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Scientists have been left stumped by a giant jellyfish with no tentacles that was found off the coast of Australia.

The huge creature, called Keesingia giga, has been described by experts as extremely venomous and 'as long as a human arm'.

It is thought to cause Irukandji syndrome, a condition that can be fatal to humans.

A giant new species of lethal jellyfish discovered in Australian waters. Keesingia gigas (shown) are gargantuan members of the Irukandji jellyfish family. They can grow to be as long as a human arm, but other members of its genus are only as big as a fingernail

The jellyfish was found off the north-west coast of Western Australia by Dr Lisa-ann Gershwin, the director of Marine Stinger Advisory Services in Queensland reports The Guardian.

It is a species of Irukandji jelly, which are normally only about as big as a fingernail in size.

Keesingia gigas, however, was found to be almost as big as a person's arm. 

Such animals have been spotted before but a specimen was not captured until 2013.

'It is absolutely humongous - the body is about 30 to 50 centimetres [12 to 20 inches] tall... It's an absolute whopper of a jellyfish,' Dr Gershwin told ABC Australia.

In her paper discussing the species, she mentions how the sting of the jellyfish has been studied previously.

In one instance a person was struck with severe Irukandji syndrome, while another victim had just local and groin pain.

In the past Irukandji syndrome has been fatal in humans, as it can cause strokes or heart failures in addition to nausea and vomiting.

The jellyfish has been spotted off the north-west coast of Western Australia at Shark Bay and Ningaloo Reef

Scientists are confused as the jellyfish has been found with no tentacles (seen in picture). It is also known to carry a powerful venom that can cause Irukandji syndrome. This can cause nausea and vomiting and in some instances can even be fatal in humans

The jellyfish was found off the north-west coast of Western Australia by Dr Lisa-ann Gershwin, the director of Marine Stinger Advisory Services in Queensland. Such animals have been spotted before but a specimen was not captured until 2013

Every Kessingia gigas that has been found so far, however, has been seen to have no tentacles – leading to some confusion.

It is widely accepted that the sting is transmitted through tentacles, so quite how this creature performs an attack is a mystery.

'Jellyfish always have tentacles...that's how they catch their food. The tentacles are where they concentrate their stinging cells,' Dr Gershwin told the Australian Assocaited Press.

'I think probably it does have tentacles but by random chance the specimens that we photographed and obtained don't have them anymore.'

This could have been because it sheds them on a regular occasions as a defence mechanism, Dr Gershwin added.

'I think it's probably a fairly tame explanation - I just don't know what it is.'

And Dr Jane Fromont of the Western Australian Museum, where the species will be kept, has warmed that swimmers should be wary of these creatures in future.

However, it is not necessarily the size of the jellyfish that should be feared - previous Irukandji jellyfish that have killed humans have been known to be as small as a peanut.

Pictured here is a Malo kingi jellyfish, part of the same family as Keesingia gigas, which has been known to kill a human. Irukandji jellyfish are normally about this size, that of a fingernail, making the recent finding of Keesingia gigas all the more astounding