If colossal and giant squid's basketball-size eyes—two to three times bigger than any other animal's—don't see particularly well, what's their evolutionary point?
According to a new study, big squid eyes do have a "superpower" Captain Ahab might have killed for: sperm whale vision.
(Related: "Colossal Squid Has Glowing 'Cloaking Device,' Huge Eyes.")
The discovery began with an ultra-rare catch.
In 2007 fishers in Antarctica's Ross Sea netted the largest intact squid ever captured (picture), a dead colossal squid (Mesonychoteuthis hamiltoni) stretching 26 feet (8 meters) long. They quickly froze the specimen, which would remain that way until a team of expert scientists could thaw the squid out at the Museum of New Zealand.
Upon dissection, the 1,091-pound (495-kilogram) animal stunned scientists with the size of its 10.6-inch-wide (27-centimeter-wide) eyes, which are comparable to those of the other big squid species, the giant squid. (See pictures of the colossal squid dissection.)
"We didn't know about any other animals with eyes that were even slightly smaller—it was a huge gap between them and the eyes of all other animals," said study leader Dan-Eric Nilsson, a biologist at Sweden's Lund University.
Big Eyes, but Why?
Nilsson began to wonder: Why would the two big squid species, colossal and giant—giant being slightly smaller—develop such outsize eyes, and how do the animals use them?
Since the colossal squid's habitat—around 3,300 feet (1,000 meters) underwater—is pitch black, it seems obvious that their eyes would be big to pick up more light.
But Nilsson's team's mathematical modeling revealed that the optical properties of water would limit the eye's ability to discern typical undersea objects, like prey—the huge eyes, it seems, confer no vision advantage.
In fact, his data suggest that any growth beyond the size of an orange is wasted effort, as far as deep-sea vision is concerned. That conclusion seems to be confirmed by a look at the eyes of other animals sharing the same waters.
"There are lots of animals down there, some as large as the colossal squid, but their eyes were not nearly so large. So just seeing better in darkness down there wasn't the answer," he said.
Giant Squid See the Light
Since eyes are "expensive" for bodies to develop and maintain, however, there had to be some reason for their size, Nilsson thought.
The team's models revealed that, what the colossal and giant squid's supersize pupils and retinas lacked in close-up vision they made up for with extreme farsightedness. The cephalopods are fine-tuned to spot very large objects at a distance—such as the sperm whales that prey on the squid.
Still, no matter how large the eye, or how big the object being seen, darkness presents a visual problem.
With their great light-gathering capacity, squid eyes are able to detect even a faint glow the equivalent of a football field away, the study found.
Not coincidentally, when a sperm whale is on the move, it disturbs tiny bioluminescent life-forms, creating a faintly glowing trail in the whale's wake—and giving squid an unintentional warning sign.
Nilsson is now hoping to use the same models to uncover how other creatures use their eyes in the deep, and consequently a bit more about how they live.
"It's so inaccessible down there that this modeling is a way of gathering knowledge," he said. "I think the model that we've developed can be used to work out what sorts of things other animals would see down there. ...
"It's a way we can begin to work out the ecology in an area where it's nearly impossible for us to observe."
The colossal and giant squid eye study was published in the March 15 by the journal Current Biology.