A viral video that apparently showed a crab hilariously wielding a knife actually probably just showed humans being mean, an expert has said.

The video showed a small crab apparently threatening a person with a huge knife that it was carrying in its claw. As the person, or would-be chef, got close to the crab, it appeared to wave the knife in a threatening motion.

The crab in the video is “absolutely distressed”, according to an expert who has reviewed the footage.

It was probably carrying the weapon because a human had pushed it into its claw, an expert has said. The waving was likely just a natural reaction to being chased by a predator, in this case the human carrying the camera.

"It's a normal behaviour,” Jack Cover, of the US National Aquarium, told the Washington Post. “If the knife wasn't there, the crab would be swinging around.

“You can put a plastic gun in that crab's claw as well and it'd look like a gun-wielding crab."

The video became hugely popular last week, just after another very similar one last month. One headline described the video as “hilarious”, and most seemed to imply that the crab knew it was going to be eaten and so had grabbed the knife as a way of fending of the chef.

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Mr Cover told the Washington Post that the video was a “hoax”. But it does appear that the events depicted did happen — just that they didn’t happen for the reason everyone thought.