"How's it feel, Kingslayer, to have all the gold in the world but it won't buy your sword hand back?"

"A minor setback, if truth be told. But look - I have spares!"

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This tiny species of jellyfish is one of the most venomous creatures in the world. Thanks to its role in the death of Robert King, a Nestlé research scientist from Ohio, it has been nicknamed the Common Kingslayer.

In 2002 King had travelled to Australia with his partner to spend some time snorkelling in Opal Reef, which is just off Port Douglas in Queensland. Earlier that year, British tourist Richard Jordan had been stung by a jellyfish 700 km away on Hamilton Island. The venom from the sting brought on Irukandji syndrome, and Jordon was its first documented fatality. Named after a local Indigenous Australian tribe near Cairns in Queensland, Irukandji syndrome is brought on by a sting from one of 10 species of venomous jellyfish found in tropical and temperate oceans around the world. Many of them are found in Australian waters.

(And now would be a good time to mention that there's a book by Sydney-based author, Wendy Lewis, that recounts King and Jordon's tragic encounters with these jellyfish called See Australia and Die. SEE AUSTRALIA AND DIE. Help I live here and my cat brings spiders into my house through the window.)

While rarely fatal, Irukandji syndrome sends most of its victims to hospital. It takes 5-10 minutes after the sting for symptoms to set in, but when they do, they're excruciating. A typical set of symptoms includes severe lower back pain, vomiting and muscle cramps, and if particularly serious, could result in toxic heart failure, fluid on the lungs or a brain haemorrhage. Around 30% of cases result in some form of heart failure, and one in five victims suffer life-threatening complications and end up on life support.

In 2002 a record number of jellyfish appeared in the tropical waters of northern Australia, blown close to the coast and its tourist beaches by unusually strong and irregular winds. During the holiday season of November to May in a regular year, an average of 30 people are hospitalised with Irukandji syndrome. But in 2002 between December and January alone, that number leapt to 80 victims at a single hospital in Cairns. That year the world's first and only deaths from Irukandji syndrome would be recorded. It's possible that Jordon's death was brought on by the open heart surgery he had endured some months earlier, which could have made his symptoms much worse, but Robert King was in perfect health when he was stung. He died in hospital with toxin-induced hypertension, which led to bleeding in multiple areas of the brain.

Jellyfish deliver venom via nematocysts. When a jellyfish's tentacle touches its victim, these little harpoon-shaped darts fire into the victim's skin to deliver venom into the bloodstream. No nematocysts were recovered from Jordan's body, which made it difficult to nail down a culprit, but it's widely assumed that he was stung by Carukia barnesi, a box jellyfish species from the Whitsundays area commonly known as the Irukandji jellyfish.

A number of nematocysts were recovered from King's body and clothing, and were examined by Australian jellyfish expert and CSIRO researcher, Lisa-Anne Gershwin. In her 2007 Zootaxa paper she concluded that these nematocysts were like none she'd ever seen before. She compared cells from these nematocysts with those from several specimens of a new species she had found in 1999, and it was a perfect match. In 2007, Gershwin named this new species Malo kingi, after the scientist it felled.

Some species of box jellyfish (cubozoans) can weigh up to 2 kg, but many are absolutely tiny. Like the bell-shaped body of C.barnesi, M. kingi's bell grows to no more than 3 cm high. And not only is M. kingi tiny, it's also transparent, making it extremely difficult to spot in the water. The species is distinguished from all others by having strange halo-like rings of tissue encircling its four tentacles.

Gershwin reported that M. kingi's danger to humans has been ambiguous, citing several attacks that have resulted in only minor symptoms. In the 1960s, Australian medical toxicologist, Jack H. Barnes, tested the venom of a M. kingi specimen in his collection, and got off lightly. In his report, Barnes even named his attacker 'Pseudo-Irukandji', because of its inability to inflict the full force of Irukandji syndrome. Gershwin noted that Barnes happened to choose the smallest, and likely youngest, specimen in his collection, and argued that a life in captivity could have messed with the jellyfish's toxicity levels:

"His specimens had been in captivity for hours prior to the stinging experiment; the degree to which the condition of the animal affects its sting ability is undocumented, but I have often observed cubozoans stinging each other and the sides of the collecting container. It is not difficult to imagine that they could have expended much of their stinging ability without sufficient time to regenerate it."

In 2003, while investigating King's death, Gershwin suffered a M. kingi sting herself, and just like Barnes, her symptoms were mild. Her attacker was also small, and Gershwin noted that it was missing the distinctive halo-form tentacles, plus gonads, suggesting to her that it was a juvenile. "I was personally stung quite extensively across the palms of both hands by this species in June 2003 at Port Douglas, without systemic effects," she wrote. "However, both hands blistered badly and several layers of skin completely peeled about one week after the sting event. The specimen proved to be immature when examined, and was subsequently used for DNA analysis."

Gershwin looked into a number of other non-fatal M. kingi attacks, and found that none of them could be attributed to an adult specimen. "It is possible that as Pseudo-Irukandji matures, it concurrently changes its physical and chemical properties, becoming lethal with halo-form tentacle bands," she concluded. So if you're going to have an encounter with the Kingslayer, make sure it's a baby one.

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