The hormone melatonin is essential to maintain our daily rhythm, and European scientists have now discovered that it also governs the nightly migration of a plankton species from the surface to deeper waters.

The findings by the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany, indicate that melatonin's role in controlling daily rhythms probably evolved early in the history of animals, and hold hints to how our sleep patterns may have evolved.
    
In vertebrates, melatonin is known to play a key role in controlling daily activity patterns which get thrown out of synch when we fly across time zones, leading to jet lag. But virtually all animals have melatonin, researchers said.
    
To find out about its role in other species, and how did it evolve the task of promoting sleep, Detlev Arendt's lab at EMBL turned to the marine ragworm Platynereis dumerilii.
    
This worm's larvae take part in what has been described as the planet's biggest migration, in terms of biomass: the daily vertical movement of plankton in the ocean.

By beating a set of microscopic 'flippers' - cilia - arranged in a belt around its mid-line, the worm larvae are able to migrate towards the sea's surface every day.

They reach the surface at dusk, and then throughout the night they settle back down to deeper waters, where they are sheltered from damaging UV rays at the height of day.

"We found that a group of multitasking cells in the brains of these larvae that sense light also run an internal clock and make melatonin at night," said Arendt, who led the research.

"So we think that melatonin is the message these cells produce at night to regulate the activity of other neurons that ultimately drive day-night rhythmic behaviour," said Arendt.

Maria Antonietta Tosches, a postdoc in Arendt's lab, discovered a group of specialized motor neurons that respond to melatonin.

Using modern molecular sensors, she was able to visualize the activity of these neurons in the larva's brain, and found that it changes radically from day to night.

The night-time production of melatonin drives changes in these neurons' activity, which in turn cause the larva's cilia to take long pauses from beating.

Thanks to these extended pauses, the larva slowly sinks down. During the day, no melatonin is produced, the cilia pause less, and the larva swims upwards, researchers said.

"When we exposed the larvae to melatonin during the day, they switched towards night-time behaviour, it's as if they were jet lagged," said Tosches.

The research strongly suggests that the light-sensing, melatonin-producing cells at the heart of this larva's nightly migration have evolutionary relatives in the human brain.

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