Hidden Depths: Antarctic krill startle deep-ocean scientists

Biologists looked into the abyss and the abyss looked back, with lots of little compound krill eyes.

KRILL ZONE. This female krill, full of eggs, from the surface waters of the Southern Ocean belongs to the same species glimpsed 3,000 meters down, researchers say. British Antarctic Survey

The shrimplike Antarctic krill, a major player in polar ecosystems, is supposedly a creature of the upper ocean. Yet the first science cruise to lower a camera to the abyssal seabed of the Southern Ocean off Antarctica found what looked like krill 3,000 meters down, says Andrew Clarke of the British Antarctic Survey based in Cambridge, England.

The cruise, during the South Pole summer of 2006-2007, inaugurated the United Kingdom’s remotely operated, camera-carrying Isis vehicle. Clarke says that he and several other biologists were just piggybacking on a mission primarily designed for glaciologists and geophysicists to examine the deep continental slope and seabed beyond.

By that time of year, photosynthesizing plankton have multiplied in a great burst at the surface of the ocean and drifted down. When the scientists lowered their camera to the sea bottom, they saw a layer of still-green plankton-fall—and the krill feeding on it. These animals were the classic Antarctic krill species, Euphausia superba, say Clarke and Paul Tyler of the National Oceanography Center in Southampton, England, in the Feb. 26 Current Biology.

The Antarctic krill species matures to 6 centimeters in length, a giant among krill kind, and its red markings show up in the Isis video. The animals, including females ready to spawn, even made nosedives into the sediment, a behavior seen in shallow water that sends up puffs of fallen plankton. The krill then scooped debris out of the water with spiny structures on their legs, held to form what biologists call a feeding basket.

Based on the video evidence, “there isn’t really much else it could be” other than the Antarctic krill, says Stephen Nicol of the Australian Antarctic Division in Kingston, Tasmania. Previous camera missions at some 600 m down have sighted these krill now and then, he says.

With so few observations of krill in the deep, biologists can only speculate about what’s going on. Nicol says krill swarm in ravenous schools at the surface, reminding him of locusts. He guesses that krill feeding on a plankton bloom may have just kept eating as their lunch sank.

“Maybe what you’ve got is another link between the bottom and the surface,” Nicol says, a matter of import in the study of nutrient cycling. If masses of krill routinely do this, the already uncertain estimates of their population could be even more so, he adds.

“I have heard rumors about this finding,” e-mailed Peter Wiebe, of the Woods Hole Oceanographic Institution in Massachusetts, who is currently shipboard on a krill survey cruise. “If the observation proves true about the krill at 3,000 m, then it shows how little we really understand about how the ocean ecosystem is structured and functions.”

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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