Quantifying the light sensitivity of Calanus spp. during the polar night: potential for orchestrated migrations conducted by ambient light from the sun, moon, or aurora borealis?

Recent studies have shown that the biological activity during the Arctic polar night is higher than previously thought. Zooplankton perform diel vertical migration during the dark period/winter, with the calanoid copepods Calanus spp. being one of the main taxa assumed to contribute to the observed diel vertical migration. We investigated the sensitivity of field-collected Calanus spp. to irradiance by keeping individuals in an aquarium and exposing them to gradually increasing irradiance in white, blue, green, and red wavebands, recording their phototactic response with a near-infrared-sensitive video camera. Experiments were performed with the two oldest copepodite stages as well as adult males and females. The copepods were negatively phototactic, and the lowest irradiance eliciting a significant phototactic response was of the order of 10(-8)-10(-6) mu mol photons m(-2) s(-1) for white, green, and blue wavebands, whereas the comparative irradiance for red wavebands was up to three orders of magnitudes higher. The different copepod developmental stages displayed different sensitivities to irradiance. During the darkest part of the polar night, the lowest irradiance for significant response corresponded to 0.0005-0.5 % of the ambient surface irradiance, depending on light source. Accordingly, Calanus spp. may respond to irradiance from the night sky down to 70-80 m, moonlight to 120-170 m, and aurora borealis down to 80-120 m depth. The high sensitivity to blue and green light may explain the Calanus' ability to perform diel vertical migration during the polar night when intensity and diurnal variation of ambient irradiance is low.