Explore the response to visual changes during arctic night

New research is exploring the visual rhythms of animals during the Arctic polar night.

Around 11:30 am or so, you might want to have lunch. The reason is that our biological rhythms are trained to tell us when we are hungry, and when we have that urge, our body knows it’s time to eat. It is the same for visual rhythms.

During the day it is usually clearer than at night. For this reason, our visual system changes so that it can be ready to operate in brighter light conditions. During the night, our eyes become more sensitive to adapt to the lack of available light.

It turns out the same is happening with arctic krill. When it is clearest in the arctic polar night – a time of year at high latitudes where the sun stays below the horizon for the entire 24 hour period – usually around the middle of the day known as midday twilight, arctic krill can swim to the bottom in order to hide from predators. When it gets darkest on the Arctic polar night, that’s when they come to the surface in search of bioluminescent food.

A new study published in PLOS Biology watched this rhythm of visual sensitivity in arctic krill during arctic polar night.

Jonathan Cohen, associate professor in the School of Marine Science and Policy at the University of Delaware at the College of Earth, Ocean and Environment, was the lead author of the article and said it was linked to more work. extensive information on the biological processes that take place. during the arctic polar night.

“The dogma for a long time was that when the sun stays below the horizon, biology just switches to suspended animation and then everything starts up again when the sun comes back above the horizon in the spring,” Cohen said. “Our work has shown that this is not the case. Instead, you have organisms that are active throughout this period of polar night when the sun is below the horizon. There is also more light than you might think, even when the sun is below the horizon, and that has biological impacts. ”

One of these biological impacts concerns the visual sensitivity rhythms of krill.

To conduct his research, Cohen traveled to the Svalbard Archipelago in the Arctic Ocean – northeast of Greenland, if you’re looking for a globe – researching both on a research vessel at sea. and in a light observatory on earth.

Using underwater acoustic technology, the researchers were able to tell that krill was active and present during the Arctic polar night.

“In this article, we really ask this question: is there enough light in the arctic polar night to train their biological rhythms? And we found out there was, ”Cohen said.

Krill used outside light in different ways. This helps their visual system move up and down the water column to find food and helps them control their own bioluminescence to avoid being eaten by a predator.

Unlike lower latitudes, when there are distinct periods of a bright day and a dark night, in the arctic there are different sources during the polar night such as moonlight and the northern lights – the northern lights – which contribute light at different times. . This expands and shifts the period of time that light is available.

“You still see that difference between day and night up there, but it’s a bit blurry by the presence of the moon and the aurora because those light levels are enough to get close enough to what you might see at noon. , “Cohen said.

Even though the light in the Arctic polar night does not fluctuate as much as it does in the lower latitudes, it does experience changes. While the sun is below the horizon, it still produces an oscillation of light intensity and these changes in light are detected by the krill.

Animals and humans train their biological clocks to synchronize with visual cues such as cyclic light – light getting darker or brighter – that tells them to turn certain genes on or off.

To research these visual mechanisms in krill, the researchers used what is known as recording extracellular electroretinograms in constant darkness to get a better idea of ​​how krill adapts to changes in light. . The researchers conducted two experiments, one to expose krill in the lab to flashes of different light intensities to measure sensitivity, and the other that exposed them to flashes of the same intensity at different times of the day to measure the impact of time on visual sensitivity.

“We can determine how well a krill can detect this flash of light because we do it with animals that we have collected and kept in the dark,” Cohen said. “So we know that they were only exposed to the light of the environment they had when collecting, and it was that light that set their biological clocks.”

In addition to the lab experiment, the researchers also relied on acoustic data – which told them when krill was undergoing its vertical migrations – and light data collected both at sea and on land that told them when the levels of light were at their highest during Arctic Polar Night.

Cohen said there were some implications for this new krill discovery. The first is that arctic krill can set their biological clocks using very small changes of light over the course of a 24-hour day, among the smallest ever measured.

This study also helped them deduce how krill uses external light and its own bioluminescence to protect itself from potential predators. When swimming in water, krill will place its back up and its belly down. Their bellies produce bioluminescence, much like krill which makes small bulbs glow from their bellies.

Underwater, light is only detectable directly above the head. Thus, by projecting this light from their belly, they erase the shadow they would present when seen from below.

“It’s called backlighting, and they produce a light that scrambles their shadow so they can’t be seen by predators from below,” Cohen said. “What we think this rhythm does is that it allows them to better tune that light shadow.”

They are also able to save energy and stay safe by staying low in the water column until it’s time to feed. While other animals may move up or down in response to a passing cloud that appears as a black shadow above their head, krill is able to ignore these shadows and conserve energy.

With these new findings in hand, Cohen will return to the Arctic in January 2022 to continue his research into the implications of artificial light in the Arctic polar night on Arctic marine species.

Reference: “Photophysiological cycles of arctic krill are driven by weak midday twilight during polar night” by Jonathan H. Cohen, Kim S. Last, Corie L. Charpentier, Finlo Cottier, Malin Daase, Laura Hobbs, Geir Johnsen and Jørgen Berge, October 19, 2021, PLOS Biology.
DOI: 10.1371 / journal.pbio.3001413

Funding for this project came from the Norwegian Research Council and the UK’s Natural Environment Research Council.