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From NPR News, This is ALL THINGS CONSIDERED. I'm Robert Siegel.

MELISSA BLOCK, HOST:

And I'm Melissa Block.

Here are two animals that really live up to their names: the giant squid and the colossal squid. They can be more than 40 feet long, if you measure all the way out to the tip of their feeding tentacles. But it's the eyes that are truly huge. They're the biggest eyes in the animal kingdom.

NPR's Nell Greenfieldboyce reports on a new explanation for why these types of squid need eyes the size of basketballs.

NELL GREENFIELDBOYCE, BYLINE: It is not exactly easy for researchers to get their hands on giant squid eyes. These mysterious creatures dwell in the deep ocean, thousands of feet down. Scientist didn't even have photographs of a live one in the wild until 2004. Occasionally, a fishing boat will haul in a big dead or dying blob.

Roger Hanlon is at the Marine Biological Laboratory in Woods Hole, Massachusetts.

DR. ROGER HANLON: By the time we get the eye, it's been kind of rotted and it's not in good shape, and eventually someone gets enough bottles of gin or whatever is on board to pickle it - they never have formalin and that sort of thing on fishing boats. And so, the eyes we get are all kind of gnarled and poorly fixed, so it's hard to get accurate measurements.

GREENFIELDBOYCE: Hanlon and his colleagues wanted good measurements to help them figure out what giant and colossal squid are doing with such strange peepers that are so much bigger than the largest eyes in other animals.

HANLON: It's always been a vexing question of why they have such large eyes. Yes, the body is large but the eyes are enormous.

GREENFIELDBOYCE: The research team was finally able to get some measurements they considered reliable. Dan Nilsson is a biologist at Lund University in Sweden.

DAN NILSSON: We heard about a fisherman in Hawaii who had caught a dying giant squid, which he put into his little boat and went into the harbor. And there, there was another guy who actually took a photograph of it.

GREENFIELDBOYCE: In the photo, there's a fuel hose crossing the eye's pupil. The researchers used the size of this standard hose to calculate the eye's dimensions. The whole unblinking eye was about 10 and a half inches across. They were also able to directly study the eyes on a well-preserved colossal squid that was frozen right after being caught. Nilsson helped dissect that squid. He says holding one of its 10-inch eyes wasn't like a holding a round ball of jelly, because squid eyes aren't filled with the gelatinous stuff that's in human eyes.

NILSSON: Squid eyes don't have that. They've just got water in their eyes. So when they're dead, the eyes collapse, so it's like a collapsed plastic bag.

GREENFIELDBOYCE: These squids live so deep, there's hardly any sunlight left - it's practically pitch black. The researchers had developed a bunch of calculations to predict how well different-sized eyes would see in this dark, watery world. For seeing small objects, like nearby prey, it wouldn't make sense to have eyes bigger than an orange.

But Nilsson says if you want to see extremely large objects from a distance...

NILSSON: Then it pays to actually make the eye a fair bit bigger.

GREENFIELDBOYCE: In fact, they calculate that the squid eyes they measured are the optimal size for seeing something far away and massive, like the squid's main predator, the sperm whale. Here's what they think: Imagine a sperm whale, intent on a lunch of giant squid. As the whale dives towards its prey through the water, it disturbs tiny bioluminescent creatures that then light up. With its enormous eyes, a giant squid could see the huge glowing mass headed its way, and use its jet propulsion to make a quick escape.

Howard Howland is a scientist with Cornell University who studied the diversity of eye size in animals. He's convinced by this new theory, which appears in the journal Current Biology.

HOWARD HOWLAND: It's, I would say, an exemplary analysis. I think that this paper is going to set the path for a number of papers looking at the influence of the environment on eyes.

GREENFIELDBOYCE: Because the size of an eye could be a window to animals' secret lives.

Nell Greenfieldboyce, NPR News.

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