Why giant squid have such big eyes

Giant squids have eyes the size of basketballs. Why? In the ocean, it’s all about who wants to have you for dinner.

Giant squids have eyes as big as basketballs – measuring two or three times that of any other animal. For example, squid and swordfish are similar in size but a squid’s eyes are three times the diameter and 27 times the volume of swordfish eyes. Why? A team of biologists offered an explanation in a study published in the March 15, 2012 issue of the journal Current Biology. They say it’s not about what the squid eat. It’s more about who wants to eat the squid. In other words, according to these scientists, giant squid are most likely using their huge eyes to spot and escape their predators, sperm whales.

Artist’s concept of giant squid and sperm whale locked in mortal combat. Image via unmuseum.org

In the ocean, it’s all about eating and being eaten. These scientists created mathematical models of how and why squids use their big eyes in the pitch darkness of the ocean depths. The team now believes that the design and size of a squid’s eye lets the animal see approaching sperm whales which, as the whales zoom toward the squid with dinner on their minds, disturb some of the ocean’s bioluminescent, or light-emitting, organisms.

The team first measured squid eyes using photos and captured animals. Then they looked at squids’ ocean environments, using data on water clarity and the amount of light at depths where squids prefer to live – typically 300 to 1,000 meters below the sea surface.

Then they mathematically modeled how squids’ eye might work.

Clearly, bigger eyes let squid collect more light. The extra light lets squid detect small contrast differences in the ocean depths, the team says. This ability is critical for squid, who are big themselves – the size of five adult men – and whose predators are also big. The team explained said the large eyes of squid let them see long distances in the ocean depths.

The scientists said a sperm whale’s sonar would probably still detect the squid before the squid sees the bioluminescence. But any advance warning might help the squid escape the whale’s mighty jaws.

This video from AP in 2008 shows a giant squid caught in 2008 and talks about the size of their eyes. These creatures are amazing.

Bottom line: Giant squids have giant eyes not to seek prey smaller than themselves, but to escape their mortal enemies, sperm whales. That’s according to a team of biologists who mathematically modeled squids’ eyes and their uses. Their study appears in the March 15, 2012 issue of Current Biology.

Deborah Byrd