Scientists studying the moon’s effect on marine life during the constantly dark Arctic winter believe they have uncovered the ‘werewolves of the ocean’– which regularly gather in their billions to undertake the largest migration on Earth.
The team from the Oban-based Scottish Association for Marine Science [SAMS] have published findings in the journal Current Biology that the actions of zooplankton [small marine animals] respond to the moon as the main light source during the polar night.
Using echo sounders fixed to the seabed and analyses more commonly associated with studying the human biological clock, the scientists observed zooplankton moving deeper into the darkness in response to the full moon. The team believes this migration is to hide from light-dependent visual hunters, like the voracious centimetre-long crustacean, Themisto libellula [top photograph].
This response could be seen across the entire Arctic at all water depths, ice covered and ice free, from 70°N to 90°N.
Lead author on the Current Biology paper, Dr Kim Last [photographed above at midday in Isfjorden] – SAMS principal investigator in marine chronobiology, says: ‘It was previously presumed that there was little activity during the Arctic winter, as there is hardly any food and no light; but our recent work with partners from the University of Tromsø showed there is a surprisingly high level of activity.
‘Now we know that when the moon rises, the zooplankton drop down in the water column to around 50 metres in depth, presumably to hide from predators.’
The mass migration has been detected by the team at the North Pole, in water 4,000 metres deep and underneath thick ice. The research suggests that reducing sea-ice cover, resulting from climate change, may cause further changes in these migrations as more light penetrates the sea.
This newly-discovered response to moonlight during the Arctic winter has been described by the researchers as lunar vertical migration (LVM) and only occurs for a few days each month as the full moon rises above the horizon.
The team also discovered that zooplankton follow the rising and setting of the moon. This phenomenon results in a new kind of daily lunar migration, the cycle of which is longer (every 24.8 hours) than the standard day / night solar light response in the sunlit waters of the rest of the world.
Dr Last says: ‘Diel vertical migration (DVM) of zooplankton is one of the biggest daily migrations on the planet, a process driven by sunlight. It’s therefore a complete surprise to us to find that wherever we look across the Arctic during the winter, we witness a migration driven by moonlight.
‘Perhaps the ‘werewolf’ isn’t a myth after all?’
Laura Hobbs, a PhD student at SAMS and co-author on the paper, says: ‘The moon must have a dramatic effect on these creatures if they are undertaking such huge migrations.
‘The next step is to find out more about the response, how it varies across the region and how the behaviours might change as Arctic sea ice cover reduces.’
The work was funded by UK’s Natural Environmental Research Council (NERC) and the Research Council of Norway (NRC) under projects Panarchive and Circa.
Note: Photographs above show, from the top, show:
- Themisto libellula, an amphipod crustacean and a predatory hunter of copepods such as Calanus, is a probable werewolf of the Arctic.
- Report author, Dr Kim Last, on field work collecting zooplankton in Isfjorden, Svalbard, at midday in January.
- Moon rise over Kongsfjorden in Ny-Ålesund, Svalbard where an acoustic mooring (ADCP) is positioned on the seabed and used to track zooplankton migrations.