Asked by: Annie Morse, Oxford

Electric eels - actually a type of knifefish, not true eels - are notorious for being able to produce a hefty electric shock of up to around 600V. The source of their power is a battery-like array of cells known as electrocytes, which make up around 80 per cent of the eel's metre-long body.

Disc-like in shape, these cells each acquire a small potential difference of around 1/10th of a volt by controlling the flow of sodium and potassium ions (charged atoms) across the cell membranes. Linked together in arrays of thousands of individual cells, the result is a kind of natural car battery, which releases its charge when the eel spots predators or prey. Up to 0.5kW of electric power is released per shock - enough to inflict significant injury on a human.

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Robert is a science writer and visiting professor of science at Aston University.