A giant squid's soccer ball-sized eyeballs are three times wider than any other animal's, but explaining why has kept squid researchers busy.
New dissections and computer models offer a lead in the mystery: The enormous peepers evolved to see bioluminescent trails of light left by sperm whales, the squids' great predator.
"Sperm whales can't make sharp turns when diving for food. They have to rely on the prey being unaware it's approaching," said biologist Dan-Eric Nilsson of Lund University. Nilsson's study was published March 15 in Current Biology.
"We think giant and colossal squid eyes have enough sensitivity to see them coming from 120 meters away, and maybe scoot to the side to avoid being eaten," he said.
Giant and colossal squid are the most enormous cephalopods known. Their fully grown bodies -- minus eight long arms and two tentacles -- extend more than 8 feet long, and their eyes are the world's largest.
Of the handful of specimens recovered, the largest eye measures more than 11 inches wide. Such enormous peepers surely excel at gathering light, and one would expect to see them in other deep-sea animals with room in their skulls. Yet the eyes of swordfish and whales, for example, top out around 3.5 inches wide, or about the size of an orange.
"There's a law of diminishing returns for eyes in the ocean. Bigger eyes can detect the same objects farther away, but eventually it doesn't pay anymore because water is a light-scattering medium," Nilsson said.
Understanding the gargantuan sizes of giant and colossal squid eyes has proven extremely difficult. The biggest challenge is that the creatures live at crushing depths of more than 2,000 feet. Only a handful of specimens have ever surfaced.
"I don't think anyone will tag a live animal in my lifetime," Nilsson said. "They're one of the most difficult-to-observe animals on this planet."
Most specimens recovered are decaying corpses with their water-filled eyes collapsed like deflated balloons, making them difficult to study.
A recent break came in February 2007 when New Zealand fishermen captured and froze a colossal squid. The eyes, thawed in 2008, offered Nilsson and others an unprecedented look at their anatomy.
With accurate measurements in hand, Nilsson and four other researchers set out to model what the giant squids might see. They learned that while the eyes offer little if any gains to close-up vision, the large pupil and huge retina provide an advantage unmatched by any other eye: a light-collection device big enough to detect faint bioluminescence from 400 feet away.
Plankton in seawater emit such bioluminescent light when they're disturbed, generally by large objects such as sperm whales on a hunting dive. (This is also one way even the stealthiest submarines can betray their locations.)
The whales use ultra-loud clicks to scan for objects during dives. Because giant squid are deaf, evolution seemingly got creative with their vision.
Nilsson said the development certainly came with a cost, as eyes are expensive structures to develop and operate.
"Walking flies spend about 20 percent of their electricity bill on their eyes, so to speak, just running the neurons," he said. "We don't know what the costs are for giant squid, but they could cause some significant drag. They are just these massive things."
While Nilsson and his team waits for the world to capture more specimens, they plan to study other ocean animals' vision.
"We want to use the same model to understand other eyes in the ocean," Nilsson said. "We want to know what they can and can't see to better understand their ecologies."
Top image: The head of a giant squid captured alive but mostly eaten by pilot whales on February 10, 1981. Its intact eye spanned nearly 8 inches across. (Henry Olsen/Dan-Eric Nilsson et al., _Current Biolog_y)
Citation: "A Unique Advantage for Giant Eyes in Giant Squid." Dan-Eric Nilsson, Eric J. Warrant, So¨nke Johnsen, Roger Hanlon, and Nadav Shashar. Current Biology, Vol. 22, Pg. 1–6, April 24, 2012. Published online March 15, 2012. DOI: 10.1016/j.cub.2012.02.031
