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. Its sting is excruciatingly painful and can kill a human in just two minutes. This basketball-sized sea creature with 4.5 meter-long trailing tentacles is almost invisible in water. Box jellyfish are not aggressive towards humans. However, they live in warm shallow waters, which are also popular with swimmers. This leads to unfortunate incidents where people get entangled in the box jellyfish's tentacles and experience its fatal sting. In Australia, box jellyfish have caused at least 80 deaths since 1883. The most recent was a 14-year boy who died in 2022.

The box jellyfish, 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 have a life span of about one year. Its scientific name is Chironex fleckeri , which means "hand murderer".

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 nematocysts fire their deadly barbs, piercing their victim's flesh, and entangling their prey with threads, releasing toxins. The pain experienced by its victim is almost instantaneous. It has been described as akin to being "branded with red hot irons". This toxin attacks the heart, nervous system, and skin cells, quickly leading to cardiovascular collapse and possibly death.

Light-skinned People are More Likely to be Stung

Light-skinned people are more likely to be stung by a box jellyfish than dark-skinned people. This is because the eyes of a box jellyfish are more likely to see the higher contrast of a dark-skinned person in the water and realise that it is not prey and swims 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.

Survival from a box jellyfish sting depends on the amount of toxin injected into its victim and the speed at which detoxification occurs. 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.

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.

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Box Jellyfish Description 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).

Box Jellyfish Polyp

Photo: Box jellyfish polyp

The box jellyfish starts as an egg and hatches into a planula, or a flat, oval-shaped free-swimming larvae. After attaching itself to a solid surface, it slowly transforms into a flower-like polyp with a mouth and tentacles that are used to catch crustaceans, rotifers, water insect larvae, and aquatic mites. During this juvenile phase, the box jellyfish acts like a coral or sea anemone, staying in one spot. As spring arrives, the polyps transform into miniature versions of the adult version of the box jellyfish and break away from their rocks, beginning the life cycle of moving medusae.

Adult Box Jellyfish Medusa

Photo: Diagram of box jellyfish anatomy

The adult box jellyfish is composed of 96% water, around 20cm across, weighs 2 kilos, and has an almost transparent, pale blue, jelly-like, cube-shaped body called a bell. This bell-shaped body consists of two layers of tissue - the external ectoderm and the internal endoderm. Sandwiched between these two layers is a non-living mesoglea layer that gives shape and structure to the jellyfish's body. In addition to these two cellular layers, their bodies have a contractile ring composed of cells that provide the contracting force that propels the jellyfish through water. 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 and was opened and closed to control the weather).

The jellyfish's mouth and stomach are formed from the top of the inside of the bell. Once prey is presented to it through its velarium, the mouth expands downwards to reach and engulf the prey, pulling it back into the animal's stomach. Once digested, waste product is ejected through its mouth.

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 rhopalium (plural is rhopalia). There is one rhopalium 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. 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 upward for 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. It is considered one of the few animals with a 360-degree view of its environment.

Photo: image of box jellyfish's eye

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

The box jellyfish does not have a brain. Instead, it has an intricate 'nerve net' of neurons dispersed throughout its mesoglea. The nerve net helps the jellyfish to interact with the sensory cells on its endoderm and ectoderm layer and the contractile cells in its body, which control its movement. Furthermore, it has a complex nerve ring that connects its four eye clusters and tentacle stubs to help control its orientation. This neural network allows information to be transmitted without a centralised brain. Scientists are uncertain how exactly these jellyfish process visual, spatial, and vibration information without a unified brain structure. Further research will no doubt unravel some fascinating discoveries!

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.

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 at speeds of up to six meters/second. That is 7.5 kilometres per hour - faster than an Olympic swimmer! 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 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 coast of Australia, particularly near the Great Barrier Reef and in the Indo-Pacific Oceans to the north. They prefer shallow waters with gently-sloping beaches so they can swim unimpeded with their long tentacles trailing behind them. They don't venture into the open ocean. But do come closer to shore when it's hot and still, particularly in areas where mangrove forests and river estuaries exist as more plentiful food is available.

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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 it.

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 12in in diameter. It can weigh up-to 2kg or 2.5lbs.
  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 nearly 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.