Moonlit Migration

SCIENCE

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Discussion Ideas

According to the Christian Science Monitor, plankton usually engage in diel vertical migration. What is diel vertical migration?
- Diel vertical migration describes a daily pattern of movement from the colder, darker, deeper part of the ocean (the mesopelagic zone) to the slightly warmer, brighter, and shallower part of the ocean (the epipelagic zone). Diel vertical migration can also apply to microscopic migrations in freshwater—from a lake’s dark hypolimnion to its sunny surface.
  - Plankton’s diel vertical migration is fairly short (about 50 meters or 164 feet), but plankton are pretty tiny! “If you’re just a small little copepod, to migrate tens of meters costs a lot, in terms of energy,” says marine ecologist Kim Last.
    - Nat Geo Explorer Kakani Katija explains it like this: “We are pretty big, so when we swim through water it feels like water. But . . . if we were something smaller, like a worm, it will feel more like swimming through honey. This difference is called fluid viscosity.”

During the dark Arctic winter, plankton engage in a different sort of migration: lunar vertical migration. How are the two migration patterns different?
- Diel vertical migration follows a solar pattern, meaning it is influenced by the sun. The solar day is 24 hours long.
- Lunar vertical migration is a lunar pattern, meaning it is influenced by the moon. The lunar day is 24.8 hours long.

Why do Arctic plankton change their migration pattern during the winter?
- Arctic winters have short days when the sun is only above the horizon a brief time. Until this study, scientists pretty much thought plankton stayed put in the cold, dark mesopelagic during the winter.

Why do plankton migrate?
- “They’re not just migrating for fun,” says Last. They are moving away from “werewolves of the deep”—predators that hunt near the surface of the ocean where they can see moonlit prey more easily.

TEACHERS’ TOOLKIT