Individually, Antarctic krill (Euphausia superba) do not make much of an impression. With a maximum body length of six centimetres, a weight of just two grams and its transparent skin, it does not look very spectacular. Yet krill play a central role for life in the Southern Ocean. Billions of these small crustaceans form huge swarms that can extend over several square kilometers and are the most important food source for many predators.
A research team from Julius-Maximilians-Universität Würzburg (JMU), in cooperation with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the Helmholtz Institute for Functional Marine Biodiversity (HIFMB) in Oldenburg and the National Oceanography Institute in the UK, has now taken a closer look at the behavior of this marine inhabitant. The group was particularly interested in its “daily vertical migration in the water column”, according to the study now published in the journal eLife.
Food on the surface, protection from predators in the depths
“Antarctic krill use the cover of darkness at night to feed on microscopic algae on the sea surface. During the day, the animals then seek shelter from predators in deeper, darker layers,” says Lukas Hüppe, describing the periodic ups and downs in the Southern Ocean. Hüppe is the first author of the study and a doctoral student at the JMU Department of Neurobiology and Genetics. He was supervised by Bettina Meyer (AWI and HIFMB) and Charlotte Förster, the former holder of this chair and now a senior professor. Internal clocks have been a focus of Förster's research for many years. Accordingly, this project focused on the question of the extent to which krill migrations are determined by internal clocks.
Although krill exert considerable influence on the mixing of the water column and the transport of carbon into the deep sea with their daily migration, and despite decades of observations, the exact mechanisms of this migration behavior are not yet fully understood. For their study, the research team has therefore now for the first time examined individual wild-caught animals over different seasons in a special activity monitor.
Observations with a newly developed technology
Researchers first developed this monitor in 2024. The new device makes it possible to record the swimming activity of individual creatures in tubes filled with seawater. For his experiments, Hüppe caught krill from the Southern Ocean on a commercial fishing vessel. On board, he was able to use the new technology to study the movements of wild-caught krill under different light conditions and at different times of the year.
His observations showed that the crustaceans were most active at night, which corresponds to their natural migration patterns in the wild. These nocturnal activity patterns adapted to the changing length of the night throughout the seasons. In addition, the krill maintained a daily rhythm of activity even when kept in constant darkness for several days.
Typical rhythm even in complete darkness
The results are clear: “Antarctic krill show a daily rhythm with increased swimming activity at night, which fits very well with vertical migration in nature,” explains Lukas Hüppe. Even in complete darkness, the animals maintained this rhythm over several days - proof that they use an internal clock to adapt their ups and downs to the day-night rhythm. The experiments also showed that krill can flexibly synchronize their behaviour with very long or short days, which only occur in polar regions.
It is therefore clear: “Krill do not only react to external environmental influences such as light or food with their behavior. It also uses its internal clock to adapt to the extreme conditions of its polar environment,” says Charlotte Förster, summarizing the key finding of the study.
Significance for the ecosystem and the climate
Even if the study is primarily concerned with physiological processes inside small sea creatures, the significance of its findings goes far beyond this. “As a carbon sink, the Southern Ocean plays a central role in regulating the global climate. This function is based on a functional, productive ecosystem, at the center of which is the Antarctic krill,” explains Bettina Meyer. The optimal adaptation of krill to its environment is a basic prerequisite for healthy krill stocks.
As changes in krill populations can have far-reaching consequences for the entire ecosystem of the Southern Ocean, a better understanding of the adaptation mechanisms is crucial in order to make predictions about the future development of the populations, the research group concludes.
In their next project, the scientists therefore want to investigate the internal clock in more detail. “We want to understand where the clock ticks in the krill brain and how the mechanism works at a neuronal level,” says Charlotte Förster. This will also focus on the question of how the internal clock influences other important processes in krill - such as reproduction and its hibernation strategies.
Contact
Prof. Dr. Charlotte Förster, Department of Neurobiology and Genetics, T: +49 931 31-88823, charlotte.foerster@uni-wuerzburg.de
Prof. Dr. Bettina Meyer, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, T: +49 471 4831 1378/2535, bettina.meyer@awi.de
Lukas Hüppe, Department of Neurobiology and Genetics, T: +49 931 31-84703, lukas.hueppe@uni-wuerzburg.de