Arctic krill maintain diurnal rhythm even during polar night | Polarjournal
Arctic krill (Thysanoessa inermis) is an important component of the diet of many larger animals in the Arctic Ocean, such as seals and whales. Photo: Dr. Russell R. Hopcroft, Institute of Marine Science, University of Alaska Fairbanks, CC BY-SA 3.0

In almost all habitats on Earth, the ecology of organisms depends on light. Living creatures adapt their activities to the daily rhythm of the sun, for example, to find food and mating partners, to migrate and to avoid predators. But what guides animals that have to survive the polar winter in darkness? An international team of researchers has investigated this question and closely studied the circadian rhythm of the Arctic krill (Thysanoessa inermis) during the polar night.

While organisms at lower latitudes are exposed to a varying but still relatively regular photoperiod throughout the year, living organisms in the polar regions experience extreme differences in the course of the year, from constant daylight in summer to persistent darkness in winter. However, even during the polar night it is not completely dark; besides the moon light and polar lights, there is a slightly higher light intensity at midday when the sun is just below the horizon, known as midday twilight. The sea ice cover and the water itself then further attenuate this already very weak light for marine life in the Arctic Ocean.

The blue shaded circles on the map represent the increasing extent of the polar night with increasing northern latitude. The study area was in Svalbard at 78°N and about 80°N. Map: Cohen et al. 2021

In the current study, published in the open-access journal PLOS Biology, the scientists were able to show that Arctic krill can sense these minute changes in light intensity during the polar winter and thus maintain its daily biological rhythm. The researchers tracked the daily vertical migrations of Arctic krill, which is the food source of many large marine animals, using underwater acoustic recordings. They determined light intensity at midday during the winter months from a light observatory on land in Svalbard and from a research vessel at sea.

According to the scientists’ measurements, in the middle of the Arctic winter, light availability at midday was only two times higher than at midnight. In spring and fall, on the other hand, light intensity at midday is seven times higher. Nevertheless, krill maintain their daily rhythm – at night they migrate to the surface to feed and at midday twilight they return to the depths to avoid predators.

To measure the Krill’s visual sensitivity to light, the research team conducted experiments in the laboratory. Using the so-called extracellular electroretinogram recording (ERG), they investigated the mechanisms underlying nocturnal migrations by exposing freshly caught krill to very weak flashes of light. Their results show that krill are more sensitive to light at night than during the day, which the researchers say indicates that they are able to synchronize their circadian rhythms with small fluctuations in external light stimuli.

The eyes of Antarctic krill are very large in relation to their body size. Their extreme sensitivity to light enables krill to maintain its circadian rhythm even in the darkness of the polar night. Photo: Lukas Hüppe, University of Oldenburg

Thanks to increased visual sensitivity at night, Arctic krill are able to adapt to minute fluctuations in light intensity and maintain their daily behavioral rhythms during the polar winter. Such adaptations may also be important for regulating monthly and annual cycles of behavior and physiology, the authors said.

Jonathan Cohen, assistant professor in the University of Delaware’s School of Marine Science and Policy and lead author of the study, further explains, “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.”

Julia Hager, PolarJournal

Link to the study: Jonathan H. Cohen, Kim S. Last, Corie L. Charpentier, Finlo Cottier, Malin Daase, Laura Hobbs, Geir Johnsen, Jørgen Berge. Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night. PLOS Biology, 2021; 19 (10): e3001413 DOI: 10.1371/journal.pbio.3001413

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