The Box Jellyfish: Australia’s Other Marine Killer

The brilliant tropical sun seems to pause in its daily routine to warm the beaches of North Queensland, Australia. The turquoise water glistens, reflecting its light. A line of bodies lie in the soft sand, allowing the sun to bronze their skin. One sunbather turns over to face the warm rays. Some energetic young people play a rousing game of volleyball. An elderly couple read under a palm tree, glancing up occasionally to watch the volleyball players.

The beach seems ideal, but something is wrong. The still water seems so inviting on this hot day, but no one is swimming. Red warning flags flap in the wind, lining the beach like sentries. Where the grass meets the sand a giant yellow sign depicts a large jellyfish’s tentacles encasing an unfortunate swimmer.

From October to May, the North Queensland beaches are haunted by a deadly animal, often forcing beach closures. The creatures that cause such havoc belong to a group of animals related to the jellyfish. Scientists named them Cubozoans for their box-like shape. Local residents call them box jellyfish or stingers.


Box jellyfish have a square-shaped float with tentacles streaming from the corners of the bell. All box jellies can sting using nematocysts, tiny stinging cells that line the other surface of the animal. When an animal touches the box jellyfish, the nematocysts fire, puncture the victim and eject venom. The venom paralyzes and kills the box jelly’s prey, which are usually invertebrates.

The venom of some box jellies can be deadly to humans as well. Of the 28 known species, only three can cause death in humans. These live in the Gulf of Mexico, Japan and Australia, according to Jamie Seymour. Seymour is the leading researcher of box jellyfish at James Cook University’s Tropical Australia Stinger Research Unit (TASRU) and has been studying these animals for 10 years.

An encounter with one type of box jellyfish - Australia’s sea wasp (Chironex fleckeri) - could be fatal. The sea wasp possesses the most potent venom of any marine creature. A brush of a mature sea wasp’s tentacle causes heart failure and death within minutes.

On March 23, 2003, a seven year-old boy was swimming on an unprotected beach near Cairns, Australia. Suddenly he began screaming in pain. His grandfather rushed into the water and pulled the boy out of the water. After 10 minutes the child suffered full cardiac arrest. This boy was the 68th person to die from a sea wasp since 1883.

At a glance, the sea wasp may appear like a jellyfish, but nothing could be further from the truth. Unlike true jellyfish, all box jellyfish can swim. The sea wasp has been clocked at three knots or 3.35 miles per hour. Compared to true jellyfish that drift aimlessly on the wind and current, the sea wasp is a speed demon.

The body of a sea wasp may grow to be as large as a basketball. Long tentacles dangle from the four corners of the sea wasp’s body. Adults may have up to 15 tentacles, three meters long, on each corner.

The animal even has eyes, three on each corner. Two of the eyes in the set detect only light, giving the animal a sense of direction. The third eye is an image-forming eye complete with a lens, like our own eyes.

“The animals have eyes but no brain. No one knows how they process what they see,” Seymour says.