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Chironex fleckeri, commonly known as "box jellyfish", 'marine stinger' and formerly the 'sea wasp', is a highly venomous species of box jellyfish. It is a very fast swimmer and has quite sophisticated eyes.

Chironex fleckeri grows to approximately the size of a basketball, is nearly transparent and has four clusters of 15 tentacles. When the jellyfish are swimming the tentacles contract so they are about 15cm long and about 5mm in diameter, when they are hunting the tentacles are thinner and about three metres long. The tentacles are covered with stinging cells or nematocysts which are activated by pressure and a chemical trigger: they react to proteinous chemicals.

The polyps are found in estuaries in northern Australia, the medusa is pelagic and is found in the coastal waters of northern Australia and adjacent areas of the tropical Indo-West Pacific, and are also found in southeastern Asia. They are not usually found on the reef.

In common with other box jellyfish, Chironex fleckeri have four eye-clusters with twenty-four eyes. Some of these eyes seem capable of forming images, but it is debated whether they exhibit any object recognition or object tracking and it is not known how they process information from their sense of touch and eye-like light detecting structures. Chironex fleckeri live on a diet of prawns and small fish and are themselves prey to turtles.

Box jellyfish warning signpost at a Cape Tribulation beach in Queensland, Australia.

Chironex fleckeri appear to avoid human beings when they are close to them and so can be said to avoid stinging humans. Their sting is incredibly powerful and extensive stings can be rapidly fatal. The sting produces excruciating pain accompanied by an intense burning sensation, and the venom has multiple effects attacking the nervous system, heart and skin at the same time. While an appreciable amount of venom (about ten feet or three metres of tentacle) needs to be delivered in order to have a fatal effect on an adult human, the potently neurotoxic venom is extremely quick to act. Fatalities have been observed as little as four minutes after envenomation, notably quicker than any snake, insect or spider and prompting its description as the world's deadliest venomous animal. Although an antivenin exists, treating a patient in time can be difficult or impossible.

Broken tentacles remain active until broken down by time and even dried tentacles can be reactivated if wet.

Dousing a sting with vinegar immediately inhibits any nematocysts which have not been activated, while rubbing a sting exacerbates the problem. Water, urine and "Coke" have been shown to be ineffective. After dousing with vinegar, rescue breaths and CPR may be required. Adhering tentacles should be removed carefully from the skin using protected hands or tweezers. The Australian snake bite treatment of using roller bandages to bandage the affected limb (with the aim of preventing distribution of the venom through the lymph and blood circulatory systems) was no longer recommended for box jellyfish envenomation after 2005. The change was prompted by research which showed that using bandages to achieve tissue compression provoked nematocyst discharge, despite the use of vinegar. Antivenin must be administered urgently, therefore contacting the ambulance service is of paramount importance.

The Box Jellyfish is estimated be the cause of at least one death a year in Australia, and the record has been set to about 67 or more, however this figure is somewhat questionable, since a number of those deaths can also be attributed to heart attack or drowning during a Box Jellyfish sting.

• Chironex fleckeri on oceanfootage.com
• Chironex fleckeri on fccj.org
• Emergency treatment
• Inactivation of nematocysts using acetic acid
• Australian Institute of Marine Science - Sea wasp information page
• Marine Medic page about box jelly and other Australian poisonous marine creatures
• Management of various animal envenomations including box jelly