A giant and extremely venomous jellyfish found off Western Australia’s north-west coast has researchers stumped because it appears to have no tentacles.

Keesingia gigas is one of two new species of Irukandji jellyfish recently discovered by the director of Marine Stinger Advisory Services, Lisa-ann Gershwin.

While Irukandji jellyfish are normally only the size of a fingernail, Keesingia gigas is the length of an arm and believed to cause the potentially deadly Irukandji syndrome.

The condition can cause pain, nausea, vomiting and in extreme cases, stroke and heart failure.

Gershwin said Keesingia gigas was first photographed in the 1980s, but a specimen was not captured until 2013, near Shark Bay by the marine scientist John Keesing, after whom the jellyfish is named.

Gershwin said in all of the photos the jellyfish did not appear to have tentacles and that the specimen was also captured without them.

“Jellyfish always have tentacles ... that’s how they catch their food,” she said. “The tentacles are where they concentrate their stinging cells.

“Some of the people working with it through the years actually got stung by it and experienced rather distressing Irukandji syndrome.”

Gershwin said the species could shed its tentacles as a means of defence, like some bioluminescent jellyfish that drop their glowing tentacles in order to distract predators, but there was no evidence that any Irukandji had that capability.

“I think more probably it does have tentacles but by random chance the specimens that we photographed and obtained don’t have them any more,” she said.

“I think it’s probably a fairly tame explanation – I just don’t know what it is.”

Irukandji jellyfish have been found as far north as Wales in the northern hemisphere and as far south as Melbourne and Cape Town. Sixteen species are believed to cause Irukandji syndrome, four of which are found in West Australia