The Truth Behind the Glowing Milky Sea Phenomenon

Bioluminescence in the ocean.

A long-exposure photo shows the bioluminescence of a dinoflagellate in the port of Zeebrugge, Belgium. (Image: via Wikipedia)

The milky sea continues to fascinate sailors and scientists alike. With its mysterious luminescence, it is one of the great ocean mysteries that remain unsolved. 

Sailors in the nineteenth century always thought the origins of the milky sea were ominous, like sea monsters and mermaids. But they are not. Milky seas, also known as mareel, are popular luminous ocean phenomena. In which enormous expanses of saltwater appear to shine brightly in varying shades of blue. 

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These luminous bodies of water move with sea currents and appear as a constant glowing patch around the sea at night. Then disappear from vision during the day because of the overwhelming amount of sunshine contrasted with the modest illumination in the sea. These patches will reappear in the satellite again. 

A satellite image of a milky sea (original on the left, colorized on the right) occurring off Java on August 4, 2019.
A satellite image of a milky sea (original on the left, colorized on the right) occurring off Java on August 4, 2019. (Image: Steven D. Miller via NOAA)

When was the glowing milky sea discovered?

For many centuries, sightings of these mysterious ocean occurrences have been circulating among sailors. The first witnessing of the milky sea phenomenon is said to be as early as 1846. Even the great science-fiction writer, Jules Verne, mentioned this phenomenon in his classic novel 20,000 Leagues Under the Seas in 1870.

And that continued up to the modern era. In 2005, a group of scientists spearheaded by Dr. Steven Miller, a scientist at the Naval Research Laboratory in Monterey, California, decided to take a better look at this phenomenon. And using the data they have from satellite sensors, they confirm the milky sea incident reported in 1995 by a British merchant vessel.

The report was on January 25, 1995, when the British trader vessel the S.S. Lima recorded that “on a clear moonless night while 150 miles of the Somalian coast on the eastern part, an ivory-like glow was seen on the horizon. A sea of a bright white color surrounded the ship with a uniform luminescence after 15 minutes of steaming. And this shows that the ship was drifting over a snow field or the clouds.”

With that information, Dr. Miller and his co-workers used the Defense Meteorological Satellite Program (DMSP). Then they used this with polar satellites to determine the incident stated by Lima’s crew. That reveals a certain area with a low-level light in the northwestern part of the Indian ocean. This evidence proves that the existence of the milky sea phenomenon is not a tale.

Why does the milky sea glow?

After knowing that the milky sea exists and is not a simple fiction created by the sailors, scientists decided to figure out what was causing it to glow. They collected some water samples from the western Arabian Sea. And with these samples, they discovered a type of bioluminescent microorganism named Vibrio harveyi.

To put it simply, Vibrio harveyi is what makes the ocean glow. Scientists found out that luminous bacteria light the particles they occupy. This luminescence’s goal might be to attract fish that devour them. These bacteria flourish in fish guts, so when their populations get too large for their primary food source, a fish’s stomach offers an excellent backup.

So, if a massive number of microorganisms were distributed across a vast region of open water, all began shining simultaneously. And that results in a milky sea.

A type of bacteria causes bioluminescence in milky seas.-sea
A type of bacteria causes bioluminescence in milky seas. (Image: Steven Haddock via Monterey Bay Aquarium Research Institute)

Bioluminescent dinoflagellate vs. bioluminescent bacteria

Bioluminescent bacteria are different from bioluminescent dinoflagellates. Dinoflagellates are responsible for the glow seen in ship wakes or waves slamming on the coast during a red tide. Dinoflagellates can also create short light flashes, while bioluminescent bacteria create a mild, continuous glow.

Additionally, the light of dinoflagellates is intended to scare off predators. But bioluminescent bacteria use their light to lure fish. Since bacteria’s preferred habitat is inside a fish’s gut, they desire to be digested. These small microorganisms emit only a dim light. As a result, they must assemble to have any effect. And when their numbers reach a certain size, their glow range can expand immensely. 

Milky sea: One of the great ocean mysteries

Scientists have discovered the milky sea in a way never seen before with the help of over a decade of satellite data. They found out that there is a rare and interesting marine bioluminescent phenomenon. And this was observed by a very sensitive spaceborne low-light sensor. 

With various satellites and equipment, modern scientists gained a better knowledge of the microorganisms that produce this shimmering and glowing water.

The milky sea continues to fascinate sailors and scientists alike. With its mysterious luminescence, it is one of the great ocean mysteries that remain unsolved. 

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