Living light

23rd March 2021

This blog is about a form of bioluminescence known variously as ‘sea sparkle’, ‘milky sea’ or more poetically ‘mareel’, which is a Shetlandic name meaning ‘sea fire’, (from Old Norse marr (“sea”) + eldr (“fire”). Also “miracle”, “morrali”, “glimro”)  

A near miss:

I feel greedy, wanting to see sea sparkle. The sea provides us already with seemingly inexhaustible inspiration and possibility; to want it to also put on a light show seems ungrateful.  I must admit though, since I first read descriptions of night-time glowing seas, I have wanted to see them for myself.

I got quite close once. I was ‘crewing’ for the first leg (Eastbourne to Falmouth), of a sailing trip that was going to cross the Bay of Biscay, carry on down to north Africa and eventually cross the Atlantic. The skipper was to pick up different crew for the various stages and on this first leg it was myself, a friend, my father-in-law, and a young vivacious eastern European girl whom none of us had met before called Lenka1.

This was to be my first time sailing at night and I was full of romantic expectation of seeing the mast ‘stir a sky full of stars’, the ‘loom’ of the lighthouses we would pass and maybe even sea sparkle. However, although we had good wind and a fast trip2 none of this happened. The sky was overcast, and the veiled moon and shore lights reflecting from the clouds prevented it from ever getting properly dark. There was not a star to be seen and certainly no glowing seas. With a strong north wind and a rough sea, it was also extremely cold and uncomfortable3.

We arrived in Falmouth tired and the three of us were relieved to climb into the warmth and comfort of a modern car and head for home.  

Lenka was staying on for the next leg and, after a day recovering, was to be joined by two new crew members for the trip across Biscay.  We swapped email addresses so we could keep track of how the rest of the journey went and said goodbye.

A couple of days later a wonderfully joyous email arrived in my inbox. High on tiredness and excitement, writing from the plane on the way back from Spain, Lenka described, in ecstatic broken English, how they had met not only bad weather and rough conditions but large areas of sea sparkle4. As the boat had ploughed into each glowing wave, cascades of glimmering sea spray had been fired back over the deck, drenching the crew, who, excited and laughing, had also begun to glow. Lenka described how the luminous water had run into their eyes, and mouths, picking them out and making them shine with sea fire.

As you might guess, I was sorry not to have been a part of this; to be aboard a boat sailing through a spectral sea with a maniacally laughing and glowing crew is something you will not get to do every day.

While I am unlikely to get the chance of experiencing sea sparkle in quite such dramatic circumstances I would still like to see it.  I have therefore made these few notes, partly out of curiosity and partly in the hope of increasing my (and anyone else who is interested) chances of seeing it in the future.

What is sea sparkle:

Nature has learned, through the guided play of evolution, to perform various tricks with light. Some of these modify light present in the environment, while with others generate ‘new’ light. The bright blue flash of a kingfisher, the shimmer of certain butterflies’ wings or the rays of the blue-rayed limpet5 are all examples of a phenomenon known as iridescence, in which the microscopic structure of the wing, feather or shell modifies the light falling on them, and causes them to shine and shimmer. Although iridescence generates no new light the effect can be surprisingly bright; catch the sun on the back of a kingfisher as it flies low over a river and you would think it was lit from within. 

Blue-rayed limpets – an example of iridescence in marine life.

Bioluminescent creatures on the other hand generate light and hence shine or glow in the dark. 

Instinctively, forgetting science for a moment, the ability to produce light seems to be somehow miraculous. There is something elemental about light.  It seems more than just another physical phenomenon. Light and its counterpoint darkness are a primeval pair of opposites.  ‘Let there be light’, the ‘light of reason’ and the ‘dark ages’, the ‘glimmer of consciousness’, the depiction of holy people as having haloes and the very word ‘enlightenment’, all contain light at their core. In children’s books as well, the ability to generate light is depicted as magical and firmly in the domain of wizards or supernatural beings, such as Harry Potter, Gandalf, Ged8 or ET. 

Such connotations seem to give light, and the ability to generate it, a mystical or even spiritual aspect6,7.

However, despite all this, being able to generate light through bioluminescence, is in fact very natural and commonplace. Many, many creatures, especially marine creatures, bioluminesce and it is thought that nature has reinvented bioluminescence fifty or so times during the course of evolution. The tree of life is, like a Christmas tree, decorated with many twinkling and flashing lights. 

The bioluminescent creature responsible for sea sparkle is a tiny single celled organism that drifts around the oceans as a component of plankton. It has the pretty Latin name ‘Noctiluca scintillans’, which translates as ‘sparkling night light’.

Undisturbed Noctiluca produces an extremely pale glow that is hard to see, but this changes if the water is disturbed by wave action or by someone walking or swimming. The cells then flash and sparkle with a blue-green light9 as a defence mechanism, to put predators off feeding, or to attract larger predators which will, in turn, feed on the predators of Noctiluca10

Swimming dolphins stimulate Noctiluca scintillans as they disturb the water11.

The mechanism by which bioluminescence is generated is chemical and is similar across many bioluminescent species. It involves an enzyme called Luciferase, and a light emitting compound called Luciferin12,13. In sea sparkle the Luciferase-Luciferin reaction is stimulated when the shape of the cell is distorted by the forces within the disturbed water. I have read that cells only sparkle at night and that even if you take a jar of sea water containing sea sparkle into a dark room it will not sparkle unless it is also night-time. It seems from this that although only a single cell Noctlluca must contain an internal clock, (but then you wonder if they also take account of the different lengths of night and day in winter and summer in which case, they also need to keep track of the time of year as well as the time of day). 

Finding sea sparkle

To be able to see sea sparkle it is necessary for Noctiluca to be present in large numbers; the conditions for this to occur therefore provide a guide for when it might be best to go hunting for it. 

These conditions include:

  • Warmth:  Noctiluca reproduces more readily in warm water, so looking after periods of warm weather might be a good idea.
  • Shelter:  Sheltered, calm bays are good hunting grounds as here the plankton stay near the surface of the water rather than becoming distributed throughout its depth. 
  • Onshore breeze: A gentle onshore breeze may also help as this will push the surface water containing the creatures toward the shore.

Even under these conditions I get the impression that the appearance of sea sparkle is quite erratic and unpredictable. There are, however, various internet groups dedicated to reporting sightings, including one dedicated to sightings around the Welsh coast14. Keeping an eye on such sites could significantly increase the chances of catching this elusive phenomenon.

If you go hunting, good luck!


  1. Not her real name.
  2. Approx. 280 miles in 34 sailing hours (8.3 knots approx. average).
  3. It is one of the strange things about sailing that despite uncomfortable experiences like this it draws you back time and again.
  4. I do not know how large the areas Lenka sailed through were, but I have read that sea sparkle can cover thousands of square miles and be visible from space.
  5. There is a short blog I wrote about Blue-rayed limpets here.
  6. It seems to me a particularly charming aspect of bioluminescence that it is what might be considered more lowly creatures, such as glow worms, fireflies, jellyfish, and single celled plankton drifting on sea currents, that have the ability generate light, while the ‘higher’ mammals, including man, are denied it.
  7. This aspect was nicely brought out in the Disney film ‘Avatar’ in which the fantasy world of Pandora was lush with bioluminescent plants and creatures.
  8. Ged was a wizard in Ursula le Guinn’s excellent Earthsea Trilogy (which included the original school of wizardry on ‘Roke Island’).
  9. This shade of blue-green light is the best colour for penetrating sea water, (the colour of the rays of the blue rayed limpet is a similar shade, for the same reason).
  10. It is easy, when casually browsing on the internet, to get the impression that such interpretations are established facts – but other sources indicate that such ideas are just hypothesis and that research in many areas is still ongoing.
  11. Screen capture from full video at
  1. This is the subject of research inspired both by curiosity and the search for possible practical applications. The American university MIT are looking into the possibility of engineering bioluminescent plants to replace electrical lighting ( )
  2. ‘Lucifer’ is Latin for ‘bringer of light’.
  3. For example,

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