Antarctic krill keep time in a world of endless days, or nights

Krill may be small, but they are certainly not insignificant. They measure just six centimeters in length, have a mass of only two grams, and are nearly invisible due to their transparent skin.

Yet these modest crustaceans play a vital role in the Southern Ocean. Billions of them gather into massive swarms that stretch across square kilometers. They are a cornerstone of the marine food web, and are preyed upon by numerous other organisms, from fish to whales, in one of the harshest environments on Earth.

Scientists from Julius-Maximilians-Universität Würzburg (JMU), in collaboration with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the Helmholtz Institute for Functional Marine Biodiversity (HIFMB), and the UK’s National Oceanography Institute, have now studied krill more closely.

Their focus has been to understand the crustaceans’ daily vertical migrations – how and why krill move up and down the water column in a pattern that tracks the day-night cycle.

Surfacing for food, diving for safety

Antarctic krill feed at night and swim to the ocean’s surface under the cover of darkness to graze on microscopic algae and other phytoplankton. Come daylight, they descend into the depths to avoid becoming lunch themselves.

“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,” said Lukas Hüppe, the study’s first author.

These migrations aren’t just important for the krill. As they travel up and down each day, they stir the ocean and move carbon from the surface into deeper waters. This helps regulate carbon levels and plays a part in Earth’s climate system.

But even after decades of observation, scientists didn’t fully understand what controls this behavior.

A new way to monitor krill

That’s why the team developed a new technology in 2024: an activity monitor that tracks the swimming behavior of individual krill inside seawater-filled tubes.

Hüppe and colleagues collected krill from the Southern Ocean during a commercial fishing trip. Onboard the vessel, they used this new monitor to record their activity under varying light conditions and throughout different seasons.

What they discovered was striking. The krill were most active at night, matching their natural migration cycles.

Even more impressive was that the krill adjusted their nighttime activity depending on the season. In summer, when nights are shorter, their activity window shrank. In winter, when nights stretch for hours, it expanded. This rhythm didn’t disappear when the animals were kept in complete darkness for several days.

The clock inside krill

“Antarctic krill show a daily rhythm with increased swimming activity at night, which fits very well with vertical migration in nature,” said Hüppe. The consistency of this activity pattern – even in total darkness – suggests the presence of an internal biological clock.

The krill didn’t just respond to light or food cues, but also time. They appeared to have their own built-in schedule.

“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,” said Charlotte Förster.

Even in the unusual light cycles of the poles – where summer days can last 24 hours and winter nights stretch on endlessly – krill were able to adjust. Their internal clocks adapted to both very long and very short days.

Tiny swimmers, big impact

While the study focused on the inner workings of krill, its significance reaches much further.

“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,” said Bettina Meyer.

Healthy krill populations are vital for maintaining this ecosystem. If krill decline, the ripple effects could be felt all the way up the food chain – from fish and penguins to whales.

Understanding how krill adapt to their environment is a key step in predicting how climate change might affect them – and the rest of the ocean.

Cracking the code of krill time

The team isn’t stopping here. Their next goal is to explore 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,” said Förster. They’ll also investigate how this internal timing system affects other parts of krill life, like reproduction and hibernation.

Even in one of Earth’s harshest climates, krill have found a way to stay in sync. And as researchers learn more about their hidden rhythms, they’re uncovering a story that links these small crustaceans to the health of the entire planet.

The full study was published in the journal eLife.

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