The pain wasn’t too bad, he said. The whipping sensation felt like a wet towel, and he has experienced worse stings from Bluebottle jellyfish, he said.
Still, he thought it was best to pack up the tent and return to their resort to monitor the lash, which left clearly visible red marks, and make sure it didn’t get worse, he said.
As a volunteer lifeguard for many years, he would normally suggest the sting be treated with vinegar, he said. But since there wasn’t any available at the resort, they were forced to improvise with another acidic substance: His stood in the bath while his wife poured soda down his back, he said.
“The stinging sensation went away almost instantly,” he said.
Judit Pungor, who researches octopuses at the University of Oregon, suggested that Mr. Karlson may have coincidentally been struck by “one of the many stinging, tentacled, jellyfish that are abundant in Australian waters.”
Octopuses, she said in an email, “do not have venom in their suckers, and any venom they do have (in their bites, not on their arms) would not be alleviated by pouring something acidic over it.”
The animals, which are usually solitary, have been captured on video winding up and punching fish, however. Peter Ulric Tse, a 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 under threat,” he wrote over email.
“My guess is that the octopus here is sending a warning meaning ‘back off,’” he said after watching Mr. Karlson’s video. “Octopuses will lunge or shoot an arm out when they feel a fish, another octopus or a human is in their space. I think this is often pre-emptive aggression, meant to signal ‘don’t mess with me,’ rather than aggression seriously meant to harm the ‘invader.’”