Milky seas effect

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

Milky seas, also called mareel, is a luminous phenomenon in the ocean in which large areas of seawater (up to 6,000 sq mi or 16,000 km2) appear to glow translucently (in varying shades of blue). Such occurrences glow brightly enough at night to be visible from satellites orbiting Earth.

Mariners and other seafarers have reported that the ocean often emits a visible glow which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow.[1] It is most likely caused by bioluminescence.[2][3][4]


Between 1915 and 1993, 235 sightings of milky seas were documented,[5] most of which are concentrated in the northwestern Indian Ocean and near Indonesia.[6] The luminescent glow is concentrated on the surface of the ocean and does not mix evenly throughout the water column.[7]

This long-exposure photo shows the bioluminescence of Noctiluca scintillans in the yacht port of Zeebrugge, Belgium

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 bacterium Vibrio harveyi.[8] Mareel is typically caused by Noctiluca scintillans (popularly known as "sea sparkle"), a dinoflagellate that glows when disturbed and is found in oceans throughout much of the world.[9][10][11] In July 2015, at Alleppey, Kerala, India, the phenomenon occurred and the National Institute of Oceanography and Kerala Fisheries Department researched it, finding that the glittering waves were the result of Noctiluca scintillans. 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 (5,900 sq mi) (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 Steven 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."[12] In Shetland (where generally caused by Noctiluca scintillans), mareel has sometimes been described as being green,[13] 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. ^ Britt, Robert Roy (October 4, 2005). "Mystery Ocean Glow Confirmed in Satellite Photos". Live Science.
  2. ^ Holladay, April (November 21, 2005). "A glowing sea, courtesy of algae". USA Today.
  3. ^ "Sea's eerie glow seen from space". New Scientist. October 5, 2005.
  4. ^ Casey, Amy (August 8, 2003). "The Incredible Glowing Algae". NASA Earth Observatory. NASA.
  5. ^ "The Marine Observer". 1993.
  6. ^ Miller, S. D.; Haddock, S. H.; Elvidge, C. D.; Lee, T. F. (2005). "Detection of a bioluminescent milky sea from space". Proceedings of the National Academy of Sciences. 102 (40): 14181–14184. Bibcode:2005PNAS..10214181M. doi:10.1073/pnas.0507253102. PMC 1242338. PMID 16186481.
  7. ^ Lapota, David; Galt, Charles; Losee, John; Huddell, Howard; Orzech, John; Nealson, Kenneth (1988). "Observations and measurements of planktonic bioluminescence in and around a milky sea". Journal of Experimental Marine Biology and Ecology. 119: 55–81. doi:10.1016/0022-0981(88)90152-9.
  8. ^ Ramaiah, N. D. Chandramohan (1992). Desai, B. N. (ed.). "Ecology and biology of luminous bacteria in the Arabian Sea". Oceanography of the Indian Ocean. New Delhi: Oxford and India Book House: 11–23.
  9. ^ Tada; Pithakpol; and Montami (2004). Seasonal variation in the abundance of Noctiluca scintillans in the Seto Inland Sea, Japan. Plankton Biol. Ecol. 51(1): 7-14.
  10. ^ Buskey (1994). Growth and bioluminescence of Noctiluca scintillans on varying algal diets. Journal of Plankton Research 17(1): 29-40.
  11. ^ Oceana: Noctiluca scintillans. Archived 2012-11-11 at the Wayback Machine Marine Animal Encyclopedia. Retrieved 27 December 2014.
  12. ^ David Pescovitz at 9:51 am Tue, Oct 4, 2005 (2005-10-04). "First milky sea photo". Boing Boing. Archived from the original on 2011-07-09. Retrieved 2014-07-17.{{cite web}}: CS1 maint: multiple names: authors list (link)
  13. ^ Moncrieff, Helen. "Mareel - Lights From The Sea". The RSPB Community. Retrieved 2 July 2014.

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