A gruesome video appears to show a slice of yellowfin tuna thrashing about on a fishmonger's tray - long after death.

The unsettling footage sees the chunk of fish come close to escaping the tray all together, as it flounders around for a minute and-a-half.

Those who witnessed the bizarre spectacle unfolding before them were clearly shocked by the strange sight.

The gruesome footage was uploaded by a Twitter user. Photo / via Twitter
The gruesome footage was uploaded by a Twitter user. Photo / via Twitter

But Twitter users were quick to point out that the fish could not possibly have been alive.

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Others theorised that the fishermen likely failed to sever the fish's nerves - which then caused involuntary muscle reflexes.

Muscle motor neurons are triggered by electrical signals, and they can still be active even after the brain has died.

Fishermen in Japan usually prevent this by inserting a spike in the fish's hindbrain, in a method called ikejime.

Some even suggest that the taste of the fish can be affected by failing to carry out ikejime, because the fish's muscles produce lactic acid as they flounder about.