Box Jellyfish and Irukandji: Characteristics, Species and Venom

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Chironex box jellyfish
The box jellyfish — which is found off mostly of the north and northeast coast of Australia and to a lesser degree in Southeast Asia — is arguably the world’s most venomous creature. Victims sometimes die within three minutes and over 80 people have died from their stings off of Queensland alone since 1880. What makes them particularly dangerous is the fact that their translucent box-shaped bodies and 60 spaghetti-like tentacles are difficult to see. They are also known as marine stingers or sea wasps. [Source: William Hammer, National Geographic August 1994]

Box jellyfish, named for their body shape, have tentacles covered in biological booby traps known as nematocysts — tiny darts loaded with poison. People and animals unfortunate enough to be injected with this poison may experience paralysis, cardiac arrest, and even death, all within a few minutes of being stung.But of the 50 or so species of box jellyfish only a few have venom that can be lethal to humans.

Very little is known about box jellyfish. Scientists have traditionally observed them underneath piers feeding on fish attracted to light, and no one had ever kept one alive in captivity until 1977. Now scientists studying keep them in disk-shaped aquariums with circulating in a current that is similar to natural conditions. Strangely enough the box jellyfish was not even identified until 1956. Before that time there were reports of swimmers leaping out of the water in agony not having any idea what caused the stringy red welts and the inferno of pain.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio

Box Jellyfish Characteristics

Box jellyfish are known scientifically as Cubozoans. Their common name comes comes from the fact the transverse section of their bells appear to be square. Tentacles are located at the corners of the square umbrella margin, and the base of each tentacle is distinctively flattened. The edge of the umbrella turns inward to form a rim called a velarium, much like the velum of hydromedusae. /=\

parts and life cycle of a Cubozoa Lebenszyklus box jellyfish in German

Box jellyfish have traits that set them apart from other jellyfish. Most notably, box jellyfish can swim — at maximum speeds approaching four knots — whereas most species of jellyfish float wherever the current takes them, with little control over their direction. Box jellyfish live for only a couple of months. They feed on small fish and shrimp and can double in size in one month. They travel at speeds of up to five feet per second in their quest for prey.

Box jellyfish are venomous, ectothermic (use heat from the environment and adapt their behavior to regulate body temperature), have radial symmetry (symmetry around a central axis), polymorphic (“many forms”, species in which individuals can be divided into easily recognized groups, based on structure, color, or other similar characteristics). [Source: Vishal Patel and Selina Ruzi, Animal Diversity Web (ADW) /=]

Box jellyfish are colorless and difficult to see in the water Even though they don't have conventional eyes or a brain box jellyfish can "see." Box jellyfish have 24 eyes clusters on each corner of their box shape, giving it 360 degree vision both horizontally and vertically. Some of these eyes are surprisingly sophisticated, with a lens and cornea, an iris that can contract in bright light, and a retina. Their speed and vision leads some researchers to believe that box jellyfish actively hunt their prey, mainly shrimp and small fish. [Source: NOAA]

Dan Nilsson, a zoology professor at Sweden’s Lund University, is an expert on animal eyes. On research he is doing with irukandji at Palm Cove, Queensland, Paul Raffaele wrote in Smithsonian magazine: He plucks one from the bucket and shows me the four tiny black dots, containing the jellyfish’s 24 eyes, on strands connected to each side of the cube of jelly. Under the microscope, Nilsson has detected in each dot something he calls a sensory club, which is an organ with a set of six eyes, including four that are — much like the eyes of other jellyfish — simply pits, limited to detecting light intensity in various directions. But the two other eyes in each sensory club have more in common with human eyes than the eyes of other jellyfish...One eye, which points obliquely downward at all times, even has a mobile pupil that opens and closes. The other major eye points upward. “We’re not exactly sure what these eyes are doing,”

range of Chironex fleckeri box jellyfish
some species of box jellyfish are found as far
west as South Africa and as far north and
east as Hawaii
Nilsson says, although he believes they may help the jellyfish “position itself in the right place where there is plenty of food.” They also help the animal situate the shoreline and the horizon — to avoid being dumped on the beach by a wave — and see obstacles that would tear its delicate tissue, such as a coral reef, a mangrove tree or a pier [Source: Paul Raffaele, Smithsonian magazine, June 2005].

If box jellyfish eyes are a puzzle, its four primitive brains — positioned on each side of its body and attached to it by the same strand that anchors its eyes — are an enigma. Can the four separate brains communicate with each other? If so, do they merge the images they receive from the 24 eyes into one image? And how do they manage if different eyes detect radically different images? Nilsson shrugs. “They’ve evolved a rather advanced system unlike any other animal on earth,” he says. “But we have no idea what’s going on in their four brains, and I suspect it will be a long time before we find out.”

Box Jellyfish Movements, Feeding and Naps

According to Animal Diversity Web: Although box jelly have the ability to move up to 4 knots, during the day they typically travel at 1 knot or less. Box jellyfish usually swim slower during the day than at night, which is likely due to hunting and consuming of prey. Unlike other jellyfish, box jellyfish rest on the sea floor, not moving unless disturbed. Box jellyfish may require this rest phase to energetically compensate for the time they spend actively swimming. Box jellyfish also display this type of behavior when seas are rough, during which they sink to the sea floor until the water calms. [Source: Timothy Schmidt, Animal Diversity Web (ADW)]

Paul Raffaele wrote in Smithsonian magazine: Because a box jelly, as expert Jamie Seymour puts it, “charges around the ocean all day hunting mobile prey, prawns and fish,” its metabolic rate is ten times that of a drifting jellyfish. So, to swiftly access the energy it needs, the box jellyfish has developed a unique digestive system, with separate stomachs in each of its tentacles. All box jellies turn their food into a semi-digested broth in the bell, and then feed it down through the tentacles to be absorbed. Since a Chironex can have up to 60 tentacles, each as long as 3 meters, in effect it has up to 180 meters of stomach. [Source: Paul Raffaele, Smithsonian magazine, June 2005].

The complex rhopalial ocelli of the box jellyfish Tripedalia cystophora

Box jellyfish take naps. ABC reported: Marine biologist and PhD student Matthew Gordon says scientists at James Cook University in Cairns have found that the animals are extremely active most of the day. But in the afternoon they "tilt their bodies down and rest on the bottom of the sea with their tentacles spread out". "They are really good swimmers," Gordon says. "They have a high metabolic rate. They run around all day at a high speed. But they can't swim 24 hours a day. They catch the food and need to digest it ... at around 3 to 4pm they sink to the bottom of the sea and stay motionless for hours at a time." [Source: Diana Plater, ABC, August 24, 2005

As part of a three-year study that could help to manage the country's northern coastline, the university's Tropical Australian Stinger Research Centre has been tracking the movements of box jellyfish using ultrasound tags, for 24 hours at a time. Gordon says researchers don't know much about the animals' movements. But the centre has tracked one in particular that swam up and down 5 kilometers of the coast, for around 20 hours continuously. It was also thought that box jellyfish swim up creeks towards the end of the season to reproduce but there was no concrete evidence of this until late April 2005 when one jellyfish was followed up an estuary.

Box Jellyfish Reproduction and Development

Box jellyfish engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body. There is no parental involvement in the raising of offspring. During the later summer, or when stressed, box jellyfish release sperm and eggs into the water. As is true with other species of jellyfish, the fertilized eggs multiply into "balls of cells" called planulae that settle onto a hard surface like a rock and then grow into a polyp. The polyp reproduces asexually into several creeping polyps which later metamorphose into adult jellyfish (called medusas) in a 12-day period. These migrate out to sea before the early summer monsoon rains starts and return in large numbers to river mouths and estuaries when they are ready to spawn.

According to Animal Diversity Web: In some cubazoan species the adults release both sperm and eggs into the ocean where fertilization will occur. Mature females of Carukia barnesi are defined as having fully developed oocytes. This typically occurs when the bell height exceeds eight millimeters. Males are considered mature when bell height is comparable to that of mature females.

group of box jellyfish at Geldkis

Cubozoans have a two-stage life cycle consisting of a medusa and polyp. Fertilized eggs develop into swimming planulae, which settle after a few days. The planulae develop into motile (move around as opposed to being stationary), feeding polyps, which produce other budding polyps. Polyps take a few months to mature, then begin metamorphosis by resorbing tentacles. four new tentacles and four rhopalia are formed. When the single juvenile medusa has fully metamorphosed, it contracts and swims away. [Source: Vishal Patel and Selina Ruzi, Animal Diversity Web (ADW)]

Judging from the fact that small medusas were found close to shore, scientist surmised that box jellyfish spawned in coastal areas and estuaries. To prove this Australian biologist Robert Hartwick spent six years looking at rocks and mollusks shells from estuary streams for box jellyfish polyps. "We looked at thousands of rocks from 14 rivers," he told National Geographic, "sometimes taking as long as four hours to examine a four-inch rock...It was like searching for a small shrub in a forest." Finally in 1986 he thought he found one. "I tried not get too excited. Polyps often look alike. We had to see what it grew into." After 12 days "the medusa emerged, and they were unquestionably tiny box jellies."

Species of Box Jellyfish and Irukandji

Of the 50 or so species of box jellyfish only a few have venom that can be lethal to humans. Box jellyfish are found in warm coastal waters around the world, the lethal varieties are found primarily in the Indo-Pacific region and northern Australia. This includes the Australian box jellyfish (Chironex fleckeri), considered the most venomous marine animal in the world. They are able ensnare prey as large as mullet. Tentacles draw the fish into their bell where the stomach begins ingestion and the tentacles absorb the liquified nutrients,

Box jellyfish are in the Class Cubozoa, Order Carybdeida, and Family Carybdeidae. Chironex fleckeri) is considered the most venomous marine animal. Carukia barnesi is named after Dr. Jack Barnes, who connected the jellyfish with Irukandji syndrome. It can be found along the coastline of Northern Australia, from Broome along the western side of Australia to Rockhamptom, Queensland on the eastern side. This includes Port Douglas in North Queensland to the Whisunday Islands near Mackay. It is also found in the Cairns regions and the Great Barrier Reef. Carukia barnes is typically found in reefs or other coastal areas. Unlike its congener Chironex fleckeri, it is usually found in deeper waters along reefs around 10 to 20 meters (33 to 66 feet). [Source: Vishal Patel and Selina Ruzi, Animal Diversity Web (ADW) /=]

Carybdeidae (left), Chironex (middle) and Tripedalia (right)

Chironex fleckeri is the largest of the box jellyfish, with body sizes reaching up to 30 centimeters (one foot) in diameter and thick, bootlace-like tentacles up to three meters (10 feet) long. It is typically found in warm, relatively shallow water off northern and northeastern Australia. These jellyfish are 97 percent water, are virtually invisible and have no brain. Their 60 or so tentacles contain five billion stinging cells or nematocysts, that can kill an adult human in less than four minutes. According to Animal Diversity Web: The name box jellyfish is derived from the shape of their bell, which is box-shaped when healthy. The bell is transparent and is usually between 16 and 24 centimeters (6.3 to 10 inches) though some reach a diameter of 35 centimeters (14 inches). The tentacles of box jellyfish dangle from pedalia, the corners of the bell. There can be as many as 15 tentacles hanging from each of the four pedalia. Each of the tentacles has a slight blue-gray tint

The Irukandji box jellyfish (Malo spp.) is the smallest jellyfish in the world. It ranges from 20 to 35 millimeters (0.8 to 1.38 inches) in length, with their average length being 25 millimeters (one inch). It also has one of the strongest toxins, which have been fatal to humans According to the University of Hawaii: Although the main bell of the box jelly is about the size of a sugar cube, its stinging tentacles can stretch for one meter The venom of Irukandji jellies, which are found off the coast of Australia, acts on the nervous system and paralyzes the lungs and heart. Some parts of the body are also more susceptible than others to stings. The many nerve endings in our face and lips mean stings to those locations are more painful than they would be elsewhere. [Source: University of Hawaii]

According to Animal Diversity Web: The Irukandji jellyfish is a carybdeid cubazoan, which tend to be smaller than the other type of cubozoa, the chirodropids. Individuals of this species typically reach 25 millimeters in diameter, however it has been documented at a diameter of 35 millimeters. Carukia barnesi consists of a transparent bell that is cuboidal in shape that narrows slightly towards the apex. Extending from each of the four corners of the bell is a retractable tentacle that varies in length from 5 to 50 centimeters. Both the tentacles, as well as the body, are covered in stinging cells called nematocysts, however, the type of stinging cells differs on these two parts of the body. This box jelly also has a primitive and transparent eye on each side of its bell. The Irukandji jellyfish has been found to be both fast and agile while swimming. They sense using vision. Carukia barnesi have image-forming eyes that respond to images, but have no brain to process the visual information.

Box Jellyfish Venom, Tentacles and Stinging Cells

Irukandji box jellyfish (Malo spp.)

Box jellyfish feed mostly on small fish and crustaceans. They use venom to paralyze their prey. The top part of box jellyfish is safe to touch but stay clear of the tentacles. The toxin is injected into the prey by the prey triggering one of the stinging cells (nematocysts) on the jellyfish’s tentacle. Once the stinging cell is triggered, a harpoon looking coil is released which stings the prey and the toxin then flows through this hollow harpoon into the prey. The tentacle can then be retracted back into the jellyfish, bringing the prey with it towards the jellyfish’s mouth, which is located inside the bell. [Source: Vishal Patel and Selina Ruzi, Animal Diversity Web (ADW) /=]

Box jellyfish tentacles can be three meters (15 feet) long and a half a centimeter (quarter inch) thick. A single tentacle contains millions of venom capsules that fire with a mechanical trigger stimulated by chemicals on the surface of a fish, shellfish and "unfortunate humans." Box jellyfish need a venom this powerful. The shrimp and fish they eat are killed almost instantly. If the they weren’t they could easily tear the jelly's fragile tissue. Some species of animals are not affected by the venom. Hawksbill turtles, for instance, gobble up box jellies like their was no tomorrow and inside the stomachs of some sea turtles have been found the remnants of dozens of jellyfish. Sea turtles have also been found with pounds of sharp silicone spicules from glass sponges.

Box jellyfish venom might contain a neurotoxin that is a neural Na+ channel activator. No antivenom has yet been developed for this species. The venom in a box jellyfish is powerful enough to kill a human in four minutes of less. "Australia most dangerous snake, the taipan," UCLA biologist William Hammer wrote in National Geographic, "has enough venom to kill 30 adults, but its bite is not very painful, and it can take several hours for untreated victim to die. A large box jellyfish, however, has enough venom to kill 60 adults, and the pain of its sting is instant and unbearable. Breathing may quickly become distressed as venom is absorbed into the circulatory and lymphatic systems; in some cases, the heart's pumping slows or stop almost immediately."

Researcher Teresa Carrete discovered that box jellyfish become more lethal with age. Juveniles that hunt shrimp have venom in only five percent of their stinging cells while adults have 50 percent in theirs, allowing them to catch much larger prey. Paul Raffaele wrote in Smithsonian magazine: Ken Winkel, director of the Australian Venom Research Unit, has conducted experiments on anesthetized and ventilated piglets and concludes that Carukia barnesi venom “fires the sympathetic nerves, pushing up dramatically the blood pressure and heart rate. That’s why you get sweating, nausea, anxiety and a feeling of doom” — the latter effect caused, Winkel believes, by the triggering of the stress hormones adrenaline and noradrenaline. In the body, noradrenaline produces a heart-thumping, throat-tightening, fight-or-flight effect. It’s what you would feel, Winkel says, “if you were put in a cage with a hungry lion.” [Source: Paul Raffaele, Smithsonian magazine, June 2005]

Chironex venom, by contrast, attacks the heart directly, which can cause dramatic and rapid cardio-respiratory arrest, says Darwin-based professor Bart Currie, a specialist in treating Chironex victims. “Ahealthy heart contains millions of muscle cells that all beat to the same rhythm to pump blood through the body,” he says. “For reasons we don’t yet know, Chironex venom makes the heart cells beat irregularly. If enough venom is injected, the heart shuts down altogether.” Death comes quickly to Chironex victims because — unlike venomous snakes, which inject a glob of venom that must pass through the lymphatic system before draining into the rest of the body — Chironex shoots its venom into the bloodstream, giving the venom a direct pathway to the heart.

Irukandji jellyfish

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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