Box jellyfish are cube-shaped marine invertebrates in the class Cubozoa. They are best known for the powerful venom of a few species, but their biology is just as remarkable: they can swim with directional control, use multiple types of eyes, and actively hunt prey in tropical and subtropical coastal waters.
Box jellyfish quick facts
- Scientific class: Cubozoa
- Animal group: Cnidarians, related to other jellyfish, corals, and sea anemones
- Body shape: A roughly cube-shaped swimming bell
- Eyes: Many species have 24 eyes arranged in four sensory structures called rhopalia
- Diet: Carnivorous; prey can include shrimp, prawns, small fish, and other marine animals
- Habitat: Mainly warm coastal and estuarine waters
- Human risk: Most species are not lethal, but several can cause severe or fatal envenomation
What makes a jellyfish a box jellyfish?
Box jellyfish belong to the class Cubozoa. Their swimming bell is square or box-like when viewed from above. Tentacles arise from muscular structures near the lower corners of the bell. As cnidarians, their tentacles contain nematocysts—microscopic stinging cells used to capture prey and defend against threats.
The name “box jellyfish” covers an entire group, not one species. The Australian box jellyfish, Chironex fleckeri, is the best-known large and dangerous species. Smaller cubozoans include species associated with Irukandji syndrome. Risk, appearance, habitat, and behaviour differ among species.
Appearance and size
Most box jellyfish are transparent or pale blue, making them difficult to see in the water. Chironex fleckeri can have a bell up to about 30 centimetres across and tentacles extending as far as three metres. Smaller species may measure only a few centimetres.
Learn how to distinguish the major features in our guide to what box jellyfish look like and our illustrated explanation of box jellyfish size.
Eyes and navigation
Many box jellyfish have 24 eyes grouped in four rhopalia. The eyes are not all identical: some detect light levels, while others have lenses and more complex structures. Box jellyfish do not have a vertebrate-style brain, but their nervous system can integrate visual information and coordinate swimming behaviour.
Explore the visual system in Do Box Jellyfish Have Eyes? and How Many Eyes Do Box Jellyfish Have?.
Swimming and hunting
Unlike jellyfish that mostly drift with currents, box jellyfish can swim with speed and directional control. Their vision and muscular bell help them navigate obstacles and pursue prey. NOAA reports that some box jellyfish can approach four knots and may actively hunt shrimp and small fish.
Read more about box jellyfish diet and feeding.
Habitat and distribution
Cubozoans occur in warm waters in several parts of the world. Species capable of causing fatal human envenomation are concentrated mainly in the Indo-Pacific. Chironex fleckeri inhabits tropical northern Australian coastal waters, including some estuaries, creeks, and coastal river environments influenced by the sea.
Distribution and season vary by species and location. Never use a general calendar as a substitute for local beach warnings. See our guides to box jellyfish habitat and Australian stinger season.
Life cycle and reproduction
The cubozoan life cycle usually includes a small bottom-dwelling polyp and a free-swimming medusa. Sexual reproduction produces larvae that settle and develop into polyps. Depending on the species, a polyp later transforms or buds to produce a juvenile medusa, which grows into the adult jellyfish.
Our box jellyfish life-cycle guide explains each stage and the differences researchers have observed among species.
Venom and danger to people
Only a minority of box jellyfish species are considered life-threatening to humans. Severe stings can cause overwhelming pain, extensive skin injury, breathing difficulty, cardiovascular collapse, and cardiac arrest. Some smaller cubozoans can cause delayed Irukandji syndrome.
Risk depends on the species, the amount of tentacle contact, body size, and how quickly rescue and medical care begin. Read How Dangerous Are Box Jellyfish? and Why Is Box Jellyfish Venom So Dangerous?.
How to reduce your risk
- Follow local marine-stinger warnings and beach closures.
- Swim at patrolled beaches and use designated enclosures where available.
- Wear a full-body stinger suit in known risk areas.
- Do not touch jellyfish or detached tentacles.
- Learn the local emergency protocol before entering tropical waters.
Authoritative sources
- Australian Museum: Box Jellyfish
- NOAA National Ocean Service: Box jellyfish
- ANZCOR Guideline 9.4.5
- Healthdirect Australia: Jellyfish stings
Editorially reviewed: 22 August 2026. Independent scientific and clinical review is pending. See our Editorial Policy and Corrections Policy.