Aussies stumped as rare lethal octopus washes up on Darwin beach: 'Much bigger'

The size of the blue-ringed octopus left some in disbelief and unable to identify the dangerous sea creature.

The distinctive blue markings on a deadly blue-ringed octopus make the marine animals easily recognisable, even if its average size of just seven centimetres can make them easy to miss.

But the sight of an octopus washed up on a beach in Darwin on Sunday stunned many who saw it and agreed the sheer size of the colourful mollusc was "much bigger" than a typical blue-ringed species.

"A friend of mine found this washed up on a beach in Darwin, Australia," a woman shared on Facebook. "Much bigger than what a blue-ringed octopus should be, so stumped on what it is".

Suggestions came in thick and fast from some who attempted to identify the creature. Many of them said the deceased octopus was a Hapalochlaena lunulata, also called the Greater blue-ringed octopus due to the size of its blue rings.

Unnamed blue ringed octopus on Darwin beach.
The large blue-ringed octopus appeared about 20cm in size, much bigger than any other species of blue-ringed octopus. Source: Facebook

Size of octopus confuses social media

Commonly known as H. lunulata, this is one of four blue-ringed octopus species found in Australia — all of which are deadly. They each contain a venom known as tetrodotoxin.

But the Darwin beach find "seems too large" to be an H. lunulata some said, which is only known to grow as big as 12 centimetres. The poster said this find was about 20cm, further baffling many Aussies.

Other recognised species include the southern blue-ringed octopus (maculosa) and the blue-lined octopus (fasciata), which are commonly found in rocky reefs and seagrass beds across Australia. But this octopus does not appear to match. The fourth, and far less common is the nierstrasziis, which has never been sighted in Australia.

The greater blue-ringed octopus (Hapalochlaena lunulata) Source: Google Images
The greater blue-ringed octopus (Hapalochlaena lunulata) Source: Google Images

Mysterious blue-ringed octopus species identified

But as it turns out, there are far more than just the four species that exist, and the one shown in the image is pretty common – at least in the northern parts of Australia, including Darwin, one octopus researcher told Yahoo. Although it remains "undescribed" it's never been formally named.

"It's the main species that you would get up there. People get it confused with the Hapalochlaena lunulata," Julian Finn, Senior Curator for Marine Invertebrates at the Museums Victoria Research Institute told Yahoo.

"The reason it gets confused with that species is because it looks similar in photos [because of it's large rings]. But that species is only, at most, about 12 centimetres long. It's a small octopus."

The lunulata is often seen in coral reefs and normally a little deeper in the ocean. This octopus on the other hand lives on the soft sediment around Darwin and is often exposed to tidal zones and harbours.

The southern blue-ringed octopus (Hapalochlaena maculosa) (left) and the blue lined octopus (Hapalochlaena fasciata) (right) are two common species. Source: Google Images
The southern blue-ringed octopus (Hapalochlaena maculosa) (left) and the blue-lined octopus (Hapalochlaena fasciata) (right) are two common species. Source: Google Images

"It'll come out when there's low tide and it'll hunt for things trapped in rock pools and catch them quickly and incapacitate them and eat them. So they're there, it's just people don't always encounter them," Mr Finn explained.

Speaking of its large size, Julian presumed the octopus in the image was a female as "males tend to be considerably smaller". He also said the body appears to be swollen and full of eggs.

To summarise, Julian said, while similar in appearance, it's incorrect for it to be called the lunulata. However, one thing remains the same.

"I can say quite confidently that it does have the same venom and it is a venomous species, there's no doubt about that," Julian warned.

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