Such adaptations may also be essential in regulating monthly and annual cycles of behavior and physiology, the authors say.Ĭohen adds, "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. Increases in visual sensitivity at night allow krill to acclimatize to tiny variations in light intensity and maintain their daily rhythms of behavior throughout the polar winter. They found that krill were more sensitive to light at night than during the daytime, indicating that they are able to synchronize their circadian rhythms with small variations in external light cues. To investigate the mechanisms underpinning these nightly migrations, the researchers used a technique called extracellular electroretinogram recording (ERG) to measure the visual sensitivity of krill in the laboratory.
Nevertheless, the krill exhibited a strong circadian rhythm, migrating to the surface to feed during the polar night and retreating to the depths to avoid predators during the midday twilight. Light availability was just 2-fold higher at midday than at midnight in the middle of the arctic winter, compared to a 7-fold difference during spring and autumn. Krill are a type of small shrimp-like crustacean that form an important part of the diet of many large marine animals such as whales. The researchers measured midday light intensity during winter months in the Svalbard archipelago in the Arctic Ocean, from a land-based light observatory and a marine research vessel, and used underwater acoustic recordings to monitor the daily migrations of Arctic krill ( Thysanoessa inermis).
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