Seasonal and daily rhythms in behavior and physiology of Northern krill (Meganyctiphanes norvegica)

Northern krill (Meganyctiphanes norvegica) represents a significant component of the zooplankton community in terms of abundance and biomass in the north-east Atlantic. A significant behavioral trait of zooplankton is diel vertical migration (DVM), known as a rising from the depth at dusk for feeding and returning to deeper layers at dawn. Generally, the variance in DVM in terms of depth or timing at specific locations is a trade-off between feeding at the most preferred site and a reduction of predation risk. Recent investigations of these rhythms have demonstrated that an endogenous circadian clock exists in Antarctic krill (Euphausia superba), synchronized by the photoperiod and linked to metabolic processes. To gain a general knowledge on the existence of an endogenous timing system in M. norvegica as well as on if and how it mediates physiological and behavioral functions such as respiration rates and DVM in different seasons we conducted field sampling every 4 hours during a 24-hour sampling campaign in May and November. Oxygen consumption was measured using freshly caught animals reared under different photoperiods and under constant darkness. In addition, daily and seasonal clock and metabolic gene expression was investigated sex and tissue-specifically. Our findings indicate that the endogenous clock of M. norvegica is possibly activated by light as Zeitgeber, but is not necessarily driving the rhythm as observed by the maintenance of patterns in oxygen consumption under constant darkness, although at lower amplitudes. Additionally, M. norvegica are able to adapt their daily rhythms in oxygen consumption to a change in light regime, despite the putative presence of an internal rhythm. The nocturnal increase in oxygen consumption matches the time when krill performs DVM, the migration to the surface for feeding. This is also related to the daily patterns observed for abundances. Accordingly, clock genes (Cyc, Cry2 and Vri) showed an upregulation during the day (after sunrise) and the night (after sunset), indicating a corresponding time to the upregulation or downregulation of oxygen consumption. Daily expression patterns of genes involved in metabolic key processes (Cs/mMDH/CoI/Fas and Hoad) generally correspond to the findings on organismic level under natural light conditions. However, significant rhythms detected in the expression profiles of metabolic genes were mainly periods of different durations. In addition, no clear rhythmicity patterns could be detected between sexes and tissues, suggesting that ultradian periods of less than 24 hours may be harmonics of a circadian clock and that these rhythms could be interpreted as ultradian patterns of one circadian output. It can also not be ruled out, that different oscillatory patterns may reflect autonomic functional traits, that may be controlled by additional and/or other than the circadian clock.