Melatonin links life rhythms to swimming plankton

Portfolio, Science Writing

“Hormone of darkness” drives rhythmic behavior in marine organism

Ragworm larva brain. Photo credit to EMBL/M. A. Tosches.

Researchers at the European Molecular Biology Laboratory in Heidelberg, Germany, found that the marine invertebrate Platynereis dumerilli (ragworm) has brain cells that produce the hormone melatonin in response to darkness, causing it to rise to the surface of the ocean at night and sink during the day.

Melatonin is a key hormone in the regulation of life rhythms, such as daily sleep and seasonal behavior, in nearly all animals, including this marine zooplankton. The ragworm larvæ have invisible “flippers” called cilia, which beat at night and pause intermittently during the day. This allows them to sink into deeper, darker waters, thus limiting exposure to the sun’s damaging UV rays. Since light oxidizes melatonin, its presence signals darkness to the organism.

The research, published last week in Cell, detected melatonin-producing cells in the plankton’s brain and measured the activity of its cilia during the day and the night. The lab, headed by Detlev Arendt, used more accurate tools and advanced analyses than past research. They found that melatonin drives the activity of motor neurons, which cause the cilia to stop beating, thus allowing the organism to sink. When the senior researcher Maria Antoinette Tosches exposed P. dumerilli to a chemical that simulated the effects of melatonin during the day, it switched to night-time behavior––quite like your body when you fly from Los Angeles to Tokyo.

Previously, Tosches says, “people assumed that melatonin was present in animals besides vertebrates, but the topic was controversial. The function of this hormone was simply not well understood in invertebrates.” Now that she and her team have established this foundation, future research will look at the function of melatonin in other invertebrates, particularly those which have neither melatonin nor circadian rhythms. “By studying this particular invertebrate, we hope to broaden the focus of melatonin research to include many more types of animals.”

The sensitivity of the marine larva to melatonin parallels that of the human brain, as reflected in our sleep–wakefulness cycle. The fact that melatonin functions similarly in tiny zooplankton and complex mammals highlights the evolutionary origin of important features of the human brain. Could the cells that now determine our daily rhythms have first evolved in the ocean? Dr. Florian Raible, who studies life rhythms in marine invertebrates at the University of Vienna, cautions that while “melatonin functions to encode representations of night and day in invertebrate brains, sleep itself is not well understood. The actual data in this study don’t address an evolutionary link between the larvæ’s water-column migrations and vertebrate sleep.”

Tosches describes the team’s findings as “basic research, driven by curiosity. Our primary interest is in evolution, so we hope that the work is useful to other evolutionary biologists.” Although there is still work to do in this field, melatonin clearly represents a conserved pathway from plankton to people. So tonight when your head hits the pillow, remember: even whale food has a daily biological clock.

Citation

M. A. Tosches et al. “Melatonin signaling controls circadian swimming behavior in marine zooplankton.” Published online in Cell on 25 September 2014. doi:10.1016/j.cell.2014.07.042.

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