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Scientists unravel mystery of humongous squid eyeballs

Living at extreme ocean depths, the colossal squid and its cousin, the giant squid, has unusually large eyes. A group of researchers say they now know what accounts for the animals' huge peepers. 

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In 2007, a New Zealand fishing crew caught the best specimen of an adult colossal squid in the Ross Sea near Antarctica. The eyes of this squid were confirmed to be the largest of any known animal.

New Zealand Ministry of Fisheries

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The basketball-sized eyeballs of the colossal squid are good for more than just staring down Captain Nemo. A new study suggests that they evolved to keep a lookout for sperm whales. 

Despite being the largest known invertebrate, the colossal squid, which lives 2,000 feet below the surface of the Antarctic Ocean, has long eluded scientific observation. Until 1981, the only evidence of the giant cephalopod's existence could be found in the stomachs of what is thought to be its chief predator, the sperm whale. Since then, four complete specimens have been captured, the largest being hauled aboard by a New Zealand fishing boat in 2007.

This squid, which measured over 30 feet and weighed half a ton, was frozen, thawed and examined by Dan-Eric Nilsson, a marine vision expert at the University of Lund in Sweden. He confirmed that the squid's eyeballs, which measured 11 inches across, were the largest of any known animal.

Indeed, they were so big that they presented a puzzle. Other squids had eyes that were much smaller in proportion to the rest of their bodies, and no other animal, regardless of its size, had eyes that were even close to those of the colossal squid, and its slightly smaller cousin, the giant squid. The eyes of swordfish and whales, for instance, tend to top out at about 3.5 inches in diameter. 

"You can find everything up to the size of an orange, which are in large swordfish," Nilsson told BBC News's Richard Black. "So you find every small size, then there's a huge gap, then there are these two species where the eye is three times as big – even though squid are not the largest animals."

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