Internal Clocks Control Activity and Seasonal Changes

Jim Crocker
1st May, 2025

Internal Clocks Control Activity and Seasonal Changes

Hydroacoustic recordings confirm that wild swarms of Antarctic krill (Euphausia superba) perform synchronized diel vertical migration in summer (a), late summer (b), and autumn (c), providing real-world evidence of the daily activity patterns driven by their internal circadian clock.

Image adapted from: Hüppe et al. / CC BY (Source)

Key Findings

  • In the Southern Ocean, Antarctic krill have an internal biological clock that controls their daily swimming patterns
  • This internal clock allows krill to adjust their movements with changing seasons, helping them adapt to environmental shifts
  • Understanding krill’s clock is key to predicting their response to climate change and its effects on the marine ecosystem
Antarctic krill play a crucial role in the Southern Ocean ecosystem, serving as a primary food source for whales, seals, seabirds, and various fish species. Understanding the behavior of these small but abundant organisms is essential for comprehending the broader dynamics of marine life and carbon cycling in this region. One key behavior exhibited by krill and many other zooplankton is vertical migration. This involves moving up to surface waters at night to feed on plankton and descending to deeper waters during the day to evade predators. This daily movement, known as diel vertical migration (DVM), significantly influences the distribution of nutrients and carbon in the ocean[2]. Additionally, seasonal vertical migration (SVM) occurs over longer timescales, allowing zooplankton to adapt to changing environmental conditions throughout the year[2]. Previous research has highlighted the complexity of krill migration patterns. A study using echosounder data from commercial fishing vessels revealed that krill swarms migrate deeper during winter but also display substantial variability in their vertical movements within the same season and location[3]. This variability suggested that factors beyond immediate environmental cues, such as life stage and predator presence, might influence krill behavior. Building on these insights, researchers at the University of Würzburg conducted a groundbreaking study to unravel the mechanisms driving krill vertical migration[1]. They employed a newly developed activity monitor called AMAZE, which allows for the precise tracking of individual krill movements in controlled settings. By capturing wild krill and observing their swimming patterns under various light conditions and across different seasons, the team aimed to determine whether krill rely solely on external environmental cues or possess internal biological clocks that regulate their behavior. The study revealed that krill exhibit a distinct bimodal pattern of swimming activity, with increased movement during night hours. Remarkably, this nocturnal activity persisted even when krill were kept in constant darkness, indicating the presence of an internal circadian clock. Additionally, the activity patterns adjusted to the changing lengths of day and night across seasons, further supporting the role of an endogenous biological clock in regulating migration. These findings align with the broader understanding of diel vertical migration as a behavior influenced by both external light cues and internal rhythms[2]. However, the discovery of a robust circadian mechanism in krill adds a new layer of complexity, suggesting that krill can maintain synchronized movements even in the absence of immediate environmental signals. This internal clock likely enhances their ability to respond to seasonal changes, ensuring that migration patterns are finely tuned to the varying conditions of the Southern Ocean. Moreover, the high degree of behavioral plasticity observed in the study echoes previous findings that krill behavior is not solely dictated by environmental factors[3]. The presence of an internal clock provides a potential mechanism for this flexibility, allowing individual krill to adjust their activity based on both internal states and external influences such as predator activity and food availability. The implications of this research are significant, particularly in the context of climate change. As the Southern Ocean undergoes rapid environmental transformations, understanding the internal mechanisms that govern krill behavior is crucial for predicting how these organisms will adapt. The ability of krill to maintain synchronized migration patterns through internal clocks may offer them resilience against changing light conditions and other environmental stressors. However, disruptions to these internal rhythms due to altered temperature regimes or altered predator-prey dynamics could impact their survival and, by extension, the entire Southern Ocean food web. Furthermore, the study demonstrates the potential of innovative tools like the AMAZE monitor to advance our understanding of marine biology. By enabling the tracking of individual organisms in real-time, researchers can gain unprecedented insights into the behavioral ecology of key species like krill. This approach complements large-scale observational studies and provides a more detailed picture of the factors influencing vertical migration. In summary, the University of Würzburg's study provides critical evidence that Antarctic krill possess internal circadian clocks that regulate their vertical migration patterns. This discovery not only enhances our understanding of krill biology but also underscores the intricate interplay between internal mechanisms and external environmental factors in shaping the behavior of marine organisms. As climate change continues to impact the Southern Ocean, such insights will be invaluable for predicting and mitigating its effects on this vital ecosystem[2][3].

BiochemEcologyMarine Biology

References

Main Study

1) A circadian clock drives behavioral activity in Antarctic krill (Euphausia superba) and provides a potential mechanism for seasonal timing

Published 29th April, 2025

https://doi.org/10.7554/eLife.103096

Related Studies

2) Two hundred years of zooplankton vertical migration research.

https://doi.org/10.1111/brv.12715

3) Plasticity and seasonality of the vertical migration behaviour of Antarctic krill using acoustic data from fishing vessels.

https://doi.org/10.1098/rsos.230520


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