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The most venomous jellyfish

Jellyfish are very simple animals which have been around for 700 million years (before the dinosaurs). They have no bones or cartilage, no blood, no heart and no brain, and are made up of 95% water.

Jellyfish move through the water by opening and closing their bodies, or bells, using muscle contractions. Most species have tentacles (anything from 1 to hundreds) that are attached to the edges of the bell, or subumbrella.

Because jellyfish are carnivores and feed on a large variety of prey - smaller fish, other jellyfish, plankton and other small organisms -, they have a stinging apparatus which, in 70 of the 200 known species, can affect humans.
In most cases, and for most species, the sting is not deadly, but causes anything from a mild skin irritation, to an excruciatingly pain. A few species, though, are considered extremely dangerous, and can potentially be fatal. Many of those dangerous jellyfish fall under the Box Jellyfish family (class Cubozoa), which comprises some 20 species. Many species, notably the two deadliest, have recently proliferated and considerably extended their natural range, probably benefiting from the global warming of the oceans, and are likely to cause serious problems in their interaction with humans in years to come.

(Note: for two venomous species that are not bow jellyfish, see the linked article :"the largest jellyfish in the world".
The venomous Physalia species are not exactly jellyfish and have been treated in two other linked articles: "the Portuguese Man o' War or Bluebottle", and "How to Treat a Bluebottle Sting")

- Box Jellyfish, Boxfish, Deadly Sea Wasp, Sea Wasp (Chironex fleckeri)

This species occurs, almost invisible, in shallow water at the edge of beaches in Northern Australia (from Exmouth, Western Australia, to Gladstone, Queensland) and in the Indo-Pacific region, notably in Thailand waters and Malaysia.

Adult jellyfish spawn at river mouths in late summer, before dying. The fertilized eggs become tiny polyps and attach themselves to rocks in estuaries. In spring, these polyps develop into small swimming jellyfish that migrate downstream, especially with rains.

They feed on shrimps and, unfortunately, often frequent beaches that are attractive to humans. These animals hunt passively, waiting for their prey to bump into their tentacles. Since a struggling shrimp can tear a young jellyfish, it needs to be killed instantly with a very strong poison.

This box jellyfish is able to see through four eyes, one at the center of each side of the bell. It is not known how the animal processes the visual information without a brain, however, it can see better than most birds and manages to avoid even the tiniest objects. Stings usually occur when people bump into the jellyfish.

In each corner of the box-shaped bell (often as large as a basketball) , which can weigh up to 2 kg, there is a bundle of ten to fifteen stinging tentacles, which extend for up to 5 m.

The tentacles are armed with up to 5,000 million stinging cells, or nematocysts. These are triggered into action when in contact with certain chemicals found on the surface of fish, shellfish and humans - but turtles are not affected by their sting and feed on this jellyfish.

Chironex fleckeri is the most lethal member in the Cubozoa family, possibly the most venomous marine creature - some even say the deadliest animal on Earth -. It can kill more people than stonefish, sharks and crocodiles combined.
Contact with only 3 m of tentacles may be fatal for an adult, and even less is enough to kill a child. Stings are fatal in 20% of cases and a single jellyfish is venomous enough to kill 60 humans.

Furthermore, even if the sting doesn't kill its victim, it is likely to make it go into shock from the overwhelming pain, which can cause this person to drown, if swimming  alone.
There are many (about 70) reported deaths that have occurred in northern Australia, mostly in summer (between November and April).

The venom of this box jellyfish has cardiotoxic, neurotoxic and highly dermatonecrotic components. It is rapidly absorbed by the body and causes extreme pain. The tentacles that adhere to the skin should not be removed before their nematocysts are inactivated - by pouring vinegar on them - otherwise they may worsen the sting.

Severe stings result in necrosis of the affected skin area. If untreated, the pain may last for weeks and often stings leave significant scars. Other symptoms might include difficulty with breathing, talking or swallowing, and respiratory or cardiovascular failure.

A moderately effective antivenin is indicated for serious envenomations and should help in reducing the pain and the scarring. If it is not available, pressure-immobilization of the limbs may be used after inactivation of nematocysts while the patient is transported to hospital. Local pain is best treated with ice packs.

- Chiroplasmus quadrigatus

Another box jellyfish and Sea Wasp, Chiropsalmus quadrigatus, is generally less common than Chironex fleckeri, although it may outnumber them on Cairns to Port Douglas beaches, in Queensland, Australia.

It is smaller, with slimmer tentacles, but the two species are difficult to tell apart.

The Chironex fleckeri antivenin is also partially effective against Chiroplasmus quadrigatus.

- Irukandji Jellyfish (Corukia barnesi and Malo kingi)

Those two tiny species of bow jellyfish are found mostly near Australia. The name Irukandji comes from the Irukandji aborigines who inhabit the coastal strip north of Cairns, Queensland.

Irukandji Jellyfish are usually found near the coast, where they are attracted by the warmer water, and most stings occur in summer in shallow waters. However, this jellyfish can be found as far as five kilometers offshore and all year round.

The tiny Irukandji barely measure 2 cm across the bell, no more than the fingernail of an adult's little finger.

Very little is known about the life cycle of these small, inconspicuous jellyfish. Furthermore, they are so fragile that, in captivity, they must imperatively be kept in a bowl that has rounded edges, like a circle, because in a normal bowl the impact if they hit the side would kill them.

It is believed that the Irukandji's venom is particularly potent - some say it is even stronger than the Sea Wasp's one - to enable it to quickly stun the small fast fish it preys on.

The Irukandji has one tentacle at each corner of its bell and stinging cells (nematocysts) not only on their tentacles, but also on the bell.
Since the venom is injected only from the tip of the stinger rather than the entire length, the initial sting is mild and only moderately irritating. However, after a delay of some 5-120 minutes, the venom starts acting.

These extremely venomous jellyfish cause symptoms collectively known as Irukandji syndrome.

The first-known of these jellyfish, Carukia barnesi, was identified in 1964 by Dr. Jack Barnes, who stung himself, his son, and a lifeguard, to prove the tiny jellyfish was the cause of Irukandji syndrome. The Malo kingi species, and possibly other jellyfish, may also produce the Irukandji syndrome.

Only a very small amount of venom can cause severe pains at various parts of the body, notably excruciating muscle cramps in the limbs, severe pain in the back and kidneys, and a burning sensation of the skin and face. Other symptoms include headaches, nausea, restlessness, sweating, vomiting, high heart rate and blood pressure, anxiety and the feeling of impending death, and occasionally acute cardiac failure. The syndrome is in part caused by release of catecholamines but the cause of heart failure is undefined. The venom contains a sodium channel modulator.

The symptoms last hours, or even days, and victims often have to be hospitalized.

Vinegar can be applied to the stung site to deactivate any remaining enmatocysts on the skin, but there is no antivenin for the venom that has already penetrated the body. Magnesium sulfate is used to treat Irukandji syndrome.

When properly treated, a single sting is normally not fatal, yet the resulting high blood pressure can be dangerous to vulnerable victims - at least one person died from brain haemorrhage within 30 hours after being stung-, and the delayed severe symptoms  might put victims into trouble if they are then in deep water or driving.

Furthermore, two people are believed from to have died from Irukandji stings in 2002, in Australia. It is now thought that these very small jellyfish might, in fact, have been responsible for a number of other deaths wrongly attributed to other causes.

- Fire Jelly, Tamoya, Moreton Bay Stinger (Morbakka fenneri)

The name Moreton Bay Stinger comes from the Australian bay where it is commonly found. This dangerous jellyfish, a close parent of the Irukandji, occurs in tropical Australian waters, all over Queensland and in northern New South Wales. Though it rarely occurs in the Gold Coast, the Fire Jelly, like for other jellyfish, tends to move further and further south, as the sea temperatures heat up.

It prefers calm waterways, such as marinas, and avoids rough waves.

The stinger's box-shaped bell is between 6 cm and 18 cm. It has four tentacles up to 1m long. It could be mistaken for Irukandji because of its shape and the identical number of tentacles. However, it is much bigger, since the Irukandji is no larger than a thumbnail.

Consequently, Fire Jellies are usually seen by their victims, unlike Irukandji's victims.

Fire jelly stings are painful but not as dangerous as Irukandji, and are normally not life-threatening. Stings leave a red mark on the skin, whereas stings by Irukandji don't.

- Carybdea alata

Along with Carybdea rastonii, this small box jellyfish often "swarms" to Hawaii's Leeward shores 9 or 10 days after the full moon. They occur mainly at Waikiki Beach, Ala Moana Beach Park, as well as Hanauma Bay, and sometimes in ocean areas and beach  front up to and including the Waianae Coast.
Both species - and especially Carybdea alata - have a very painful sting and can even cause anaphylactic shock in some individuals.

The sting of box jellyfish found in Hawaii, Carybdea alata and Carybdea rastonii, is very painful and can even cause anaphylactic shock in some individuals.

In case of a sting, it is advised to douse or spray the sting liberally with vinegar. Then to pluck off any visible tentacles carefully - using fins, a towel, or anything else, rather than fingers -. A hot pack, or a cold pack or ice, can help reduce the pain. Immediate medical attention may be required in cases of severe reactions.

- Carybdea rastonii

See the preceding one. Carybdea rastoni's toxins, CrTX-A and CrTX-B are large proteins. Like numerous carybdeids, some unidentified, Carybdea rastonii causes less severe illness.

Carybdea jellyfish are also found along all the coasts of Australia (especialy in South and Western Australia) where they are commonly called Jimble.

- Tripedalia cystophora

This box jellyfish lives in tropical mangrove swamps in Central America, sheltered in between the roots of the trees, probably to avoid predators.

This tiny box jellyfish is a highly poisonous predator which possesses 24 eyes, four parallel brains and 60 arseholes.

The eyes are arranged in clusters on the four sides of the cube-like body. Sixteen are simply pits of light-sensitive pigment, but one pair per cluster, though measuring hardly 0.1 mm in diameter, has a a sophisticated lens, retina, iris and cornea. The photoreceptors in their eyes are similar to those of vertebrates.

Those complex eyes probably help this jellyfish to hunt. They are very effective for spotting large, stationary objects, while filtering out unnecessary detail such as plankton drifting with the current.

Tripedalia cystophora is today threatened by the loss of its habitat. Indeed, mangrove forests are everywhere being cleared for development, agriculture, and fish and shrimp farms. These fragile shallow coastal environments and their rich marine life are also threatened by pollution and the sediment deposits that erosion upstream brings along.

Australian venomous jellyfish, envenomation syndromes, toxins and therapy.
J Tibballs
Irukandji jellyfish
Introduction to Dive Medicine Dangerous Creatures of the Sea
Box Jelly
Fire jelly fish on Coast
Kathleen Donaghey, February 3rd, 2009
Dangerous Jellyfish : Wildlife Australia