Box jellyfish venom is the most deadly in the animal kingdom and has caused at least 5,568 recorded deaths since 1954. Each tentacle has about 500,000 cnidocytes which are harpoon-shaped needles that inject venom into the victim. Most often, these fatal envenomations are perpetrated by the largest species of box jelly, Chironex fleckeri, owing to its high concentration of nematocysts, though at least two deaths in Australia have been attributed to the thumbnail-sized Carukia barnesi. Those who fall victim to C. barnesi suffer severe physical and psychological symptoms known as Irukandji syndrome. The box jellyfish actively hunts its prey, rather than drifting as do true jellyfish. It is capable of achieving speeds of up to 4 knots (1.8 m/s).