Box Jellyfish

The box jellyfish is the most venomous animal in the world. Its sting can kill a human in less than two minutes. Contrary to popular belief, a box jellyfish does not bite - it stings with its tentacles. Nor does a box jellyfish have poison - its tentacles inject venom. Found in the warm coastal waters off the coast of Australia and the Pacific and Indian oceans, the box jellyfish is not aggressive towards humans. However, they live in warm shallow waters, which are also popular with swimmers. In Australia, box jellyfish have caused at least 80 deaths since 1883. The most recent box jellyfish sting death was a 14-year-old boy who died in 2022.

The box jellyfish has a pale blue, almost transparent, jelly-like, cube-shaped body called a bell that can grow to over 30cm in diameter, weigh 2kg and is composed of 96% water. From each corner of its bell hang clusters of tentacles up to 4.5 meters long lined with thousands of deadly stinging cells called nematocysts, which fire toxic darts that can kill its victim in seconds.

The box jellyfish's scientific name is Chironex fleckeri , which means "hand murderer". The box jellyfish is also known as a Cubozoa, Sea Wasp, Fire Medusa, and Stinger. It belongs to a group of animals called cnidarians (pronounced ‘cni·dar·i·an'), which also include corals and sea anemones.

Box Jellyfish Sting Box Jellyfish Attack

Photo: Diagram of cnidocil, nematocysts and barbs

The box jellyfish's sting can be lethal. Its 4.5 meter-long tentacles are covered with hundreds of specialised stinging cells called nematocysts. Each nematocyst contains a tiny toxic barb attached to a long thread. When triggered by touch or motion, thousands of these nematocysts fire their deadly barbs, piercing their victim's flesh, releasing toxins and entangling their prey with their threads which can quickly lead to cardiovascular collapse and death. The pain experienced by its victim is almost instantaneous. It has been described as akin to being "branded with red hot irons". Survival depends on the amount of toxin injected and the speed of detoxification. CPR must be administered immediately until professional help arrives. The only known remedy is vinegar, which can neutralise the effect of the stings and relieve the pain. Interestingly, light-skinned people are more likely to be stung than dark-skinned people because the jellyfish can see the higher contrast of a dark-skinned person in the water and may swim away.

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Box Jellyfish Life StagesBox Jellyfish Lives as Two Different Animals

Photo: Diagram of two life stages of box jellyfish

Box jellyfish has two very distinct life stages. In its juvenile stage, it lives as a stationary polyp similar to a coral or sea anemone. But as an adult, when it is referred to as a medusa (plural: medusae), it takes on a totally different form and floats about in the water. (Think of it as similar to a caterpillar transforming into a butterfly).

Box Jellyfish Polyp

Photo: Box jellyfish polyp

The life cycle of a box jellyfish begins with an egg that hatches into a planula, a flat and oval-shaped larvae that swims freely. Once it attaches itself to a solid surface, it transforms into a polyp that resembles a flower, complete with a mouth and tentacles used to capture prey. During this stage, the box jellyfish remains stationary, much like a coral or sea anemone and feeds on crustaceans, rotifers, water insect larvae, and aquatic mites. As spring approaches, the polyps transform into miniature versions of adult box jellyfish detach from their surface and begins the next stage of its life as moving medusae, or adult box jellyfish.

Adult Box Jellyfish Medusa

Photo: Diagram of box jellyfish anatomy

The adult box jellyfish's body consists of two layers of tissue, the external ectoderm and the internal endoderm, with a non-living mesoglea layer sandwiched between them that gives shape and structure to the body. The jellyfish's body also has a contractile ring composed of cells that provide the force that propels the jellyfish through water. The lower part of the bell rolls inward and joins a membrane with a central opening called a velarium that can open and close — like the iris of an eye. The jellyfish's mouth and stomach are formed from the top of the inside of the bell that expands downwards to reach and engulf its prey, pulling it back into the animal's stomach. Once digested, waste product is ejected through its mouth.

Photo: image of box jellyfish's eye

The box jellyfish has four stalks, called pedalium, hanging from each of the lower corners of the bell. These stalks have up to 15 long, slender, hollow, and venomous tentacles that can grow up to 4.5 meters long. The box jellyfish also has 24 eyes located in a circular band around the bottom edge of its bell, grouped into four clusters of six eyes each. These eyes are sensitive to light, odour, and orientation and contain pacemakers that regulate the expansion and contraction of the bell, controlling the jellyfish's movement. The box jellyfish also has gravity sensors called statocysts, which detect the earth's gravitational pull, helping the animal balance and orient itself. It can also sense vibrations, allowing it to detect prey and water turbulence.

The box jellyfish does not have a brain but instead has an intricate 'nerve net' of neurons dispersed throughout its mesoglea. This neural network allows information to be transmitted without a centralised brain. The box jellyfish also has no gills or lungs and absorbs oxygen and releases carbon dioxide into the water by diffusion through its ectoderm and endoderm. Nutrients are transported throughout its body by diffusion from cell to cell.

Box Jellyfish Swimming Box Jellyfish Movement

Box Jellyfish Swimming Video

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The box jellyfish is a graceful swimmer. To swim it contracts its bell and ejects water through a constricted opening in its velarium to propel itself. Using this water-jet propulsion the box jellyfish can move at speeds of up to six meters/second. That is 7.5 kilometres per hour - faster than an Olympic swimmer! It can also change direction by adjusting the rhythm and strength of the contraction of its body. This squeezing action is the pulsating motion often seen in jellyfish. Because of their superior mobility, they are rarely found washed up onshore.

Box Jellyfish Habitat Where Do Box Jellyfish Live?

Photo: Box jellyfish habitat map

Box jellyfish are primarily found along the north-eastern coast of Australia, particularly near the Great Barrier Reef and in the Indo-Pacific Oceans to the north. They usually inhabit shallow waters near estuaries and bays with gently-sloping beaches where they can swim without entangling their long trailing tentacles. They prefer warm, tropical waters with temperatures between 20°C and 30°C. During mating season, they can also be found in deeper waters up to 100 meters below the surface.

During the night, box jellyfish slowly sink to the bottom of the ocean and settle on the ocean floor to sleep.

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Box Jellyfish Diet What Food Do Box Jellyfish Eat?

Photo: Box jellyfish consuming a fish

The box jellyfish diet consists primarily of small fish, crustaceans, marine larvae, and plankton. It also preys on other jellyfish, not of its own species. Given its good eyesight and its ability to propel itself from place to place, the box jellyfish, unlike other jellyfish, actively hunts prey. They are carnivorous and feed during the day. The box jellyfish captures its prey by trapping them in its tentacles and incapacitating them. The prey is then hauled up by its tentacles into its mouth and stomach, where it is digested.

Box Jellyfish Reproduction & Life Cycle Sexual Reproduction in Box Jellyfish

Photo: Diagram of Box jellyfish life cycle

Box jellyfish reproduce by releasing eggs and sperm into the water. This process, called spawning, generally happens once a year in estuaries where many jellyfish gather. A few days later, fertilised eggs develop into flat, oval-shaped planulae larvae. These tiny larvae drift down to the estuary floor, attach themselves to a hard surface such as an overhang or crevice, and grow into a flower-like polyp and eventually a fully grown box jellyfish.

Box Jellyfish Predators and Threats What Kills Box Jellyfish?

Photo: Green turtles eat jellyfish

Because of its deadly sting, box jellyfish have few predators. Green turtles, with their thick skin, are the only animals that are immune to the stings of this jellyfish and eat them.

Box Jellyfish Conservation Status Are Box Jellyfish Endangered?

Box jellyfish are not endangered. There is anecdotal evidence to suggest that the warming of the oceans caused by global warming is providing ideal conditions for the box jellyfish to thrive in.

What is a Group of Jellyfish Called?

A group of jellyfish is known as a bloom or swarm. Box jellyfish blooms usually occur during the wet season, from about November to April. Blooms are generally a result of behavioural and ecological causes, such as an increase in water temperature or food supply.