The milky sea phenomenon, sometimes known as mareel, is a very rare form of bioluminescence in which luminous bacteria communicating with each other produce a sustained glow that makes the water light up like softly-glowing snow. The perplexing display is poorly understood, but a serendipitous encounter by a boat in Indonesia with a mareel has given scientists a unique opportunity to study the milky sea phenomenon and the tools we use to identify it.

A milky sea was detected by satellite between July and September 2019 south of Java, Indonesia, but without surface observations to corroborate the images, it was difficult for scientists to confirm the finding. If only, say, a private yacht could have happened to have been drifting through that exact stretch of ocean at that time of year with some eyewitnesses, a bucket, and a digital camera to boot.

As luck would have it, that’s exactly what happened.

The Ganesha vessel, a private yacht captained by Johan Lemmens with a crew of six, was crossing between Lombok, Indonesia, and the Cocos (Keeling) Islands when something unusual occurred on August 2.

“When waking up at 2200 the sea was white,” read the ship’s log. “There is no moon, the sea is apparently full of ? plankton ? but the bow wave is black! It gives the impression of sailing on snow!”

Their accounts were able to confirm the satellite observations made in 2019, marking an incredibly rare event that’s expected to occur globally just once or twice a year. Photographs and interviews about the rare encounter were analyzed by scientists and their findings were published in the journal Proceedings of the National Academy of Sciences.

Curious and enamored by the glow they described as “akin to glow-in-the-dark stars/stickers” (some ship log entries simply read “glowing :)”), the crew decided to scoop up a bucket of water to get a closer look. Collecting the sample didn’t disrupt the glow and, interestingly, stirring it actually caused the water to darken, which is the opposite response to that of “normal” bioluminescence.

Captain Lemmens believed that the glow sat around 10 meters below the water’s surface, contradicting a previously suggested surface slick hypothesis that stated milky sea movement may be the result of luminous bacteria sitting in a thin film. That the bioluminescence sat deeper in the water than this was supported by the fact that the yacht’s movement didn’t disrupt the glow.

You may be thinking that bioluminescence occurs all the time and what is so special about this phenomenon, but throughout history, it has only ever been observed by people in chance encounters. The only time it has ever been studied in the field was 40 years ago when a research vessel happened upon one off the coast of Yemen in 1985 and had equipment onboard to take samples. 

While many questions remain about the formation, structure, and inhabitants of milky seas, the Ganesha’s observations are pivotal in that they have confirmed that satellite imagery can be used to identify mareels. In future, this could enable marine scientists to swoop in at the opportune moment and get a clearer idea of what’s going on.

“With a newfound confidence in our spaceborne lookouts, a directed expedition to a milky sea enters the realm of possibility,” concluded the study authors.