Arctic: Marine Life Migrates According to Moonlight in Dark Arctic

Billions of zooplankton, or tiny marine animals, migrate from the top of the ocean to its depths during hours of daylight. In an attempt to escape predation, zooplankton swim downward -- because their predators are visual feeders and need to be able to see in order to eat. But, what happens when there is no day-night cycle?

In the Arctic, the sun is unseen for six months out of the year-prompting the question, do the zooplankton still migrate when there is no sunlight and are they still being eaten by predators if those animals are unable to see?

A team of scientists from the Scottish Association for Marine Science in Scotland tackled this question by studying the migratory cycles of zooplankton during the frigid Arctic winter. Lead researcher and author of the study, published in Current Biology, Kim Last said that during these dark days the zooplankton actually respond to the light of the moon by vertically migrating downward in masses. As the moon rises above the horizon line, the zooplankton sink downward; then, when the moon settles below the horizon the zooplankton migrate upwards once more. When the Arctic shifts from a 24-hour solar day to a 24.8 hour lunar day, the zooplankton follow.  

The scientists found that these migrations also directly coincided with the phases of the moon. When there was a full moon and it was high in the sky for six days out of the month there was a "massive migration of zooplankton down to about 50 meters," Last said in a film about the study.

Zooplankton is the basis of the food chain with many animals, such as whales and polar cod, depending on them to survive. Therefore, the zooplankton migrations structure the migrations of their predators. 

The daily movement of the zooplankton is likely due to copepods, according to Last. Copepods are small crustaceans who actively pursue zooplankton for consumption. Last knows that copepods are able to visually perceive the zooplankton in moonlight because the team measured the copepods' visual spectral sensitivity.

"The most surprising finding is that these migrations are not rare or isolated to just a few places," Last stated in an official release. In every section of the Arctic--the fjord, shelf, slope and open sea-the scientists saw the same vertical migrations associated with the lunar cycle, even in locations covered in sea ice.

When zooplankton migrate downward they move carbon with them, making them a key component in global carbon cycling. This finding is significant considering the implications of global climate change. "Since there is no photosynthesis during the polar night, carbon is only moved into the deep by predators feeding on prey," Last explained. As more Arctic sea ice melts, there will be more sunlight and moonlight penetrating the ocean surface prompting greater zooplankton migrations. However, scientists do not yet know the repercussions of this.

For a full look at a video on this study, click here.

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