Scientists solve mystery of Arctic krill’s ability to sync daily rhythms in darkness

20 Oct 2021

Arctic krill. Image: Geir Johnsen (CC-BY 4.0)

Researchers have discovered how this krill species maintain their circadian rhythms throughout polar winters.

A new study by researchers at the University of Delaware suggests that Arctic krill have one of the most sensitive circadian rhythms studied to date.

Krill are small crustaceans that live in large swarms and migrate to the surface of the water to feed at night. During the polar winter months, the sun never rises above the horizon in the Arctic krill’s habitat, which is inside the Arctic Circle. Despite this, the krill continue to migrate to the surface to feed during the night, before retreating to the depths during the day to avoid predators.

The researchers measured light intensity in the Svalbard archipelago using both a light observatory on land and a marine research vessel. They discovered that light availability was only two times higher at midday than midnight during the winter months, compared to seven times higher during spring and autumn. They also used underwater acoustic recordings to determine the migratory patterns of the krill.

To investigate how the krill maintain their daily rhythms even with such little difference in light levels, they used a technique called extracellular electroretinogram recording to measure the visual sensitivity of krill in a laboratory setting. They discovered that krill have higher light sensitivity at night than during the day, allowing them to acclimatise to tiny variations in light intensity and keep their daily routine in sync.

Jonathan Cohen of the University of Delaware said, “We found that the light environment during the high Arctic polar night has a complex timing of ‘light’ and ‘dark’ due to light coming from the sun below the horizon, the moon, and the aurora borealis. While this light is dim and unlike the typical photoperiod at lower latitudes, we show that it is sufficient to set a biological clock in krill, showing this animal has one of the more sensitive biological rhythms studied to date.”

This study, published in the open-access journal PLOS Biology, has both physiological and ecological relevance. The researchers believe that the impact of the climate crisis may force other marine species to evolve similar adaptations to thrive in extreme environments.

By Méabh Lunn

Méabh Lunn is a transition year student at Sandymount Park Educate Together. They write for the school newspaper, The Sandymount Park Press, and have a keen interest in journalism. Science has always been a key interest of theirs, especially genetics, marine biology and space exploration.

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