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A peek behind the science curtain

Bethany Brookshire

Melatonin and the watery beginnings of sleep

The nightly behaviors of plankton may give insights to how sleep evolved

Platynereis dumerilii

It may not seem like humans would have much in common with ragworms, but both species use melatonin to help control daily behaviors.

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This rise and fall takes place on a nightly basis. As dusk arrives, zooplankton propel themselves up toward the surface. At the night and into the dawn, they sink back down into deeper water. The biological mechanisms underlying this daily routine might also be the basis of other circadian rhythms, including our own.

“[The larva is] pretty far away from us evolutionarily,” Arendt explains. “So whatever characteristics it has that are conserved between it and vertebrates like us has been present in all other animal groups as well.” He and his group were particularly interested in the potential role of melatonin in the larvae. In previous studies, he and his laboratory had found light-sensitive cells in the middle of the zooplankton’s developing brain. The cells were similar to cells in the human pineal gland. In people, the cells produce melatonin and help to control circadian rhythm. So Arendt’s group became interested in whether the cells in the zooplankton larvae might also express melatonin and have a similar function.

Maria Tosches, an evolutionary and developmental biologist at the University of Heidelberg with Arendt, set out to determine whether these ragworm larvae synthesized melatonin in their tiny brains, and if they did, if it was involved in their nightly movements up and down the water column. Melatonin also soaks up unpaired, reactive ions that can cause damage to DNA or proteins in a cell. So its mere presence in larvae would not be enough to determine the cause of the zooplankton’s evening rhythms.

She found that the larvae not only produce melatonin in their brains, they also use it to control their evening rise and fall in the ocean. At night, melatonin rises, and the openings through which cilia project into the water close. With the cilia retracted, the larvae sink lower in the water. An extra dose of melatonin, even during the day, decreased the number of cilia working away at the water. Conversely, blocking the receptors for melatonin at night prevented the cilia openings from closing, leaving more cilia in the water to propel the plankton about . Tosches, Arendt and their colleagues published their findings September 25 in Cell.

Tosches showed the hormone activates two specific neurons in the larval ragworm’s brain, spurring the cells to fire in rhythmic bursts. These neurons release the chemical messenger acetylcholine, and the signal increased ciliary closure. As the cilia close, the zooplankton sink.

So melatonin played an important role in regulating the daily behavior of the ragworm larvae, just as important a role as the hormone plays in our own internal clocks. But this sinking is not quite plankton sleep. “It is more complex than that,” explains Tosches.

While the larvae don’t sleep in the traditional sense, their nightly rise and fall might be the first instance of something “sleep-like.” “All phenomena have a first instance when they occur,” he explains. “We know that organisms came from the ocean, so it’s possible we are finding and characterizing the first niche where sleep played a role.”

The finding highlights what we can learn about how we came to be from seemingly humble model species. “Life evolved in the ocean, but unfortunately we cannot go back 600 million years and watch what happened,” Tosches says. “But studying marine ecology and neurobiology can still tell us something about where our brains come from.”

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