Box Jellyfish (Chironex fleckeri): Facts, Venom and Range

What does the Australian box jellyfish look like?

The body of an adult box jellyfish is a pale blue, almost clear bell shaped like a cube with rounded edges. From each of its four lower corners grows a fleshy, hand-like arm called a pedalium, and each pedalium carries a cluster of up to 15 ribbon-like tentacles. That gives a large animal up to 60 tentacles in total.

Inside the bell hangs a short, trunk-like tube called the manubrium, with the mouth at its tip. Around the opening of the bell runs a thin shelf of tissue, the velarium, which narrows the opening like the nozzle of a hose. On each of the four sides of the bell, a little above the rim, sits a small sense organ called a rhopalium, which carries six eyes.

Because the bell and tentacles are so transparent, the box jellyfish is almost impossible to see in murky coastal water. That invisibility is a big reason why it is so dangerous to swimmers.

The species was named in the mid-1950s. In January 1955 a five-year-old boy died after a sting in shallow water at Cardwell, Queensland. The North Queensland doctor Hugo Flecker collected jellyfish at the scene and sent a box-shaped one to the zoologist Ronald Southcott, who described it as a new genus and species and named it after Flecker. The genus name combines Chiron, the centaur of Greek myth, with the Latin nex, meaning “murder.”

How big is a box jellyfish?

Adult Chironex fleckeri bells usually measure about 16–24 cm (6–9.5 in) across, and the largest reach about 30–35 cm (12–14 in). A big one can weigh up to about 2 kg (4.4 lb). The tentacles change length dramatically: while the jellyfish swims they contract to about 15 cm (6 in), but when it hunts they relax and stretch to as much as 3 m (10 ft).

Most other box jellyfish are far smaller. Many species have bells only a few centimeters across, and the notorious Irukandji jellyfish are about the size of a fingertip.

Where does the Australian box jellyfish live?

Chironex fleckeri lives along the tropical northern coast of Australia, from around Exmouth in Western Australia, across the Northern Territory, to about Agnes Water in central Queensland. It has also been reported from New Guinea north to Indonesia, the Philippines, Malaysia and Vietnam, but some of those records may belong to close relatives, such as the Japanese Chironex yamaguchii and the Thai Chironex indrasaksajiae, described in 2009 and 2017.

The jellyfish prefers calm, shallow water off sandy beaches, near river mouths, estuaries and mangrove channels. It is rarely found on the outer Great Barrier Reef. During storms and rough seas it seems to move into deeper, calmer water. Our guide to where box jellyfish live covers the other species found around the world.

How box jellyfish swim and sense the world

Unlike most jellyfish, which drift with the currents, box jellyfish are strong, active swimmers. The bell contracts powerfully, and the narrowed opening forces water out in a jet. Chironex fleckeri can reportedly reach about 4 knots (7 km/h, 4.5 mph) in short bursts, though it usually cruises much more slowly, and it can turn sharply in a few strokes.

Box jellyfish have 24 eyes, six on each of their four rhopalia. Two of the six on each rhopalium have a lens, a cornea and a retina, much like a simple camera eye. In experiments, Chironex fleckeri is attracted to light and swims away from dark objects, which helps it avoid obstacles such as posts and rocks. It has no brain, so the eyes and other sense organs are linked by a ring of nerves around the bell. Read more in do box jellyfish have eyes?.

Field studies in Queensland suggest that box jellyfish are most active by day and spend much of the night resting on the sandy bottom.

What do box jellyfish eat?

The Australian box jellyfish is a predator. Young jellyfish feed mainly on small shrimp, especially the tiny Acetes shrimp that swarm in shallow coastal water. As they grow, they take more prawns and small fish.

The jellyfish hunts by swimming with its tentacles trailing behind it. When a shrimp or fish touches a tentacle, thousands of stinging cells fire at once. Each cell, called a cnidocyte, contains a coiled capsule (nematocyst) with a hollow thread that shoots out like a microscopic harpoon and injects venom. The prey is paralyzed or killed almost instantly, and the pedalium folds up to carry it to the mouth.

Fast-acting venom matters to an animal with soft, fragile tentacles. A fish that struggled for long could tear them, so the box jellyfish needs to stop its prey within seconds.

How dangerous is the box jellyfish’s venom?

Chironex fleckeri venom is a complex mixture of proteins. Some of the most important are pore-forming toxins that punch holes in cell membranes. In laboratory animals this lets potassium leak out of cells, which can disturb the heartbeat and lead to sudden cardiac arrest. The venom also causes intense, burning pain and can damage the skin badly enough to leave lasting scars.

In the worst cases, a person can die within two to five minutes, sometimes before reaching the shore. At least 64 deaths have been recorded in Australia since the 1880s, and rough estimates suggest that box jellyfish stings kill more people each year in parts of Southeast Asia, such as the Philippines, where records are incomplete.

Yet most stings are not life-threatening. A study of 225 box jellyfish stings in Australia’s Northern Territory from 1991 to 2004 found that only 8% of patients needed hospital admission and only one person died, a three-year-old child. Serious stings usually involve long lengths of tentacle across a large area of skin. The full picture is in our article how dangerous is a box jellyfish?, and the species tops our list of the most venomous animals.

Life cycle and lifespan

Box jellyfish have two body forms. Adults (medusae) are thought to spawn in or near estuaries, releasing eggs and sperm into the water. A fertilized egg develops into a tiny larva that settles on a hard surface, often the underside of a rock in a creek or estuary, and grows into a polyp only 1–2 mm long with a couple of tentacles.

The polyp feeds on plankton and buds off copies of itself. Then, at the start of the warm season, each polyp transforms completely into a single young medusa, which swims out to the coast. True jellyfish such as the moon jellyfish work differently: their polyps split into stacks of many baby jellyfish.

The adult stage is short-lived. Tiny crystals in the rhopalia, called statoliths, add growth layers that appear to form daily, and they show that wild box jellyfish grow fast and live only a few months. The adults die after spawning at the end of the season. In captivity, the longest-lived individuals have survived about nine months.

Predators and conservation status

Few animals can eat a box jellyfish safely. Sea turtles are the best known exception. The green sea turtle eats box jellyfish without apparent harm, and some fish and crabs are also reported to feed on them.

The Australian box jellyfish has not been assessed for the IUCN Red List, so its status is Not Evaluated. It is common in season across a large range, and no threats to its survival are known. Scientists do watch how warmer seas might change where and when dangerous jellyfish appear, since some Irukandji species have been reported farther south in recent years.

Box jellyfish and people

In northern Australia, the box jellyfish shapes beach life. During the “stinger season,” roughly October or November to May or June, warning signs go up, and at many popular beaches swimmers use netted enclosures that keep large box jellyfish out. Lifeguards recommend swimming only at patrolled beaches and wearing full-body Lycra “stinger suits,” because box jellyfish stinging cells fire mainly in response to chemicals on human skin and cannot get through the fabric.

Avoiding the water during warnings, keeping away from beached jellyfish (detached tentacles can still sting) and watching children closely are the most effective protection. If anyone is stung, seek emergency medical help immediately: call 000 in Australia, 112 in the EU, 999 in the UK or 911 in the US.

Box jellyfish venom is also a subject of medical research. An antivenom has been made in Australia since 1970, although doctors still debate how much it helps, and researchers are studying how the venom attacks cells in the hope of better treatments.

Myths and facts

Myth: box jellyfish are true jellyfish. Fact: they belong to their own class, Cubozoa, separate from the true jellyfish (Scyphozoa). Their eyes, active swimming and one-polyp-one-medusa life cycle set them apart, and like all jellyfish they are not fish at all but relatives of corals and sea anemones.

Myth: box jellyfish just drift with the tide. Fact: they swim actively, steer around obstacles and choose where to go.

Myth: every box jellyfish sting is fatal. Fact: most stings cause pain and skin marks but are not life-threatening. Deaths are rare and usually involve heavy stings, often to children.

Myth: one box jellyfish has enough venom to kill 60 people. Fact: this figure is repeated widely but has never been reliably measured. What is certain is that a large box jellyfish can kill an adult.