In the polar ‘twilight zone,’ these unusual sea creatures outshine the starlight

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The ocean’s twilight zone is a spooky place, where creatures like krill and “werewolf” plankton hunt—and hide—using only the light they themselves emit. In most seas, the zone is deep, stretching from 200 to 1000 meters beneath the surface. But new research has found that in the winter waters near Norway’s Svalbard archipelago, the zone shifts upward during the long polar night, bringing some of these creatures close to the surface. What’s more, the denizens of the zone live in distinct layers, with some dominating the upper levels and others ruling below. The findings could lead to a new understanding of polar marine ecosystems, even as they are endlessly transformed by melting sea ice.

“This is a new look at the structure of the water column in the polar night,” says Edith Widder, an oceanographer and biologist who studies bioluminescence at the Ocean Research & Conservation Association in Fort Pierce, Florida. “[This] looks at the impact of one of the most important organizational principles in the open ocean, light, including the light animals make themselves.”

The polar night was long considered a time of hibernation. For nearly 4 months, the sun disappears entirely; tiny animals at the base of the food chain were thought to die off or go dormant. But that thinking changed in 2007, when sonar equipment used by marine ecologist Jørgen Berge picked up on a strange signal: an echo that descended predictably during the day and rose at night. In warmer waters, a similar pattern marks the daily movement of millions of marine creatures up and down the water column. The sonar echo meant this mass migration was also happening near the poles, triggering a complete re-evaluation of Arctic ecosystems. Since 2012, Berge, at the University of Tromsø - The Arctic University of Norway, has led annual expeditions examining everything from “werewolf” plankton to clams with internal clocks timed to an absent sun.

In their dark journeys, Berge and his team discovered an unexpected abundance of bioluminescence. They began wondering where the creatures’ own light took over from the virtually nonexistent daylight and how that glow affected predator-prey relationships. But to do so, the team had to first quantify how much light was being produced—and by what.

The scientists started by netting a variety of organisms, from krill to comb jellies to copepods, small crustaceans that form the base of the Arctic food chain. Then, they used a special device to measure the emitted light: an Underwater Bioluminescence Assessment Tool (UBAT), designed in part by team member Mark Moline, a marine ecologist at the University of Delaware (UD) in Newark. Like a mini vacuum cleaner, the breadbox-sized black box sucks water into a chamber and whips it, prompting the creatures inside to light up.               

“It measures every 1/60 of a second, so you get to see the kinetics of the actual flash,” Moline says. “Each organism emits a different signal in terms of intensity and duration. It almost looks like a Morse code or a heartbeat.”

In the lab, the scientists examined 17 species and came up with distinct signals for seven of them. Then, over the course of two 3-week cruises in 2014 and 2015, they used their key to map the entire column in 20-meter increments down to 120 meters. Finally, they calculated the total bioluminescence of each level and compared it with the light that should have reached that depth from the atmosphere. The brightness from bioluminescence surpasses starlight, moonlight, and even the nearly imperceptible noontime daylight about 20 to 40 meters below the surface, they report this week in Scientific Reports. Further, dinoflagellates, microscopic creatures that can selectively photosynthesize, dominate the upper ranges, whereas copepods rule the deeper realms.

“We tried to … break out who’s there and where they are and how much light are they producing,” says paper author and visual ecologist Jonathan Cohen of UD. “We put that together with where atmospheric light and bioluminescence actually flip roles.”
 

The team’s next step is to explore the role bioluminescence plays in predator-prey dynamics. Cohen, who specializes in krill bioptics, has already mathematically modeled how far away and at what depth a keen-eyed krill can detect a predatory bird diving into the twilight zone, especially if that bird happens to be trailing a line of brightly lit dinoflagellates. The findings suggest that extra light from other creatures may save the krill from becoming an untimely snack. But the research has other ramifications.

“One of the major implications of climate change in the Arctic is thinning ice and a changing light climate,” Cohen says. Sea ice blocks daylight, so less frozen ocean makes seas a brighter place, possibly posing problems for animals adapted to darkness in the polar night. “Changing the atmospheric light portion even in times of twilight will influence the role bioluminescence can play.”