Circadian Rhythms in Respiration Rates of Juvenile Antarctic Krill

Studies of the winter physiology of juvenile Antarctic krill (Euphausia superba) are scarce and knowledge of the krill winter behavior patterns is limited. As oxygen consumption varies in response to behavioral changes within an organism, analyzing patterns within respiration rates can give us important insight into organismal daily behavioral changes. Further, these behavioral changes can be indicative of mechanisms krill employ to survive the winter, thus effecting overwinter physiology. During this study, we investigated the circadian patterns in oxygen consumption of fifteen juvenile krill by using new high-resolution PyroScience oxygen sensors over the course of 48 hours. From these data, we calculated hourly respiration rates, applied a Gaussian smoothing filter and de-trended the rates to account for natural decrease in respiration rate over time in a confined space. We determined the periodicity of the data using BioDare2 and tested for significance in the circadian rhythms detected using the R RAIN package. Using these fine-scale data we found that the majority of krill exhibited significant unimodal respiration rhythms, with an average peak in respiration occurring just before local dawn, at about 07:00, and experienced an average period of 16 hours. However, there was evidence of within stage variability. Four juveniles had significant bimodal respiration rhythms with a period of 12 hours and primary peaks occurring around local dawn and secondary peaks occurring in the evening between 18:00 and 00:00. We discuss these results in the context of juvenile krill behavior and winter energy demand.