When they're not stealing homes or plotting world domination, octopuses are constantly wowing us with their colours and camouflage. But explaining one cephalopod's recent inflation demonstration is proving a bit more challenging than expected. 

Image: Pink Tank Scuba/YouTube

YouTuber and underwater videographer PT Hirschfield encountered what has been nicknamed the "balloon" octopus while diving off the coast of Melbourne, Australia. The animal had been hunting crustaceans when Hirschfield made her way towards it – to be greeted with the peculiar display you see above.

"This octopus was not impressed when I interrupted its morning feeding stroll," she recalls on YouTube. "It blew itself up like a parachute multiple times to try to intimidate me, before trying to torpedo me like a bowling ball!"

It's possible that the Sydney octopus (Octopus tetricus) was trying scare off the human interloper – many animals, like pufferfish, employ the "make yourself big" manoeuvre to get out of dodge. But scientists have put forward other explanations since the video was posted.

In an interview with LiveScience, Dr Kathleen Sullivan Sealey, an associate professor of biology at the University of Miami, suggests the octopus was merely continuing its foraging mission. By spreading its arms like a large net, it might be trying to trap potential prey near its crushing beak.

What's more, by channelling water through its mantle (the bulbous structure above the eyes), it could be forcing shrimp and other small invertebrates out from under nearby rocks – essentially using its body like a power washer. 

Dr Crissy Huffard of the Monterey Bay Aquarium Research Institute (MBARI) agrees. "Many shallow-water octopuses do a version of this. They 'cast' their web over rocks or seaweed and catch the prey that gets trapped underneath." Similar foraging behaviour has been observed in other octopus species, too.

Hirschfield suspects it may have been a combination of the two – feeding and posturing – but it's also possible that this was something else entirely. 

"Towards the end of ten minutes it definitely seemed to want its own space, and made no apologies for hunting for crabs right next to my body in a way that I'll admit was a bit intimidating!" she told LiveScience. "[It] decided to send me a strong message by torpedoing directly at me like a bowling ball with tentacles. I got the hint and let it go."

We'll post updates if more information about this behaviour comes to light. If you've encountered something similar on a dive, let us know in the comments below!


Top header image: John Turnbull/Flickr