Box Jellyfish What is a Box Jelly Fish?

The box jellyfish is the most venomous animal in the world. Found along the north-eastern coastline of Australia its sting is lethal and can kill a human in less than four minutes. The box jellyfish is almost transparent, pale blue in colour, and hardly visible in water. It gets its name from its unique four-sided box-like shape.

Why is it called a Box Jellyfish? What Does Its Name Mean?

The box jellyfish is so named because of its cube-like shape that looks like a domed four-sided tent. It is a member of the Cubozoa class of jellyfish. Its scientific name is Chironex fleckeri. This name is derived from the Greek words 'chiro' meaning hand and the Latin 'nex' meaning murder. The word 'fleckeri' is in honour of the Australian radiologist and naturalist Dr Hugo Flecker who first identified this jellyfish in 1955.

Also known as the Sea Wasp, Fire Medusa and Stinger the box jellyfish is actually not a fish at all; strictly speaking it isn't even a true “jellyfish”. The reason it is not a fish is that it does not have a backbone. By definition, all fish are marine animals with backbones. It isn't a true ‘jellyfish’ either because by current definition a jellyfish drifts along in its environment. The box jellyfish on the other hand is capable of propelling itself through the water. Because of this some people have started calling them "Jellies" which is, quite frankly, a very uncreative name. So for now the name Box Jellyfish will have to do.

The box jellyfish has two body forms during its life cycle. As an adult it is a free roaming animal known as a medusa and as a juvenile it is a stationary creature known as a polyp.

Note: There is much inconsistency in the terminology used to refer to this animal's upper body. Some call it a 'medusa' while others refer to it as a 'bell'. In order to differentiate the adult animal from its body form, we will use the term 'bell' to describe the upper body of this jellyfish.

Box Jellyfish Life Stages It is Lives as Two Different Animals

Jellyfish belong to a group of animals called cnidarians (pronounced ‘cni·dar·i·an’). These animals, which also includes corals and sea anemones, have simple sac-like bodies with a single opening surrounded by stinging tentacles.

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

Box Jellyfish Anatomy What Do Box Jellyfish Look Like?

Bell (Medusa)

In its adult stage box jellyfish has an almost transparent, pale blue, jelly-like, cube-shaped bell which can grow as large as a basketball. It can weigh as much as 2 kilos and is composed of almost 96% water.

The box jellyfish is a diploblastic animal — it has only two layers of tissue. The outer layer is referred to as the ectoderm, or epidermis and the inner layer is called the 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 like 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. If we compare this to a human. Our outside skin is its ectoderm and the lining of our mouths, stomachs, intestines, etc is its endoderm. Unlike humans, however, the jellyfish has no specialised organs like a heart, bones etc in its mesoglea. Its basically just jelly.


The lower part of the bell rolls inward and joins a membrane with a central opening which can open and close — like the iris of an eye. This membrane is referred to as 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).

Mouth and Stomach

On the inside towards the dome of the bell, facing the velarium, is an extension of its endoderm which forms the stomach and mouth of the jellyfish. 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, nutrient extracted and the waste product ejected through its mouth.

Gonads (Sex Organs)

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

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 reside in a little silo (like 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 long thread, both of which are 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.

Box Jellyfish Eyes (Rhopalia)

The box jellyfish has 24 eyes located in a circular band around the bottom edge of its bell. They are grouped in 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 which 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 are callable of seeing images, are known as the upper-lens and lower-lens eyes. Regardless of the jellyfish’s position, the lens of the upper eyes are always pointed upwards. This suggests that the jellyfish is capable of orientation and navigation. The lower eyes are focused downward and helps 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 to have a 360-degree view of its environment.

Gravity & Vibration Sensors

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

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 overlap 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 are 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 and in this way information is transmitted and shared without a centralised brain. Since it does not have a central nervous system, it is not certain 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.

No Gills, No Heart, No Liver, No Nothing

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 has no need for specialised organs.

Box Jellyfish Movement Faster then a Speeding Swimmer

By squeezing its bell and ejecting water through a constricted opening in its velarium the box jellyfish uses water-jet propulsion to move itself through the water at speeds of up to six meters/second. That is 7.5 kilometers 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 therefore their ability to move away from danger, they are rarely found washed up on shore.

Box Jellyfish Diet What Do a Box Jellyfish Eat?

The box jellyfish is a carnivore that feeds during the day. Its diet consists primarily of small fish, crustaceans, marine larvae and plankton. It even preys on other jellyfish not of its own species. Given its eyesight and its ability to propel itself from place to place, the box jellyfish, unlike other jelly fish, actively hunts prey.

It captures its prey by ensnaring them in its tentacles. Even the slightest touch causes thousands of little 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, guided through the open velarium by its four pedalium and into its mouth which engulfs its victim and passes it on into its simple tubular stomach were it is digested. Digestive waste is ejected through its mouth.

Box Jellyfish Habitat Where Do Box Jellyfish Live?

Box Jellyfish Sleep at Night

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

Box jellyfish are a coastal animals preferring shallow waters with gently sloping beaches with minimal obstructions to snag their long trailing tentacles. They are found along the north-eastern coastline of Australia including many shorelines of the Great Barrier Reef and also in the Indo-Pacific oceans further to the north. They do not venture into the open oceans. When the air is hot and still, they will come closer to the shore especially in areas with mangrove forests and river estuaries were their food supply is more plentiful.

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.

It is because they share a common fondness for shallow sloping beaches that these jellyfish and humans confront each other.

Related Article: Great Barrier Reef — Coral, Animals, Plants and Attractions

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 be contain a toxic substance that is harmful if touched or eaten. Because the box jellyfish injects its toxins it is venomous.

Box Jellyfish Attack Killer Jellyfish?

The box jellyfish species Chironex fleckeri is considered to be the most venomous animal in the world. In Australia it has caused at least 64 deaths since 1883. However this is much less than are killed by sharks. It is claimed that the each box jellyfish has enough venom to kill 60 people.

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 of it is almost transparent with tentacle up to 5 meters long trailing behind, it is very hard to see in the water. In most instance a swimmer only becomes aware of it once they have been entangled in the jellyfish’s tentacles and stung.

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 as quickly as 2 to 5 minutes.

Treatment of Jellyfish Attack What to Do If Stung by a Jellyfish?

The Boy Who Died Twice and Survived

Did You Know

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 the jellyfish will 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.

Survival from a box jellyfish sting depends on the amount of toxin injected into its victim and the speed in 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 administer an antidote quickly enough.

The only certain 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 relive the pain. Since cardiac arrest is like to occur very quickly it is vital that CPR is administered until profession help arrives.

Box Jellyfish Reproduction Sexual Reproduction in Box Jellyfish

The adult box jellyfish, depending on its sex, releases eggs or sperm it into the water. This release, known as spawning, usually occurs once a year in late summer in river estuaries where a large population of jellyfish gather to release their eggs and sperm at the same time to ensure cross-fertilisation. 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 preferable under an overhang or crevice and slowly transform themselves into flower-like polyps. Theses polyps have a mouth and tentacles that they use to capture and feed on zooplankton. At this stage of their lives their appearance behaviour is very similar to that of a coral or sea anemone polyp. By 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. They live for about a year.

Threats and Predators What Treats Do Box Jellyfish Face?

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

Conservation Status

The box jellyfish are not considered threatened.

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