Do you find the daily commute a struggle during the dark winter months? Be thankful that you鈥檙e not a microscopic Arctic organism, forced to migrate by moonlight when the sun doesn鈥檛 rise at all.
One of the biggest-scale migrations on the planet takes place every day, when billions of tiny zooplankton swim deeper in the morning to escape predators that are active in daylight. Every night, the zooplankton surface to feed on photosynthesising phytoplankton.
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You might think this couldn鈥檛 happen inside the Arctic Circle in winter, when the sun doesn鈥檛 rise at all for several months. But acoustic location readings have revealed that when winter sets in, Arctic zooplankton continue with their daily grind 鈥� although they switch to a lunar rhythm instead.
Many organisms have an internal circadian clock, but use the sun to set its rhythm, posing the question of what happens when the sun doesn鈥檛 rise at all. Analysing 20 years鈥� worth of acoustic data, the team detected the plankton switching from their usual 24-hour cycle to a lunar day cycle, which is 50 minutes longer due to the amount that the moon moves around the Earth each day.
鈥淲herever we looked with this abundance of data, we could see responses to the moon,鈥� says of the Scottish Association for Marine Science near Oban, UK.
Monthly excursions
鈥淓ven though it鈥檚 incredibly dark and cold, the amount of activity in the water column at this time of year is incredibly high,鈥� says Last.
The moon is around a million times dimmer than the sun. How zooplankton manage to perceive small fluctuations in lunar light, which are beyond human detection, is unclear. 鈥淲e鈥檙e working with electrophysiologists to work out what these organisms can perceive and at what depths, but it鈥檚 in the order of a few photons of light,鈥� says Last.
It鈥檚 possible that the behaviour isn鈥檛 triggered by light, but by an internal clock mechanism instead. 鈥淭hat is still open for debate and not solved by the present observations,鈥� says of the Royal Netherlands Institute of Sea Research on Texel Island.
In addition to the daily cycle, the team also found a monthly pattern, with the zooplankton diving 50 metres deep for six days around each full moon. They probably do this to hide from predators such as fish that hunt like werewolves by bright moonlight, Last says.
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(Image credit: Arctic-Images/Corbis)
