The moon drives the migration of Arctic zooplankton

The daily rising and setting of the sun propels what is thought to be the world’s largest migration: Tiny zooplankton move from the near-surface waters — where they spend the night feeding — down into deeper, darker waters during the day to avoid predators that rely on sight for finding a meal.

It was thought that in the perpetually dark waters of the Arctic winter that such a migration wouldn’t happen. After all, there’s no sunlight for weeks or months. And until last year, researchers believed that the Arctic pretty much shut down for the winter; it turns out that the region can be surprisingly active in the dark of the polar night.

Now a new study that combines 50 years of observations from locations across the Arctic shows that zooplankton are still migrating in the depths of winter. But with the sun gone, they have tied their timing to the next biggest source of light — the moon.

Zooplankton may be tiny — some are less than 2 micrometers long — but they are so numerous that acoustic instrumentation can detect their presence. Sound bounces off the itty bitty critters, creating what can look like a false ocean bottom (or at least it did to World War II sonar operators). And for more than five decades, scientists have monitored zooplankton with moored acoustic instruments at several locations in the Arctic.

Kim Last of the Scottish Association for Marine Science in Oban and colleagues gathered data from those instruments to look at daily zooplankton movements at locations across the Arctic. The moon plays an important role in zooplankton migration, the team reports January 7 in Current Biology.

In spring and fall, when the sun sets and rises daily in the Arctic, zooplankton follow their normal pattern of vertical migration, moving down deep in the day and rising toward the surface at night. But after the sun sets for winter, the zooplankton adjust their schedule, swimming up and down the water column not every 24 hours but every 24.8 hours, following the rising and setting of the moon. And every 29.5 days, when there is a full moon, the mass of zooplankton fall to a depth of about 50 meters, where they can keep out of the brightest moonlight. The movement may help hide the zooplankton from predators that need light to find their prey, the researchers say.

In 2013, researchers found a marine worm with a biological clock tied to the phases of the moon, but it is not yet clear if there is a similar molecular mechanism at work in zooplankton. The invertebrates could be responding to subtle changes in illumination, diving deeper to avoid getting eaten by what Last and his team call the “werewolves” of the Arctic night.