Look too quickly, and you might miss any given member of the Cubozoa class entirely. Their tentacles drift very faintly in the water, and their cube-shaped bodies are transparent enough to be compared to glass. You might know them by their colloquial name: the box jellyfish.
Up close, their bells look rather delicate, almost like a boxy parachute. Their bodies are usually just a few centimeters thick, but can spread up to a foot across. And from each corner of their boxlike bells trail long, slender tentacles that can span up to three meters long.
All 51 species of this unassuming looking jelly — including Chironex fleckeri and Irukandji jellyfish — can be found in the Indo-Pacific shallows of northern Australia and Southeast Asia. Little do swimmers know, however, how great of a risk it is to cross the box jellyfish’s path.
It may not look like the ocean’s deadliest creature. But it is.
Why The Box Jellyfish Is So Deadly
Ask anyone what they think the greatest oceanic threat is, and they’d likely name a shark; perhaps the great white, or maybe the tiger shark. But sharks usually attack less than 50 people per year. Only about five or six of these attacks end up being fatal. The box jellyfish, on the other hand, is known to kill approximately 100 people per year.
This jelly’s well-deserved reputation as the deadliest sea creature rests on two primary evolutionary advantages, the first of which being that it’s essentially impossible to see.
Even in the calmest, clearest of waters, cubozoan jelly’s bells are so transparent that they’re incredibly difficult to spot. On top of this, its long, threadlike and fragile tentacles don’t create much of a shadow, either. The transparency of their bells and tentacles alone position box jellies as highly dangerous: beachgoers could be swimming within inches of one without even realizing it — until its sting hits.
Although their transparency allows them to swim largely undetected, the box jellyfish’s second advantage is what makes them truly lethal: their venom. As a 2015 study from BMC Genomics explains, each one of its tentacles is armed with thousands of complex, microscopic intracellular structures known as nematocysts. When triggered by touch, these structures fire with explosive speed, and release harpoon-like tubes that embed into your skin.
As soon as the tubule embeds, venom is delivered within fractions of a second. And as the study notes, this venom is a highly sophisticated biochemical weapon — unlike the stings of other jellyfish, which mostly just irritate the skin. Chironex fleckeri’s sting, the deadliest of all cubozoan jellies, is known to inflict an array of horrific symptoms, including:
- Inflammation. Incredibly severe burning-like pain and swelling to the skin, which will start almost immediately post-envenoming.
- Dermonecrosis. Skin at the site of the sting may even begin to die, which usually leads to permanent scarring.
- Dyspnea. Envenoming makes it especially hard to breathe
- Hypertension/hypotension. Venom may cause blood pressure to spike or drop dangerously.
- Cardiovascular collapse. In extreme cases, the venom can cause the heart to stop working properly. This may lead to total collapse or, in worst case scenarios, cardiac arrest.
For small fish, cubozoans’ intended prey, the effect of their venom is instant. But for humans, a few meters of contact with their tentacles is enough to be utterly excruciating; in some cases, it’s even fatal. Survivors often describe box jelly stings as being so overwhelmingly painful that movement becomes close to impossible.
A Skilled Hunter
What makes the box jellyfish truly remarkable, and somewhat terrifying, is that it’s not just a hazard that drifts along at the mercy of currents like most jellies do. Instead, cubozoan jellyfish actively predate their prey.
A majority of jellyfish species are only capable of weakly pulsing their way through the ocean; they push out small amounts of water to shuffle themselves along. These jellies are more passengers than they are pilots, as the tides are largely what dictate their lives.
The box jellyfish, on the other hand, is a highly skilled swimmer. As research from the Journal of Zoology explains, cubozoan jellies make use of highly developed forms of jet propulsion. Their cube-shaped bells are intricately lined with muscular bands, each of which they forcefully contract in order to suck seawater into their bell. Once sucked up, this water is then expelled in strong jets, which shoots their bodies forward in turn.
This “inhale and exhale” technique is seen even in some of the smallest species of cubozoans, as 2024 research from Fisheries and Aquatic Sciences observes. This motion is what gives the box jellyfish its astonishing ability to swim at speeds of up to four knots — fast enough to outpace many small fish.
Even more impressively, these jellies can steer, too. Unlike their relatives, these jellies aren’t condemned to drifting in eternal circles. As the Journal of Zoology study explains, cubozoan jellyfish have a unique anatomical difference to their counterparts: their four pedalia. These muscular, paddle-like structures located at the base of their tentacles are thought to allow them to make sharp turns and deliberate movements.
Of course, their adept swimming and steering capabilities would be rendered useless if they weren’t able to see where they’re going. Here, too, the box jellyfish defies and one-ups its many relatives. As a report from Current Biology explains, box jellyfish have an astonishing 24 eyes.
Their two-dozen eyes are grouped into four separate sensory clusters, all of which are arranged evenly around its bell. And these aren’t just light-sensitive spots like those found on most jellyfish. These include pit eyes, slit eyes and lens eyes. The lens eyes in particular have bizarrely complex make-ups, including lenses, corneas and retinas — the same essential structures found in that of vertebrates and cephalopods.
For long, and even today, it’s still elusive as to why these jellies need such intricate sets of eyes, and so many of them. What we do know, however, is that the pit and slit eyes are what assist cubozoans in detecting contrasts between light and dark; this makes it possible for them to orient themselves in shallow, sunny waters.
But the lower and upper lens eyes are what allow them to form optical images of what’s around them: they can recognize shapes, the water’s surface or the shadows of their prey. In fact, the Current Biology report notes that they’ve even been observed to swim around obstacles, rather than bumping blindly into them — particularly within mangroves and between their roots.
Given their lack of a brain, these jellies don’t think in the same ways we humans do. That said, their nervous systems are advanced and specialized enough to take these visual cues in and translate them into purposeful action. Paired with its strong swimming, transparent disguise and lethal venom, this visual system makes the box jellyfish one of the stealthiest and precise hunters in the ocean.
Are jellyfish the least of your worries when it comes to the ocean? Take the science-backed Thalassophobia Test to find out how deep your fear of the ocean runs.
