Having eaten a fresh mackerel from a supermarket, I put the oily, greasy wrapping paper together with a few of the bones to one side for disposal. Later, when the room had darkened, I noticed a glow coming from the fish oil. The light seemed to come from the fish juice, not the bones. Is this a common effect in fish and if so what is the mechanism? Or was this fish caught near a nuclear power plant?
Fish, it seems, glow for all manner of reasons. Which of the processes below was responsible for this late-night light show may never be known 鈥� Ed
鈥� This is a common effect, not only found in raw, dead fish, nor just organisms found near nuclear plants. The light comes from bioluminescent bacteria. Why these bacteria behave in this manner is something of a puzzle, but they are of wide significance in biology. You also find them living on dodgy meat, wood and fungi, and in deep-sea fish. These last creatures have evolved such elaborate symbiosis with these bacteria that they cultivate them in special organs and then use them to produce controlled flashes of different colours of light.
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鈥淭he light is from bioluminescent bacteria, which can also be found on dodgy meat, wood and fungi. Deep-sea fish even cultivate them in special organs鈥�
Meat or fish glowing with some of these bacteria may still be edible, because such bacteria are not generally harmful, though the food soon becomes unappetising. And less benign bacteria may also be helping the rotting process, so be wary of eating such food just for the fun of experiencing glowing saliva.
Any nutritious organic material might harbour luminous bacteria, whether in a living organism or not. I have known earthworms emerging on a warm damp night in a forest to produce slime so bright that it seemed glow-worms were responsible, and my fingers became slimy (and shiny) trying to catch these non-existent beetles. And it took me years to realise why not all earthworms shone at night.
Jon Richfield, Somerset West, South Africa
鈥� The luminescence was probably caused by marine bacteria, most likely of the Vibrio genus, which are bioluminescent. Vibrio bacteria are responsible, for example, for the glow of the deep-sea angler fish鈥檚 lure. If the questioner is still in good health after eating the fish, we can assume it was not the closely related, but pathogenic, Vibrio cholerae.
Meredith Lloyd-Evans, Cambridge, UK
鈥� This glow is simply caused by the high content of phosphorus compounds within the fish oil. These compounds are phosphorescent and will emit at visible wavelengths of light any energy they have absorbed while exposed to other wavelengths, such as the ultraviolet from fluorescent lighting or sunlight.
Accounts of this phenomenon and other fascinating related details can be found in the book The Shocking History of Phosphorus: A biography of the Devil鈥檚 element by John Emsley and published by Cambridge University Press.
Martin Barfield, Hambleton, North Yorkshire, UK
鈥� The glow sometimes occurs in fish flesh that has spent a period of time in storage and is caused by the rapid growth of spoilage bacteria present mainly on the surface of the flesh. It is most usually a result of temperature abuse 鈥� the fish has been stored at too warm a temperature or the temperature of the fish has been allowed to rise for a period of time while being transported.
鈥淭he glow can occur in fish that has been in storage and is caused by spoilage bacteria on the surface of the flesh. It is often a result of warm temperatures鈥�
The bacteria tend to be of the Pseudomonadaceae family. They live quite happily in saltwater and the glowing effect is often found on cold smoked fish that has either not been cooled rapidly after the smoking process or where the brine for the salting operation has not been changed frequently enough. In the latter case a high number of the bacteria concentrate in the brine, are then transferred onto the fish and in turn multiply rapidly during the smoking process.
This is not related to the rainbow effect that can sometimes be seen on the surface of freshly cut tuna or other fish or meats. This effect is a result of the way in which the muscle fibres are cut by the chef鈥檚 knife, which leads to a splitting of the light that is reflected by the flesh.
Your correspondent will, I imagine, be pleased to learn that the glow is most definitely not caused by the fish being caught near a nuclear power station.
Adrian Barratt, Trade and Quality Executive, The Sea Fish Industry Authority, Hull, East Yorkshire, UK