The pain wasn’t that bad, he said. The hit sensation felt like a wet towel and he had experienced worse stings from bluebottle jellyfish, he said.

Even so, he thought it best to pack up the tent and return to their resort to watch the whip, which was leaving clearly visible red stains, and to make sure it didn’t get any worse, he said.

As a volunteer lifeguard for many years, he would usually suggest treating the sting with vinegar, he said. But since there weren’t any at the resort, they had to improvise with another acidic substance: he stood in the bathroom while his wife poured soda on his back, he said.

“The stinging sensation went away almost instantly,” he said.

Judit Pungor, who studies octopuses at the University of Oregon, suggested that Mr. Karlson was accidentally struck by “one of the many stinging tentacle jellyfish common in Australian waters.”

Octopuses, she said in an email, “don’t have poison in their suckers, and any poison they have (in their bites, not on their arms) would not be relieved by pouring something sour on them.”

However, the animals, which are usually solitary, have been videotaped with fish being coiled up and beaten. Peter Ulric Tse, professor of cognitive neuroscience at Dartmouth College who studies octopus cognition, said they “can express what we would call aggression when they feel threatened or when they feel their territory is threatened “he wrote via email.

“I suspect the squid here is sending a warning that means ‘back’,” he said after watching Mr Karlson’s video. “Octopuses rush or shoot an arm out when they feel that there is a fish, another octopus, or a person in their room. I think this is often a preventative aggression designed to signal that I shouldn’t be messed with, and not an aggression that is seriously intended to harm the intruder. “