Disproportionate Eyes Help Giant Squids Avoid Predators

Researchers from Swedish and American universities say that they have solved the mystery as to why giant and colossal squid have such enormous eyes, and perhaps unsurprisingly, the oversized ocular orbits are essentially a defense mechanism.

According to Rob Waugh of the Daily Mail, the giant squid can be upwards of 27-feet long and can weigh half a ton, or “as much as five adult men.” Even so, their eyes, which are roughly the size of a basketball, are still “far, far too big for their bodies.” Experts from Lund and Duke Universities set out to discovery exactly why that was the case.

“It doesn’t make sense a giant squid and swordfish are similar in size but the squid’s eyes are proportionally much larger, three times the diameter and 27 times the volume,” Duke biologist Sönke Johnsen, one of the researchers involved on the study, said in a press release. “The question is why. Why do giant squid need such large eyes?”

In order to probe that question, Johnsen, lead scientist Dan-Eric Nilsson of Lund University, and colleagues obtained photographs of the eye of a giant squid and examined a colossal squid corpse from New Zealand, LiveScience Senior Writer Stephanie Pappas wrote on Thursday.

They studied the specimen and the pictures first in order to confirm the purported eyeball size of both types of squid, confirming that they could reach diameters of more than 10 inches. With that knowledge, Pappas said that the team then created a mathematical model showing how well the cephalopods can see in the ocean depths.

For the most part, they found that large eyes would not be beneficial at depths of 1 kilometer (0.6 miles), as objects in motion are detected more frequently as a result of bioluminescence resulting from the provocation of minute aquatic creatures than by sight itself, BBC News Environmental Correspondent Richard Black said.

“For seeing in dim light, a large eye is better than a small eye, simply because it picks up more light. But for animals that live in the sea or in lakes, the optical properties of water will severely restrict how far away things can be seen,” Nilsson said in a March 15 statement.

“Through complex computations we have found that for animals living in water, it does not pay to make eyes much bigger than an orange. Making eyes larger than that will only marginally improve vision, but eyes are expensive to build and maintain,” he added.

There is one exception, though, according to the BBC: that of very large moving objects, such as the sperm whale, one of the squid’s primary predators. In these cases, larger eyes help creatures detect sources of bioluminescence more easily, essentially helping the squid discover the presence of the whales at distances of approximately 120 meters (394 feet) and giving them a greater chance to “take evasive action and avoid being eaten,” Black added.

“They’re most likely using their huge eyes to spot and escape their predators, sperm whales,” Johnsen, who along with her colleagues submitted their research to the journal Current Biology, said. “It’s the predation by large, toothed whales that has driven the evolution of gigantism in the eyes of these squid.”

Image 2: Two men inspect a nearly intact 9.2 meter giant squid. Credit: Photo: NTNU Museum of Natural History and Archeaology, via Wikimedia Commons

Image 3: It´s no surprise that giant and colossal squid are big, but it´s their eyes that are the real standouts when it comes to size, with diameters measuring two or three times that of any other animal. Now, researchers reporting online on March 15 in Current Biology, a Cell Press publication, have used complex computations to explain those massive peepers. Giant squids´ 10-inch eyes allow them to see very large and hungry sperm whales from a distance in the pitch darkness of their deep-sea home. According to the researchers´ calculations, animals living underwater would have no use for such large eyes if the goal were to see an average object, such as prey smaller than themselves. That´s why even the eyes of large whales aren´t much more than 3.5 inches across. Credit: Nilsson et al.: “A unique advantage for giant eyes in giant squid.” Current Biology

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