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Torben Lonne, Dive Instructor

Explore Marine Life

Meet One of the Most Toxic Animals on Earth:
Box Jellyfish

Box jellyfish, also called sea wasp or marine stinger, has received much attention as it is considered the most dangerous marine creature. It has a highly toxic venom and can inflict extreme pain in humans.

It causes dozens of deaths every year, high medical costs, closed beaches and, consequently, it leads to negative effects on tourism industry.

Box Jellyfish
Box jellyfish can have up to 40 tentacles with 5000 stinging cells each.


Jellyfish are soft-body animals which makes it extremely rare to find records of them in fossils. However, it is still possible to find fossilized remains of these invertebrates in sediments all around the world. For instance, in 2019, a huge fossil discovery was made in China’s Hubei province. Thousands of soft-body fossils were found, including jellyfish. They were estimated to be 518 million years old, which makes jellyfish even older than dinosaurs.

Biology of box jellyfish

Box jellyfish are named for the cube-shaped bell. In most cases, it is difficult to detect their presence in the water because of the pale blue and transparent color. A full grown box jellyfish can measure up to 10 feet-long (3 meters) and weight 4.4 pounds (2 kg).

As other jellyfish, they are composed of approximately 95% water. They have no brain, heart or a respiratory system. Besides, they have a central cavity that functions as mouth and anus.

Box jellyfish can have up to 40 tentacles with 5000 stinging cells each. These cells, the so called nematocysts, consist of a capsule with a harpoon inside embedded in venom, and a hair trigger. When the box jellyfish detects the presence of a chemical on the surface of the prey, stinging cells are triggered, and the toxins enter the blood of the prey. These cells are frequently used for species identification, according to their type and shape.

Another interesting fact about box jellyfish is that they have 24 well developed eyes separated in four types with different structures and functions. In fact, this group of invertebrates uses vision not only to navigate in the environment but also to find preys and mates. And because box jellyfish is usually found in kelp forests, coral reefs and sandy beaches, vision is especially important to avoid collisions with obstacles that could damage this soft-body animal. However, it is still not very clear how they process what they see because of the lack of neural branches and a nervous system to process and interpret images.

Box Jellyfish with dead fish in its stomach
Box Jellyfish are composed of approximately 95% water. They have no brain, heart or a respiratory system.

Reproduction and life cycle

Regarding reproduction, box jellyfish has a complex behavior. Usually, medusas release eggs and sperm in the water. On the other hand, some species are ovoviviparous which means that female medusae take up the sperm released in the water by males, and eggs are fertilized internally. In this case, embryos can be released hours after fertilization.

The larvae swim around for few days until they attach to a surface. Thereafter, they form a colony where they develop into polyps. Polyps reproduce asexually by budding off young box jellyfish that will further mature into medusae. They live up to 1 year, dying at the end of the summer. However, some species of box jellyfish are seen throughout the year.

Interestingly, box jellyfish has a solid balance stone that grows by accretion of daily growth rings. It is used to age the animal and scientists also use it as a taxonomic indicator.

Geographical distribution and factors affecting it

Although there is little information available concerning quantitative spatial distribution of box jellyfish, it is well established that it is a tropical and subtropical marine organism. In fact, these invertebrates are frequently detected in coastal waters of Northern Australia.

In addition, they can be found in the Indo-Pacific region, including the Philippines, Indonesia, Thailand, Papua New Guinea and Japan. There are also reports of their presence in California, Hawaii, South Africa, Caribbean and Mediterranean Seas.

Due to the fact they are coastal animals, the main variables affecting their distribution are changes in salinity, temperature and currents, variations in primary production as well as the presence of sandy bottom and/or algae.

What do box jellyfish eat?

Juvenile box jellyfish usually feed on crustaceans. As they grow, they undergo a prey shift and start feeding on planktonic invertebrates and larval fish. Box jellyfish high mobility along with their complex visual structures are thought to play an important role in prey capture. Besides, they produce neurotoxins in the tentacles that help them paralyze prey.

Even though box jellyfish have few predators, juvenile green sea turtles usually feed on this stinging animal, using its flippers and thick skin as protection from the tentacles.

The poisonous box jellyfish
The stings of Box Jellyfish species cause a set of severe symptoms that are known as Irukandji syndrome.

Box jellyfish powerful venom

Up to the moment, 50 species of box jellyfish were described, but only a few have a highly toxic venom that can be really harmful for humans. A sting from these can cause death in minutes. Particularly, the Australian box jellyfish (Chironex flackery) is considered the largest and most virulent species of this group, having killed more than 70 people since 1883.

It is though that stings occur in brief epidemics. In Australia, for example, there are two “stinger season”: one in December or January, and the other in March or April. These seasons bring consequences to public health due to high medical costs. In addition, the tourism-related businesses are also severely affected because beaches are closed, and negative publicity arises.

There isn’t a well-established method to treat a sting from box jellyfish. However, for less severe stings, it is suggested that rinsing the affected area with vinegar, removing the tentacles with tweezers and soaking the skin with hot water might help stopping the stinging. To treat discomfort, hydrocortisone creams and oral anti-histaminic are also used.

On the other hand, the stings of some box jellyfish species cause a set of severe symptoms that are known as Irukandji syndrome. It is typically characterized by severe back and abdominal pain, nausea, vomiting, cramps, spasms, headaches, sweating, difficulty breathing, anxiety, and feeling of “impending doom”.

Less common symptoms include hypertension, coughing, pulmonary edema, cerebral hemorrhage and acute cardiac failure. In extreme cases, people die due to complications arising from Irukandji syndrome.

It is estimated that this group of invertebrates causes between 20 and 40 deaths per year, just in the Philippines. Most of them are children and residents of fishing villages. In reality, box jellyfish cause more injuries and deaths in humans than sharks.

Box Jellyfish
Increasing temperature of oceans and changes in acidity might cause a shift in box jellyfish distribution and increase its abundance.

What is the impact of climate change in box jellyfish?

Climate change is endangering the survival of many species worldwide, but it is not the case of box jellyfish. In fact, researchers suggest that the increasing temperature of oceans and changes in acidity might cause a shift in box jellyfish distribution and increase its abundance.

This is especially threatening because dangerous species of box jellyfish may be moving into more populated areas. Therefore, they may become common in new ranges and disturb the balance of ecosystems.

There are reports of jellyfish blooms all around the world that damage fishing industry. Depletion of oxygen and overfishing of jellyfish predators are other possible reasons for the increase in jellyfish appearances.

Before planning vacations in countries where box jellyfish are part of the marine fauna, you should get information about when they show up. In some countries, websites and social media groups were created to share information about these animals.

The search for an antivenom and the therapeutic potential of box jellyfish

Given the severe damages that a sting from box jellyfish can cause, it is considered critical to find antivenoms to tackle it. In 1970, the first box jellyfish antivenom became commercially available in Australia, but its efficacy is still not very clear. Research is being made in order to find an antidote for box jellyfish stings and some results may be promising.

Curiously, few studies discuss the potential applications of jellyfish in the treatment of pain, osteoarthritis and antimicrobial treatments. Some researchers even suggest that jellyfish venom could be used as a cardiovascular medicine, but more studies are needed.


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