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[Origin of photograph unknown]
Photos
Box jellyfish (Chironex fleckeri)are a hazard to Pacific swimmers and divers. They are a real problem to the swimmers inshore north of the tropic of Capricorn from November through April but are not a problem at islands well offshore or at coral reefs. Chironex is an almost clear, box-shaped jellyfish, quite agile, and very difficult to see in the water, with many tentacles trailing from the four corners of its "bell". It appears characteristically after the first summer storms, probably relating to the presence of its food source, a small shrimp, Acetes australis.
Many Australian beaches have stinger-resistant enclosures for swimmers, which are basically systems of nets anchored to the bottom. There are two species of "box" jelly fish, the smaller one being Chiropsalmus quadrigatus, which has not been associated with deaths but packs a nasty sting.
Box jellyfish are often numerous after rain in the vicinity of creek outlets and are usually absent when the seas are rough. They patrol parallel to the shore gathering shrimps and small fishes beneath the surface, drop down to eat their catch, then bob back up to the surface-causing a characteristic ripple. Sea wasps are agile swimmers and can maintain a pace of 3-4 knots all day, sometimes capable of 5 knots in a sprint. They have four sensory organs that register posture/attitude, change of direction, change of light intensity, and one of their "eyes" has a convex lens capable of forming crude images. When approached they will move away and tend to avoid dark objects in the water. Diving in on top of them has caused many stings-not allowing the jelly fish a chance to get out of the way.
The nematocysts are triggered by both chemical and physical receptors, and even a thin layer of clothing or hair can be protective to swimmers. Pantyhose have been used by life guards in Australia for many years, and now colorful "stinger suits" are frequently seen on the beaches in the summer.
Box jelly fish stings are accompanied by immediate agonizing pain. One with a 75 mm (3 in) diameter bell will be potentially lethal to children; one with a bell 115 mm (4.5 in) can kill an adult. Because their surface to weight ratio is higher and they get a bigger dose, children are much more vulnerable. When extended for feeding the jelly fish tentacles may be 2.5-3 m. (8-10 ft) long; the welts produced have a characteristic ladder-like, cross-hatch pattern.
