Tiny marine creatures living in the dark, frigid waters of the wintry Arctic migrate up and down in the ocean based on moonlight, according to new findings published in Current Biology this week.
Animals living in extremely high-latitude marine environments have to navigate without any solar illumination in the wintertime. Yet, even in the darkest part of the polar night, zooplankton migrate vertically in the ocean every day: It’s quite possibly the world’s largest daily migration by biomass. Among other advantages, vertical migration helps these tiny migrants avoid visual predators that hunt near the surface.
To see what drives mass vertical migration during the Arctic winter, a team led by Kim Last of the Scottish Association for Marine Science (SAMS) studied data from moored acoustic instruments that have been deployed for a total of at least 50 years.
"During the permanently dark and extremely cold Arctic winter, [these] tiny marine creatures, like mythical werewolves, respond to moonlight by undergoing mass migrations," Last explains in a statement. And these so-called lunar vertical migrations (LVMs) occur throughout the entire Arctic Ocean – within fjords, under sea ice, and in shelf, slope, and open sea environments – from the North Pole to Svalbard, Norway, to the Canadian Archipelago.
In the winter when the Moon rises above the horizon, there’s a shift in activity from the 24-hour solar-day vertical migration to a 24.8-hour lunar-day one. Additionally, a mass sinking of zooplankton from the surface to a depth of about 50 meters (164 feet) occurs every 29.5 days – which syncs up with full moon periods.
The researchers think that moonlight helps these zooplankton avoid light-mediated visual predators, such as birds, fish, and carnivorous planktonic animals. It’s unclear how important visual predation is at such high latitudes during the wintertime, though there’s some evidence that the planktonic amphipod predator Themisto libellula (pictured above) can detect its prey (such as Calanus species) even at 80°N during the darkest part of the polar night. "Whilst the predatory zooplankton are using the moonlight as a tool for visual predation, the prey avoid the moonlit regions to reduce this threat," study co-author Laura Hobbs of SAMS explains to IFLScience.
While photosynthesis and primary production in the Arctic is almost nil during the winter, these daily lunar-illuminated migrations play an important role in the ocean’s carbon pump – moving carbon from the surface into the ocean interior.
Image in the text: Arctic moonrise. Geir Johnson/Norges Naturvitenskapelige Universitet and University Centre in Svalbard