Box Jellyfish Most Venomous Animal in the World

Photo: Box jellyfish with tentacles.

Photo: Box jellyfish with its deadly tentacles

The box jellyfish is the most venomous animal in the world. It is a basketball-sized sea jelly with 4.5 meter long trailing stinging tentacles. It is not aggressive towards humans. But it is almost transparent and difficult to see in the water. In most instances, a swimmer only becomes aware of it once they have been entangled in the jellyfish's tentacles and stung. The pain experienced by its victim is instantaneous and described like being "branded with red hot irons". This leads to cardiovascular failure and death within 2 to 5 minutes. Each box jellyfish has enough venom to kill up to 60 people.

The box jellyfish belongs to a group of animals called cnidarians (pronounced 'cni·dar·i·an'). These animals, which also include corals and sea anemones, have simple sac-like bodies with a single opening surrounded by stinging tentacles. Box jellyfish have a life span of about one year.

The box jellyfish is also known as a Cubozoa, Sea Wasp, Fire Medusa, and Stinger. Its scientific name: Chironex fleckeri which means "hand murderer".

• Learn All About the Box Jellyfish Sting


Box Jellyfish Life Stages Box Jellyfish Lives as Two Different Animals

Photo: Diagram of two life stages of box jellyfish

Box jellyfish have 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).

The box jellyfish egg hatches into a flat, oval-shaped free-swimming larvae called a planula that drifts down and attaches itself to a hard surface under an overhang or crevice. Here it slowly transforms itself into a flower-like polyp, referred to as a juvenile. These polyps have mouths and tentacles to capture and feed on zooplankton such as crustaceans, rotifers, water insect larvae and aquatic mites. In this juvenile stage, the box jellyfish lives as a stationary polyp and behaves like a coral or sea anemone. In spring, the polyps transform into a miniature version of the adult box jellyfish, release themselves from their rock, and begin life as medusae.

Photo: Box jellyfish polyp

The adult box jellyfish has an almost transparent, pale blue, jelly-like, cube-shaped bell about 20cm on each side and is about the size of a basketball. It can weigh as much as 2 kilos and is composed of almost 96% water. Its tentacles can grow to 4.5m.


Box Jellyfish Description Adult Box Jellyfish Medusa

Photo: Diagram of box jellyfish anatomy

A box jellyfish is a diploblastic animal — it has only two layers of tissue. The outer layer is referred to as the ectoderm or epidermis. The inner layer is called an endoderm or gastrodermis. Sandwiched between these two layers is the mesoglea, a non-living jelly-like substance which makes up most of the jellyfish's shape and structure. The bell contains a ring of contractile cells, which function similarly to muscles in other animals and provide the contraction force necessary to propel it through the water.

Think of the jellyfish's body as an inverted bowl. The glazing on the outside is its ectoderm. The glazing on the inside of the bowl is its endoderm. And the clay in-between is its mesoglea. Suppose we compare this to a human. Our outer skin is its ectoderm, and the lining of our mouths, stomachs, intestines, etc. is its endoderm. However, unlike humans, the jellyfish has no specialised organs like a heart, bones, etc., in its mesoglea. It's basically just jelly.

The lower part of the bell rolls inward and joins a membrane with a central opening that can open and close — like the iris of an eye. This membrane is called a velarium (In ancient Rome, a velarium covered the top of the coliseum. Its central opening could be opened and closed to control the weather).

On the inside, towards the dome of the bell, facing the velarium, is an extension of its endoderm, which forms the jellyfish's stomach and mouth. Once prey is presented to it, the mouth expands to reach out and engulf the prey, pulling it back into the animal's stomach for digestion. The prey is digested by gastric juices, nutrients extracted and the waste product ejected through its mouth.

On either side of the mouth and stomach, as slight bulges in the endoderm wall, are the jellyfish's gonads. Depending on its gender, these produce eggs or sperm.

From each of the lower corners of the bell hang four stalks, called pedalium from which hang up to 15 long, slender, hollow, and venomous tentacles that can grow up to 4.5 meters in length.

Photo: Box jellyfish eyes

The box jellyfish has 24 eyes located in a circular band around the bottom edge of its bell. They are grouped into four clusters of six eyes each. Each cluster is referred to as a rhopalia. There is one rhopalia for each of the four sides of the box jellyfish. These rhopalia are sensitive to light, odour, and orientation. They also contain pacemakers that regulate the expansion and contraction of the bell, which controls the jellyfish's movement.

Photo: image of box jellyfish's eye

In each rhopalia, two eyes are pigmented light-sensitive pits, two others are light-sensitive slit eyes, and the other two are the most sophisticated. These more complex eyes, which can see images, are known as the upper-lens and lower-lens eyes. Regardless of the jellyfish's position, the lens of the upper eyes is always pointed upwards. This suggests that the jellyfish is capable of orientation and navigation. The lower eyes are focused downward and help the animal detect obstacles and prey below it. These eyes, like those of humans, include a lens, retina, iris, and cornea. The box jellyfish's eyes allow it to navigate and hunt out its prey. It is thought to be one of the few animals with a 360-degree view of its environment.

The box jellyfish also has gravity sensors called statocysts, close to each rhopalium. These detect the earth's gravitational pull, which also helps the animal balance and orient itself. This species also can detect vibrations, allowing it to detect prey and water turbulence.

The box jellyfish has no brain. Instead, it has a network of neurons, referred to as the 'nerve net', dispersed throughout its mesoglea. This collection of nerve cells overlaps and criss-cross each other creating a microscopic net-like pattern. These neurons interact wherever they cross each other. The nerve net is connected to sensory cells on its endoderm and ectoderm and also to its contractile cells, which function similarly to muscles in other animals. Along with the nerve net, the box jellyfish also has a more complex nerve ring that connects its four eye clusters and tentacle stubs and controls the animal's orientation and movement. Signals travel from one cell to another through this rudimentary network. In this way, information is transmitted and shared without a centralised brain. Since it does not have a central nervous system, it is not sure how the box jellyfish processes visual, spatial, and vibration information it receives from its eyes and other sensors to regulate the pacemakers that control its movement.

The box jellyfish has no gills or lungs. It absorbs oxygen and releases carbon dioxide into the water by diffusion through its ectoderm and endoderm. Body waste, too, is dispensed in this manner. Nutrients are transported throughout its body by diffusion from cell to cell. Given its simple anatomy, it does not need specialised organs. (Note: In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semi-permeable membrane is present, so only the solvent molecules are free to move to equalize concentration.)


Box Jellyfish Tentacles & Sting The Most Deadly Animal in the World

Box Jellyfish Sting

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Photo: Diagram of Box jellyfish stinging cells

Jellyfish have specialised stinging cell on their tentacles and around their mouths called cnidocytes. Within these cells are stingers called nematocysts. Each nematocyst resides in a little silo (like a missile silo) with a tiny touch-sensitive hair-like trigger called a cnidocil. Each nematocyst is loaded with a minuscular harpoon around which is wrapped a long thread, both covered in toxin. When touched or otherwise triggered, thousands of nematocysts fire their deadly harpoons with threads attached. The harpoons either pierce the flesh or the threads ensnare its prey, releasing toxins into its victim paralysing, killing, or scaring it off in excruciating pain.

The toxin fired from the jellyfish's nematocysts attacks the heart, nervous system, and skin cells. The pain experienced by its victim is instantaneous and described like being "branded with red hot irons". This can lead to cardiovascular collapse and death within 2 to 5 minutes.

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Box Jellyfish Swimming Box Jellyfish Movement

Box Jellyfish Swimming Video

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The box jellyfish moves by squeezing its bell and ejecting water through a constricted opening in its velarium. It uses water-jet propulsion to move through the water at speeds of up to six meters/second. That is 7.5 kilometres per hour. By comparison, an average human swimmer can only manage 3.2 kph, and the fastest human swimmer was recorded at 8.6 kph. This squeezing action is the pulsating motion often seen in jellyfish. Because of their mobility and ability to move away from danger, they are rarely found washed up onshore.


Box Jellyfish Habitat Where Do Box Jellyfish Live?

Photo: Box jellyfish habitat map

Box Jellyfish Sleep at Night

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

Box jellyfish live along the north-eastern coastline of Australia, including the Great Barrier Reef. They are also found in the Indo-Pacific oceans further to the north.

Box jellyfish are coastal animals preferring shallow waters with gently sloping beaches with minimal obstructions to snag their long trailing tentacles. They do not venture into the open oceans. It is because of fondness for shallow sloping beaches that these jellyfish and humans confront each other.

When the air is hot, and still, they will come closer to the shore, especially in areas with mangrove forests and river estuaries where their food supply is more plentiful.

• Great Barrier Reef — Coral, Animals, Plants and Attractions


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 feeds during the day.

The box jellyfish captures its prey by trapping them in its tentacles. Even the slightest touch to its tentacles causes thousands of tiny harpoons to be fired from its neocypts, which inject venom into the victim and quickly paralysing it.

Once paralysed, the victim is hauled up by its tentacles through the open velarium by its four pedalium and into its mouth. The mouth engulfs its victim and passing it on into its simple tubular stomach, where it is digested. Digestive waste is ejected through its mouth.


Box Jellyfish Reproduction & Life Cycle Sexual Reproduction in Box Jellyfish

Photo: Diagram of Box jellyfish life cycle

The adult box jellyfish, depending on its sex, releases eggs or sperm into the water. This release, known as spawning, usually occurs once a year in late summer in river estuaries where large populations of jellyfish gather to release their eggs and sperm at the same time to ensure cross-fertilisation.

Photo: Box jellyfish free-swimming larvae (planula)

Within a few days, the fertilised eggs develop into planulae (singular: planula), which are minute, flat, oval-shaped free-swimming larvae. After a few days, these planulae drift down to the estuary floor where they attach themselves to a hard surface, preferably under an overhang or crevice, and slowly transform themselves into flower-like polyps.

They live for about a year.


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 it.

The box jellyfish are not threatened or endangered.


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.

25 Box Jellyfish Facts

  1. The box jellyfish is the world's deadliest animal.
  2. It lives along the north-eastern coastline of Australia.
  3. The box jellyfish can grow as large as a basketball. That is 30 cm or 12 in in diameter. It can weigh up-to 2kg or 2.5 lbs.
  4. From each of the lower corners of its body hang up to 15 long, slender, hollow, and venomous tentacles.
  5. These can grow up to 4.5m in length.
  6. When these tentacles are touched, they fire tiny harpoons to paralyse its prey.
  7. They prey trapped in its tentacles is then hauled up and eaten.
  8. Its venom can kill a human in 2 minutes.
  9. Its venom kills by paralysing the heart in a contracted state.
  10. The best remedy for a sting is vinegar, CPR, and urgent medical attention.
  11. The box jellyfish feeds and poops from the same multifunction opening.
  12. It is composed of 96% water.
  13. The box jellyfish has 24 eyes.
  14. But it has no brain, gills, lungs, or heart.
  15. It is near invisible in water.
  16. The box jellyfish is not a jellyfish at all because it can swim (other jellyfish can't).
  17. It has gravity sensors to help it balance and orient itself.
  18. It can swim at twice the speed of humans.
  19. It starts off life as a polyp similar to a coral.
  20. It has no interest in attacking people.
  21. Light-skinned people are stung more often than dark-skinned people.
  22. A group of jellyfish is called a bloom or swarm.
  23. Because it can move, box jellyfish actively hunt for prey.
  24. In Australia, the box jellyfish has caused at least 79 deaths since 1883.
  25. The last death was in 2021.