Australian box jelly, Sea wasp
Chironex fleckeri, commonly known as the Australian box jelly, and nicknamed the sea wasp, is a species of extremely venomous box jellyfish found in coastal waters from northern Australia and New Guinea to Indonesia, Cambodia, Malaysia and Singapore, the Philippines and Vietnam. It has been described as "the most lethal jellyfish in the world", with at least 64 known deaths in Australia from 1884 to 2021.
Notorious for its sting, C. fleckeri has tentacles up to 3 m (10 ft) long covered with millions of cnidocytes which, on contact, release microscopic darts delivering an extremely powerful venom. Being stung commonly results in excruciating pain, and if the sting area is significant, an untreated victim may die in two to five minutes. The amount of venom in one animal is said to be enough to kill 60 adult humans.
Chironex fleckeri is the largest of the cubozoans (collectively called box jellyfish), many of which may carry similarly toxic venom. Its bell usually reaches about 16 cm (6.3 in) in diameter but can grow up to 35 cm (14 in). From each of the four corners of the bell trails a cluster of 15 tentacles. The pale blue bell has faint markings; viewed from certain angles, it bears a somewhat eerie resemblance to a human head or skull. Since it is virtually transparent, the creature is nearly impossible to see in its habitat, posing significant danger to swimmers.
When the jellyfish are swimming, the tentacles contract so they are about 150 mm (6 in) long and about 5 mm (0.20 in) in diameter; when they are hunting, the tentacles are thinner and extend to about 3 m (9.8 ft) long. The tentacles are covered with a high concentration of stinging cells called cnidocytes, which are activated by pressure and a chemical trigger; they react to proteinous chemicals. Box jellyfish are day hunters; at night they are seen resting on the ocean floor.
In common with other box jellyfish, C. fleckeri has four eye-clusters with 24 eyes. Some of these eyes seem capable of forming images, but whether they exhibit any object recognition or object tracking is debated; it is also unknown how they process information from their sense of touch and eye-like light-detecting structures due to their lack of a central nervous system. They are attracted to light of different colors (white, red, orange, yellow, green and blue), but blue light seems to elicit a feeding behavior, as it slows down their pulsation rate and makes them stream out their tentacles. Black objects, on the other hand, cause them to move away.
Chironex fleckeri lives on a diet of prawns and small fish, as well as crabs and other pelagic invertebrates. Their only known predators are green sea turtles and leatherback turtles, whose thick skin is impenetrable to the cnidocytes of the jellyfish, among other pelagic predators.
The medusa is pelagic and has been documented from coastal waters of Australia and New Guinea north to the Philippines, Malaysia, Singapore and Vietnam. In Australia, it is known from the northern coasts from Exmouth to Agnes Water, but its full distribution outside Australia has not been properly identified. To further confuse, the closely related and also dangerously venomous Chironex yamaguchii was first described from Japan in 2009. This species has also been documented from the Philippines, meaning the non-Australian records of C. fleckeri need to be rechecked. Breeding occurs in lower levels of rivers and mangrove channels.
Chironex fleckeri is capable of both sexual and asexual reproduction and are oviparous.
