Des Mellican, 50, has been working on Mornington Island, 28 kilometres off the coast of northern Australia, and was exploring the Gold Cliff area when suddenly a "beautiful" creature "as big as [his] hand" caught his eye.
"I was walking along the rocks just looking for starfish when I saw a blubber-like creature. Upon inspection, I identified it as an octopus," he told Yahoo News Australia.
However, the "penny didn't drop" quickly enough for Des who didn't realise it was a toxic blue-ringed species. "It was so big. I just thought there can't be a blue-ringed [octopus] that big".
Larger than usual octopus: 'As big as my hand'
Typically just seven to 10 centimetres in size (or sometimes even smaller), this blue-ringed octopus was up to 20cm. "It was as big as my hand," said Des.
The species he stumbled upon was an "undescribed" species — not one of the current four types known— which is most common around the northern parts of Australia, octopus researcher Dr Julian Finn told Yahoo News Australia.
This other species is much larger than the typical blue-ringed octopus often seen along the east coast of Australia, including the southern blue-ringed octopus (maculosa) and the blue-lined octopus (fasciata). The same kind was spotted on a beach in Darwin this week, leaving Aussies stumped because of the larger size of the usually small marine animal.
'Fascinating' moment after lethal encounter
It was only when Des sent a video and some photos to his friends he learned of his close encounter and was told he'd just discovered a lethal octopus. "I knew they were poisonous but I didn't really know what they looked like," he said. "I just thought it was a beautiful-looking creature. When someone said it was a blue-ringed octopus thought 'of course it is'.
Being from Brisbane, Des had never seen one before, but despite this, he said he was "cautious". Using a thong, he carefully guided the octopus back into the ocean — then something "amazing" happened.
"It turned bright orange," he recalled. Before that, the octopus was almost white with "faint blue rings' and nothing near as vibrant. "It was fascinating to watch."
Octopus changes colour when threatened, says expert
Dr Finn said this is pretty normal behaviour and happens when they are "startled" or feel "threatened". "If you see this display from a blue-ringed octopus, you are probably too close," he warned.
One photo shared with Yahoo shows the creature almost camouflaging with the sand. In another, it's a brighter orange with neon-like blue rings.
"When a blue-ringed octopus is out foraging, it will generally have a more subtle colouration, blending in with the environment, to avoid being seen by potential predators, and allowing it to sneak up on prey," Dr Finn explained.
But when it feels threatened, in this case by Des's thong, "it immediately responds with an aposematic display" — a display to repel predators, and in this case indicates that it's venomous.
"The blue-ringed octopus produces this display by relaxing the muscle around the blue rings, making more of each ring visible and darkening the area around the rings to create contrast. This species also turns orange," Dr Finn added
Thankfully, Des walked away unharmed with a story to tell and the octopus was safe too.
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