Box Jellyfish

Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their cube-shaped medusae. Some species of box jellyfish produce extremely potent venom: Chironex fleckeri, Carukia barnesi and Malo kingi are among the most venomous creatures in the world. Stings from these and a few other species in the class are extremely painful and sometimes fatal to humans.

Box jellyfish most visibly differ from the Scyphozoan jellyfish in that they are umbrella shaped, rather than domed or crown-shaped. The underside of the umbrella includes a flap, or velarium, concentrating and increasing the flow of water expelled from the umbrella. As a result, box jellyfish can move more rapidly than other jellyfish. In fact, speeds of up to six meters per minute have been recorded.

The box jellyfish's nervous system is also more developed than that of many other jellyfish. Notably, they possess a nerve ring around the base of the umbrella that coordinates their pulsing movements; a feature found elsewhere only in the crown jellyfish. Whereas some other jellyfish do have simple pigment-cup ocelli, box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are located on each of the four sides of their bell in clusters called rhopalia. This enables them to see specific points of light, as opposed to simply distinguishing between light and the dark. Box jellyfish also have 20 ocelli (simple eyes), that do not form images but detect light and dark; they therefore have a total of 24 eyes. A box jellyfish has the closest thing a known jellyfish has to a brain. Box jellyfish also display complex, probably visually guided behaviors such as obstacle avoidance and fast directional swimming. Tests have

shown that they have a limited memory, and have a limited ability to learn. Research indicates that, owing to the number of rhopalial nerve cells and their overall arrangement, visual processing and integration at least partly happen within the rhopalia of box jelly fish.

Some species have tentacles that can reach up to 3 metres in length. Box jellyfish can weigh up to 2 kg.

The box jellyfish actively hunts its prey (zooplankton and small fish), rather than drifting as do true jellyfish. They are capable of achieving speeds of up to 1.5 to 2 metres per second that's about 4 knots (7.4 km/h; 4.6 mph).

A fully grown box jellyfish has a respectable size: it measures up to 20cm along each box side (or 30 cm in diameter), and the tentacles can grow up to 3 metres in length. Its weight can reach 2 kg. There are about 15 tentacles on each corner. Each tentacle has about 500,000 cnidocytes, containing nematocysts, a harpoon-shaped microscopic mechanism that injects venom into the victim. Many different kinds of nematocysts are found in cubozoans.

The venom of cubozoans is distinct from that of scyphozoans, and is used to catch prey (small fish and invertebrates, including prawns and bait fish) and for defence from predators, which include the butterfish, batfish, rabbitfish, crabs (Blue Swimmer Crab) and various species of sea turtles (hawksbill turtle, flatback turtle). Sea turtles, however, are apparently unaffected by the sting and eat box jellyfish.

Box jellyfish can see. They have clusters of eyes on each side of the box. Some of those eyes are surprisingly sophisticated, with a lens and cornea, an iris that can contract in bright light, and a retina. Their speed and vision leads some researchers to believe that box jellyfish actively hunt their prey, others insist they are passive opportunists, meaning they just hang around and wait for prey to bump into their tentacles. They certainly are very good at avoiding even tiny objects and probably at least try to avoid humans, too.