This is the sea wasp or box jellyfish, Chironex fleckeri, a kind of jellyfish of the Cubozoa class:
It inhabits shallow, turbid oceanic waters of Australia and Southeast Asia (although during the breeding season and polyp phase they can be found in rivers).
The bell (“body”) is cube-shaped and translucid (Φ15-24cm). From each of the 4 pedals (“corners”) can arise up to 15 tentacles and can grow up to 3 m long.
Each tentacle has millions of cnidocytes, the stinging cells that inject the lethal venom.
It has photoreceptors, i.e. cells that are capable of detecting variations in light.
Due to its lethality, the only known predator is the green turtle (Chelonia mydas) thanks to its thick skin, that protects it against the venom.
These jellyfish have killed 60 people in the last century, mostly children and young people.
The symptoms of a sting are extreme pain, breath shortage and purple weals… Depending on the amount of venom injected the symptoms may disappear weeks later or may lead to a heart embolism.
It is estimated that all the venom from a single jellyfish could take the lives of about 50 people.
An antidote to the venom has been developed and it can save lives if it is administered quickly.