Box Jellyfish Most Venomous Animal in the World

Photo: Box jellyfish with tentacles.

Photo: Box jellyfish with tentacles

The box jellyfish is a sea jelly with a cube-shaped body. It can grow to the size of a basketball with up to 60 venomous 3m (10ft) long stinging tentacles. The sting from a box jellyfish can kill a human in less than 4 minutes. 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.

The box jellyfish is not a fish at all, nor is it a proper ‘jellyfish’. It is not a fish because fishes have backbones, and the box jellyfish does not have a backbone. It isn't a true jellyfish either because, by current definition, a jellyfish drifts along in its environment. The box jellyfish can actively propel itself through the water. There you have it. But we still call it a jellyfish.

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

The box jellyfish lives along the north-eastern coastline of Australia and the Indo-Pacific. Box jellyfish have a life span of about a year.

• Box Jellyfish Sting — Why, How & When

Box Jellyfish Life Stages Box Jellyfish Lives as Two Different Animals

Photo: Diagram of two life stages of box jellyfish

The 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 attached to a surface

The box jellyfish egg hatches into a flat, oval-shaped free-swimming larvae called a planula, which drifts down and attach themselves to a hard surface, preferably under an overhang or crevice, and slowly transform themselves into flower-like polyps. Box jellyfish polyps have a mouth and tentacles to capture and feed on zooplankton, which are are small, aquatic micro-organisms that include crustaceans, rotifers, water insect larvae and aquatic mites.

At this stage of their lives, their appearance and behaviour are very similar to that of a coral or sea anemone polyp. By about in about spring, the polyps transform themselves into a miniature version of the adult box jellyfish, release themselves from their rock and begin lives as medusae.

Box Jellyfish - Medusa

Photo: Box jellyfish with back-lighting

The box jellyfish has an almost transparent, pale blue, jelly-like, cube-shaped bell in its adult stage. It can grow in size to 20cm on each side, which is as large as a basketball. It can weigh as much as 2 kilos and is composed of almost 96% water. Its tentacles can grow to 3m.

Box Jellyfish Description Adult Box Jellyfish Characteristics

Box Jellyfish Body

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.


Box Jellyfish Tentacles

Photo: Diagram of Box jellyfish barbs

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.

Jellyfish have specialised cells on their tentacles and around their mouths called cnidocytes ("stinging cells"). Within these are organs called nematocysts (stingers). 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. Captured prey is passed on to its mouth and stomach.

• Related Article: Box Jellyfish Sting

Box Jellyfish Eyes

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.


Box Jellyfish Gravity Sensors

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.

Box Jellyfish has No Brain

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.

Box Jellyfish Breaths by Diffusion (not Osmosis)

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.)

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.

Box Jellyfish Sting Box Jellyfish Venom Deaths & Treatment

Photo: Jellyfish warning sign

Venomous vs Poisonous What's the Difference?

Wondered why we keep using the term 'venomous' rather than 'poisonous'? Here is the reason. A venomous animal injects or otherwise delivers its toxin into another animal. While a poisonous animal's entire body or parts of it may contain a toxic substance that is harmful if touched or eaten. Because the box jellyfish injects its toxins it is venomous.

The box jellyfish is the most venomous animal in the world. It is claimed that each box jellyfish has enough venom to kill 60 people.

Box Jellyfish Deaths

In Australia, the box jellyfish has caused at least 79 deaths since 1883. The most recent was a 17 year boy who died in 2021. However, box jellyfish deaths are much less than people killed by sharks.

The box jellyfish is not aggressive towards humans. Unfortunately, it too enjoys the warm shallow waters with slightly sloping coastlines also enjoyed by people. Because it is almost transparent with tentacles up to 3 meters long trailing behind, it is very hard 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.

How Does Box Jellyfish Toxin Kill?

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”. The venom causes the victim's cells to become porous and leak potassium, causing hyperkalemia. This can lead to cardiovascular collapse and death within 2 to 5 minutes.

Light-skinned People are More Likely to be Stung

A light-skinned person is more likely to be stung by a box jellyfish than a dark-skinned person. The reason for this is that the eyes of a box jellyfish is more likely to see the dark contrast of a dark-skinned person in the water and realising that it is not pry and swim away. On the other hand, a light-skinned person is not as visible to the jellyfish and is more likely to come in contact with it.

Box Jellyfish Sting The Boy Who Died Twice

video: Boy nearly dies from box jellyfish sting

Box Jellyfish Sting Survival

Survival from a box jellyfish sting depends on the amount of toxin injected into its victim and the speed at which detoxification takes place. Unfortunately, it is very unlikely that a person would survive a full-fledged attack because in most instances it is impossible to administer an antidote quickly enough.

Best Short-term Remedy

The only remedy to date is vinegar, which prevents the discharge of any active neophytes still on the victim's skin and seems to neutralize the effect of the stings and relieve the pain. Since a cardiac arrest is like to occur very quickly, CPR must be administered until professional help arrives.

Box Jellyfish Movement Faster then a Speeding Swimmer

Photo: Box jellyfish swimming

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.

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.