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Stinger season

With marine stinger season upon us it's a perilous time to be seduced by the inviting waters of tropical northern Australia. Lurking just offshore are flotillas of almost invisible killers, with venomous tentacles trailing more than four metres behind them.

The Irukandji

The Irukandji is only 2.5cm in diameter, which makes it very hard to spot in the water (Source: /Dr Lisa-Ann Gershwin, Surf Life Saving)

Also known as the Indo-Pacific Sea Wasp, the box jellyfish Chironex fleckeri, is the world's most venomous marine creature, responsible for as many as 60 deaths in 50 years in Australia, as well as countless numbers in other tropical areas.

Between November and May, 'boxies' as they're affectionately known migrate to Australia's shallow, murky coastal waters and estuaries. Their travelling companion is the tiny Irukandji, Carukia barnesi, which, while only about the size of a thumbnail, can land an adult in intensive care, or even result in death.

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Deadly stings come in big packages

Box jellyfish and Irikandji both feature a roughly box-shaped bell. The box jellyfish has a cluster of tentacles at each corner; whilst the Irikandji has only a single tentacle.

Each of their trailing tentacles is packed with nematocysts, stinging cells that contain a tiny harpoon attached to venom-filled bulb. Each species has several types of nematocysts, reflecting their individual ways of feeding and catching prey.

The bell of the box jellyfish, C. fleckeri, can grow up to 38cm. Despite this and their often-fatal effect on humans, people are not the boxies' prey. So why are they so toxic to us?

According to Dr Lisa-Ann Gershwin, National Marine Stinger Adviser, two theories predominated until just recently. The first said that stingers are fragile animals that must quickly subdue prey to prevent damage to themselves, while the second said the toxic tentacles gave protection from predators.

Recently, however, venom researcher Theresa Carrette from James Cook University's Tropical Australian Stinger Research Unit discovered that it's only when a box jellyfish gets over a certain size that it becomes lethal to vertebrates.

"Once their bell reaches 8-10 cm, not only does the venom become more toxic per drop, but the ratio of highly venomous nematocysts per tentacle changes as well," says Gershwin. "This is related to their food preference: below 8cm they prefer to feed on prawns; above they like fish. Not only are fish larger and faster than prawns, the vertebrate nervous system is also completely different, so the jellyfish must adapt their prey submission technique to match; it's just unfortunate that they have the same deadly effect on humans."

About 2-3 metres of tentacle is needed to kill an adult human, and when the box jellyfish's bell is below 8cm there is simply not enough tentacle length to skin surface for a human to be fatally stung .

"However," cautions Gershwin, "I wouldn't take the risk of assuming that if you see a small boxy in the water with you it won't kill you - it could always be an early developer - or an Irukandji."

Fact file:

When: Sting season varies depending on location: in Darwin, the season runs from early August to late June, while in the Cairns-Townsville region, it is November to June.

Where: Box jellyfish are found in northern Australia from Gladstone in the east to Exmouth in the West.

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Irukandji syndrome

While a sting from the smaller Irukandji jellyfish may not kill outright, it causes what is known as Irukandji syndrome, a set of symptoms that includes lower back pain, muscle cramps, vomiting, fluid on the lungs, hypertension and heart failure (if untreated).

Originally named for Carukia barnesi, the first jellyfish demonstrated to cause Irukandji syndrome, it now refers to any jellyfish whose sting produces these symptoms. While Irukandji syndrome is found primarily in the waters of the Tropic of Capricorn, cases have also been reported in Sydney, Geelong and Rottnest Island. Similar symptom sets have shown up in as diverse locations as North Wales and Waikiki, leading researchers to believe that Irukandji syndrome-causing jellyfish are probably spread throughout the world.

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Active hunters

Far from being passive predators that wait for hapless prey to bumble into their deadly tentacles, all cubozoans have eyes so that they can hunt as well as avoid objects in the water.

And just to add to the boxy's fearsome reputation, it now also appears that rather than being at the mercy of wind, tides and currents like many jellyfish, the box jellyfish C. fleckeri is capable of swimming at up to 3 knots (approximately 5.5 km/h). Few people could outswim a determined boxy.

One of the big questions of boxy behaviour has been why they move onshore during the summer months, and where do they go during the 'off-season'?

Gershwin says that at the start of stinger season mostly small jellyfish are found near the shore, suggesting they breed in shallow estuaries and creeks and move into the coastal zone as they grow-at a staggering 1mm a day.

Big boxies are sometimes found near the beaches early in the season indicating that they live for at least one year (and have probably returned to the coast to spawn). Prawn trawlers are known to catch large box jellyfish out to sea during winter, suggesting that while they may move into deeper water to catch larger prey as they grow, if big fish are in short supply they will revert to feeding on prawns and other crustacea.

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Sting-free swimming

Despite the obvious dangers, people continue to throw caution to the winds and swim in stinger-infested waters, in the mistaken belief that they won't be stung - even, according to Gershwin, when researchers are catching dozens of jellyfish just metres away.

"The simplest way to avoid being stung is to use common sense," she says. Stay out of the water when a beach is closed, or swim in the stinger proof enclosures that are located on many of the popular beaches.

Protective clothing like a full-body lycra suit can also help: "We have a saying that if you can't see through it, a jellyfish can't sting through it," says Gershwin.

If you do get stung, your only hope of survival might come from a humble bottle of vinegar, which disables the nematocysts and stops them firing.

"Supported by Surf Lifesaving Australia and local businesses, bottles of vinegar are found on many beaches in North Queensland," she says. "We also encourage people to be pro-active and not only pack their sunscreen, but to pack their vinegar too - it might save more than just their own life.

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Further info and credits

Tropical Australian Stinger Research Unit

CRC Reef Research Centre

Thanks to Dr Lisa-Ann Gershwin from the Surf Life Saving/QLD Government Irukandji Task Force for the fantastic photographs.

Tags: marine-biology, invertebrates

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Published 26 January 2006