Milky seas effect

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Milky sea effect off the coast of Somalia in the Indian Ocean

Milky seas, or mareel, is a condition on the open ocean where large areas of seawater (up to 6,000 square miles) are filled with bioluminescent bacteria, causing the ocean to uniformly glow an eerie blue at night. The condition has been present in mariner's tales for centuries – notably appearing in chapter 24 of Jules Verne's Twenty Thousand Leagues Under the Sea – but until recently it has not been rigorously documented.


There have been 235 documented sightings of milky seas since 1915 - mostly concentrated in the north-western Indian Ocean and near Indonesia.

In 1985 a research vessel in the Arabian Sea took water samples during milky seas. Their conclusions were that the effect was caused by the bacteria Vibrio harveyi.

In 2005, Steven Miller of the Naval Research Laboratory in Monterey, California, was able to match 1995 satellite images with a first-hand account of a merchant ship. U.S. Defense Meteorological Satellite Program showed the milky area to be approximately 15,400-km² (roughly the size of Connecticut). The luminescent field was observed to glow over three consecutive nights.

While monochromatic photos make this effect appear white, Monterey Bay Aquarium Research Institute scientist Steve Haddock (an author of a milky seas effect study) has commented, "the light produced by the bacteria is actually blue, not white. It is white in the graphic because of the monochromatic sensor we used, and it can appear white to the eye because the rods in our eye (used for night vision) don't discriminate color."[1] In Shetland, mareel has sometimes been described as being green,[2] rather than the traditional blue or white milky seas effect seen by the rest of the world. It is not known whether this difference depends on the area, or simply a perception of a cyanic colour as being green.


The phenomenon is known as mareel in Shetland. This term is derived from the Norn word *mareld, which is itself derived from the Old Norse word mǫrueldr, which is a compound of marr (mere, sea) and eldr (fire). [a]


  1. ^ Cognates of mareel include the Icelandish term maurildi which refers to both the mareel itself, and also to the Noctilucales order of marine dinoflagellates that bring about the mareel, Danish morild, Norwegian morild or moreld, Swedish mareld, and Finnish merituli.


  1. ^ David Pescovitz at 9:51 am Tue, Oct 4, 2005 (2005-10-04). "First milky sea photo". Boing Boing. Retrieved 2014-07-17. 
  2. ^ Moncrieff, Helen. "Mareel - Lights From The Sea". The RSPB Community. Retrieved 2 July 2014. 

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