Light Sensitivity of the Arctic Copepod Metridia longa during Midnight Sun and Polar Night

The high Arctic is defined by an annual light regime ranging from 24-h light (Midnight Sun) to 24-h dark (Polar Night). Light acts as an important cue for marine zooplankton, influencing their orientation and vertical migration, prey detection and predator avoidance, and population dynamics and reproductive strategies. The spectrum and intensity of underwater light differ between Midnight Sun and Polar Night, and these differences are relevant to zooplankton visual processes. Here, we determine behavioral responses of the Arctic copepod Metridia longa, measured as swimming activity in a novel laboratory apparatus, to spectral- and irradiance-controlled light stimuli during times of Midnight Sun and Polar Night. Metridia longa maintains a consistent blue-green spectral response, from 400 to 550 nm, during both times of year. However, peak spectral response shifted between seasons, with Midnight Sun individuals showing increased activity at 501 nm compared to 473–490 nm during Polar Night. Additionally, Polar Night M. longa showed heightened irradiance sensitivity by an order of magnitude as compared to Midnight Sun individuals. Their irradiance response was also consistent across varying temperatures. We show that spectral and irradiance responses in M. longa are seasonally adapted and temperature compensated, suggesting that this copepod maintains a consistent light-mediated predator avoidance capacity, despite predicted seasonal light and temperature shifts in the high Arctic.