So we've been sent this one a LOT recently, with everyone asking for an explanation – how and why is this fish still flailing about and seemingly trying to escape – minus his head and guts?
The explanation is a reasonably simple one, and also a pretty common phenomenon as well – we also see the same jerking movements from fresh raw frogs’ legs and diced up squid.
Frogs' legs dancing after being laced with salt. Posted by Thearchipelagos via YouTube.
Described by the uploader as a squid, although it appears to be a cuttlefish, the chef dices the animal in a particular fashion in order to serve as sashimi. The Cephalopoda seems to perform an eerie post mortem jig after its body has been removed. Soy sauce, which is rich in sodium, can also be used to exacerbate muscle movement after death. Ahbtsang/full video via YouTube.
Sashimi, a Japanese dish typically served as a starter, is defined as a thinly sliced raw food that has been prepared in this manner for centuries. It may shock some viewers as the chef slices into the live fish that it continues to move after being chopped into pieces – however, the Cephalopoda dies relatively quickly, and its continued movements post-mortem are explained below.
Human corpses are also known to randomly jerk and move limbs for hours after death – although this is due to a different mechanism from that in the dancing squid.
So why, without signals sent and received via the brain and nervous system, or a beating heart, does dead tissue continue to move? Although the brain and heart are not functioning, there are cells that can still respond to stimuli, for example, added sodium. Immediately after death, muscle motor neurons (the nerves that create movement within the tissue), which are triggered by electrical signals, still contain some membrane potential (difference in ion concentrations).
All cells are polarized, which means that there is a high-to-low gradient of charged atoms, or ions, from inside cells to outside them. The difference between these concentrations is what creates a charge across a membrane.
When not being activated by the nervous system, neurons maintain their membrane potential by pumping out a balance of sodium and potassium ions (both needed to instigate neurons firing). However, when the neuron is activated with an electric signal, specific channels within the cell open up, allowing sodium ions to flood in – and as equilibrium of charge in the cell to its environment is required, potassium channels are, as a result, also opened up, causing them to flood out of the cell.
Eventually the channels close and the neurons work to restore balance between concentrations of sodium and potassium inside and outside them – but not before triggering nearby channels to open, causing a chain reaction within the muscle.
This is basically how neurons create movement within a tissue.
As previously mentioned, immediately after death, motor neurons maintain some membrane potential, or difference in ion charge, which then starts a domino effect down neural pathways causing movement. Adding additional sodium, in the form of table salt or soy sauce, will significantly increase the reaction.
So Mr. Frog and Mr. Fish are definitely dead – their muscles just haven’t got the memo yet.