You don’t get much deadlier than box jellyfish. In fact, they’re widely deemed the most venomous creatures in all the Earth’s oceans. While there is no definitive record of how many lives they have taken, it’s estimated that around 20 times more people are killed by them each year than sharks.

One of the serial offenders in the ‘boxie’ fraternity is Chironex fleckeri. Although sometimes referred to as sea wasps, compared to their insect namesakes, this seriously underplays the potency of their sting. They are known to be responsible for 63 deaths between 1884 and 1996 in Australia alone.

So what is it that makes box jellyfish so dangerous? As is often the case, it’s a lethal cocktail of factors. First and foremost is its unique venom, which affects vital bodily functions including our nervous and coronary systems. Sometimes the excruciating pain alone can be the cause of death, with the victim fainting and then drowning, or even suffering a heart attack on contact. Another characteristic that contributes to its deadliness is its ability to propel itself in any direction – a rare trait among jellyfish – making it unpredictable. Plus its translucent body and fine tentacles are difficult to spot until it’s too late. Habitat also plays a part, as they favour warm, shallow waters, often bringing them near bathers and snorkellers.

DANGER FACTOR: About 100 deaths globally per year. Survivors can experience weeks of pain and permanent scars.

Bell diameter: 20cm (7.9in)

Tentacle length: < 3m (9.8ft)