The box jellyfish is the world's most venomous animal, equipped with millions of microscopic venomous harpoons on its tentacles.

The Australian box jellyfish (Chironex fleckeri) is widely considered the most lethal marine animal known to science. Its venom delivery system relies on roughly 5,000 stinging cells, or cnidocytes, located on each of its 15 tentacles. When triggered by touch or chemical signals, these cells launch microscopic venomous harpoons at an acceleration of 40,000g, completing the strike in under 700 nanoseconds.The venom is a complex mixture of proteins that causes rapid hyperkalemia—a dangerous surge of potassium into the bloodstream. This disruption prevents heart muscles from relaxing, leading to cardiac arrest. Research from the University of Sydney has shown the toxin is potent enough to cause death in humans within two to five minutes of contact.Beyond its sting, the box jellyfish possesses a sophisticated visual system with 24 eyes arranged in four clusters called rhopalia. Two of these eye types—the upper and lower lens eyes—can form images and detect color, enabling the creature to navigate through mangrove roots and hunt effectively. This sensory complexity is unique among cnidarians, which typically lack centralized nervous systems or true eyes.