The Sea wasp the largest species of box jelly, Chironex fleckeri, has caused the death of nearly 6,000 people.
The box jellyfish actively hunts its prey and is capable of achieving speeds of up to 3.5 knots (1.8 m/s).
The Sea wasp venom is used to catch prey (fish and small invertebrates) and for defense from predators. The Sea wasp is among the most venomous creatures in the world. Stings from such species are excruciatingly painful, either initially or as an after-effect, and are often fatal to their prey and sometimes even for humans.
Sea turtles eat box jellies and are not effected by its venom.
Sea wasp is abundant in the warm waters of northern Australia and drives away most swimmers. Australian researchers have used ultrasonic tagging to learn that these creatures sleep on the ocean floor between 3 am and dawn. It is believed that they sleep to conserve energy and to avoid predators.
The box jellyfish is the only jellyfish with ‘ eyes’ an active visual system. It has four sets of eyes, in clusters one centered on each of the four lateral faces of its bell. Each set consists of six eyes: Four simple photoreceptors capable of distinguishing light from dark, and two more sophisticated “camera eyes”–an “upper” and a “lower”–both comprising a cornea, lens and retina. The lower camera eye also possesses a mobile iris, which expands and contracts in response to light.
This camera eye reveals a spherical lens capable of forming a near-perfect, almost distortion-free image. Despite this, box jellyfish is very farsighted.
Excerpts courtesy of en.wikipedia.org/wiki/Box_jelly