Why Do Giant Squid Have Huge Eyes?

Scientists delve into details of the vision of giant squid.

The world’s biggest squid come in two types: giant and colossal. They can grow to the combined weight of five adult men, which corresponds to that of a large swordfish. But in comparison, swordfish eyes are about the size of a softball, measuring just 3 inches in diameter. Those of the squid, however, correspond to the size of basketballs.  “It doesn’t make sense that a giant squid and swordfish are similar in overall size but the squid’s eyes are proportionally much larger – three times the diameter and 27 times the volume,” says Duke biologist Sonke Johnsen. “The question is why? Why do giant squid need such large eyes?” 

Finding an Explanation
Johnsen collaborated with a group of biologists to model, both physically and biologically, how and why a squid uses such big eyes. As a starting point, they first measured giant and colossal squid eyes using photos and captured animals for this purpose. They also found data on the water clarity and amount of light at the ocean depths where the squid live, typically between 1,000 to 3,250 feet. Using this information, the scientists began to devise a mathematical model as to how the creatures’ eyes would work and what they could see.  The team discovered that the large eyes of the squid collect more light, compared with animals of similar size but with smaller eyes. The extra light intake improves the squid’s ability to detect small contrast differences under the dim conditions of the deep ocean. Johnsen said this capability doesn’t matter much to the majority of deep-sea animals, which are looking at small objects that become too small to see before they fade away.  But the boost in being able to sense contrast, which large eyes provide, is critical for detecting the low light differences of large, distant objects, with the most important one, according to Johnsen, being the bioluminescence in other animals stimulated by large animals such as approaching sperm whales. “The squid are most likely using their huge eyes to spot and escape their predators – sperm whales,” he explained.  

How Squid Use Their Eyes
The team realized that sperm whales dive and swim continuously while emitting sonar to track the squid. The cephalopods are deaf to the sonar, but the whale’s wake triggers small organisms like plankton to produce light. Based on the design of the squid’s eye, the animal could see this light over “freakishly long distances” corresponding to about 400 feet, which is equivalent to the length of an American football field, Johnson explained.  The contrast is low because water absorbs and scatters the light as it travels from the glowing plankton to the squid’s eye. Bigger eyes mean seeing more of the faint light and predicting a predator’s approach. But Johnsen said a sperm whale’s sonar would probably still detect the squid before it sees the light. As a result, the squid’s basketball-sized eye, and the body to carry it, isn’t necessarily for moving out of the whale’s detection range, but for planning a well-timed escape.  “It’s the predation by large, toothed whales that has driven the evolution of gigantism in the eyes of these squids,” suggests Johnsen. The team plans to publish its complete theory on underwater vision later this year.
Reference: Nilsson et al. A Unique Advantage for Giant Eyes in Giant Squid. Current Biology, March 15, 2012 DOI: 10.1016/j.cub.2012.02.031

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