In the Arctic winter, plankton switch from a solar to a lunar clock

Every day, in every ocean, an enormous mass of small animals rises towards the surface after dark and sinks again at first light. Diel vertical migration is a simple trade: the food is near the surface, so is anything that hunts by sight, so go up only when it is dark. It is the largest synchronised movement of biomass on the planet, and it runs on a 24-hour clock because the sun provides one.

Take the sun away for several months and the arrangement should collapse. There is no dawn to descend before. In the Arctic Ocean through the polar night, the prediction is straightforward: migration stops, and the animals distribute themselves wherever the food is.

That is not what happens.

A 24.8-hour day

Kim Last of the Scottish Association for Marine Science, with Laura Hobbs, Jørgen Berge, Andrew Brierley and Finlo Cottier, reported the result in Current Biology in 2016 under a title that states the finding plainly: moonlight drives ocean-scale mass vertical migration of zooplankton during the Arctic winter.

The key observation is a change of period. Through the winter, once the moon rises above the horizon, the migration shifts from the solar day of 24 hours to the lunar day of 24.8 hours. Those two numbers are close enough that a casual look at a record would not separate them, and far enough apart that over a few weeks the two schedules drift completely out of step with each other. The animals are following the second one.

During full moon periods, the migrations track moonlight in synchrony with both the altitude and the phase of the moon. The authors named the pattern lunar vertical migration.

What the animals are avoiding

The behavioural logic is unchanged from the summer version. Descend when there is light, because light means a visual predator can see you against the brighter water above. What changes is the threshold.

Moonlight is faint. A full moon at high latitude, filtered through cloud and often through a skin of sea ice, delivers a light level far below anything that reads as brightness to an eye adapted to daylight. The animals are treating it as sufficient reason to go deeper, which implies that something up there can hunt by it. That inference, rather than the timing itself, is the part with consequences: it says the Arctic winter is not a refuge where visual predation switches off, only a period when it operates at a much lower light level and on a different schedule.

Why the polar night is the right place to look

This experiment could not be run anywhere else. In temperate water the solar signal swamps everything; a lunar rhythm, if present, is buried under a much stronger daily cycle. The polar night removes the dominant variable for months at a stretch, which turns the Arctic into a natural laboratory for asking what organises behaviour when the obvious cue is absent.

The details of that absence matter, because “polar night” covers several quite different light regimes depending on latitude. Just inside the polar circles the midday sky still reaches civil twilight; above roughly 84°34’ there is no astronomical twilight at solar noon at all. We set out the four grades in a separate polar night explainer.

The carbon question

There is a second inference in the paper that reaches beyond behaviour. Animals that feed near the surface and then descend to depth are transporting carbon downwards, in their bodies and in what they excrete, on whatever schedule they keep. If the schedule persists through the winter rather than pausing, then so does that transport.

The authors put it as a possibility rather than a measurement: moonlight may play a central role in carbon sequestration during the Arctic winter as well as in predator and prey interactions. Turning that into a number would require grazing and flux measurements that, as things stand, are rarely made directly. The zooplankton explainer covers why that gap persists.

What it does not say

It does not say the animals are navigating. The word does a lot of work in popular coverage of this result, and the measurement supports something narrower: the timing of a vertical movement tracks the moon’s altitude and phase. Whether any individual is orienting to the moon in a directional sense is a different question, and not one a moored acoustic record can answer.

Nor does it say every species does it. A signal in the aggregate biomass of a water column is compatible with a great deal of variation among the animals producing it, and the paper is about the aggregate.

What survives is a clean, testable claim with an awkward implication for anyone modelling the polar winter as a dormant season: the largest migration on Earth does not stop when the sun does. It changes clocks.