Meta data for behavioral experiments.

For each experiment (Experiment ID), the meta data show the light regime during the experiment in the activity monitor (LD: light-dark cycle, DD: constant darkness), the duration of the experiments under the respective light condition, the local date and time, Region (BS: Bransfield Strait, SOI: South Orkney Islands), exact location, and depth for krill sampling, the local natural photoperiod at the time of krill sampling,the number of krill used for each experiment, and their mean length and standard deviation.

Krill swimming activity persists under constant conditions.

Swimming activity under 5 days of DD conditions in experiment 1 of individual #9 after entrainment to simulated short-day conditions (a) and group mean swimming activity of rhythmic individuals under 5 days of DD conditions from the same experiment (n = 6, c). The result from LSP analysis shows significant circadian rhythmicity of swimming activity of individual #9 during DD conditions of experiment 1 (period: 24.2 h, b). Grey shading represents standard error of the mean (s.e.m.). Color bars at the top indicate light conditions (constant darkness).

Light and the circadian clock shape the daily activity profile of krill.

Average day analysis of the group mean swimming activity over 3 days of short-day simulations (experiment 1, n = 11, a) and 5 days of long-day simulations (experiment 2, n = 9, b). Color bars at the top indicate light regime under day-night simulations. Shading around the line represents the s.e.m.. Data in (b) from Hüppe et al. (43).

The amplitude of swimming activity decreases in DD conditions.

Distribution of the daily amplitudes of swimming activity visualized with boxplots for every day of the LD-DD experiments for short-day (experiment 1, a), and long-day simulations (experiment 2, b). Differences in the daily amplitude of swimming activity between LD and DD conditions for short-day (b) and long-day (d) experiments. Lower and upper hinges of the boxes correspond to the 25th and 75th percentiles, the upper and lower whiskers extend to the largest and smallest value no further than 1.5 of the interquartile range, respectively, the horizontal line shows the median. Points represent daily amplitudes of individuals for each day and light condition. Differences in amplitude between first and last day under DD tested with paired t-test for experiment 1 (a; n = 11, t = 3.197, df = 10, p-value = 0.005) and experiment 2 (c, n = 9, t = 2.731, df = 8, p-value = 0.012), and between LD and DD conditions with Mann-Whitney-U-Test for experiment 1 (b; n = 88, p-value = < 0.001) and experiment 2 (d; n = 90, p-value < 0.001). Significance levels: p < 0.05: *, p < 0.01: **, p < 0.001: ***.

Circadian swimming activity persist across seasons.

Group mean swimming activity of rhythmic individuals during the first four days under DD conditions for krill sampled in summer (experiment S1, n individuals = 5, a), late summer (experiment S2, n individuals = 8, b), autumn (experiment S3, n individuals = 10, c), and winter (experiment S4, n individuals = 4, d), as well as corresponding average day analysis of the first four days in summer (e), late summer (f), autumn (g), and winter (h). Grey shading represents the s.e.m.. Color bars at the top indicate the natural photoperiod at the day of sampling.

Krill display synchronized DVM in the field across a wide range of photoperiods.

Hydroacoustic recordings showing the vertical distribution of krill swarms in the upper water column (<220 m) below the vessel, visualized by the mean volume backscattering signal (200 kHz), on the days preceding krill sampling for experiments S1-S4, in summer (a), late summer (b), autumn (c), and winter (d). Diffuse blue shading between ∼100-220 m represents instrument noise. Grey areas depict missing data. Color bars at the top indicate the natural light regime.

Locations of krill sampling and hydroacoustic recordings for behavioral experiments.

Colored dots indicate sampling locations for the respective behavioral experiments. Colored lines indicate the cruise track of the fishing vessel during recording of hydroacoustic data prior to sampling for behavioral experiments S1-S4. BS: Bransfield Strait. SOI: South Orkney Islands. Bathymetric data were retrieved from the International Bathymetric Chart of the Southern Ocean (500m× 500 m resolution, https://ibcso.org/).

Krill group activity under LD conditions.

Group mean swimming activity of krill under long-day (experiment 2, n=9, A) and short-day (experiment 1, n=11, B) simulations. Shading around the line depicts the standard error of the mean (s.e.m.). Color bars at the top indicate the light regime provided. Data for (A) from Hüppe et al..

Swimming activity of individual krill under DD conditions.

Normalized and detrended swimming activity under 5 days of constant darkness (DD) after entrainment to simulated short-day conditions (experiment 1) for each individual. Color bars at the top indicate subjective day and night.

Swimming activity of individual krill under LD conditions.

Normalized swimming activity under 5 days of simulated short-day (experiment 1) conditions for each individual. Color bars at the top indicate the light regime provided.

Comparison of daily swimming activity between LD and DD conditions.

Group mean swimming activity of rhythmic individuals over an average day under 3 days of simulated short-day conditions (A) and 5 days of constant darkness (B) during experiment 1. Color bars at the top indicate the light regime provided, as well as subjective day and night. Arrows indicate times of increased or decreased activity which lead to the characteristic activity pattern during the dark phase. Shading indicates the s.e.m..

Group swimming activity under DD conditions across seasons.

Group mean swimming activity of rhythmic individuals under DD conditions after individuals have been sampled from their natural environment during summer (experiment S1, A), late summer (experiment S2, B), autumn (experiment S3, C), and winter (experiment S4, D). Grey shading indicates the s.e.m..

Individual swimming activity under DD conditions of krill sampled in Summer.

Normalized and detrended swimming activity under 8 days of constant darkness (DD) after individuals have been sampled from their natural environment during summer (experiment S1, photoperiod: 17.5 h).

Individual swimming activity under DD conditions of krill sampled in Late Summer.

Normalized and detrended swimming activity under 6 days of constant darkness (DD) after individuals have been sampled from their natural environment during late summer (experiment S2, photoperiod: 14 h).

Individual swimming activity under DD conditions of krill sampled in Autumn.

Normalized and detrended swimming activity under 7 days of constant darkness (DD) after individuals have been sampled from their natural environment during autumn (experiment S3, photoperiod: 12.6 h).

Individual swimming activity under DD conditions of krill sampled in Winter.

Normalized and detrended swimming activity under 4 days of constant darkness (DD) after individuals have been sampled from their natural environment during winter (experiment S4, photoperiod: 5.9 h).

Rhythm analysis of group swimming activity under DD conditions across seasons.

Periodogram derived from Lomb-Scargle Periodogram analysis (LSP) of group mean swimming activity under DD conditions after individuals have been sampled from their natural environment in different seasons (experiments S1-S4, see suppl. fig. 6).