10 inches) in size – but its tentacles can extend up to three metres (10 feet) in length. It is possible to be stung without even seeing the jellyfish itself. Its prey is prawns and small fish that swim in shallow water.
The name comes from the box-shaped transparent body, from each corner of which hangs a bunch of 15 tentacles. The flattened sides of the box contain sense organs that include remarkably complex eyes.
Should a swimmer be unfortunate enough to be stung, and not be able to get hold of anti-venom, the least he or she can expect is excruciating pain and skin damage that can leave permanent scars. Heart failure is entirely possible, as is drowning following loss of consciousness.
Despite the extreme dangers presented by contact with this animal, some sea turtles are able to eat box jellyfish without suffering any harm.