Squids evolved giant eyes to watch out for sperm whales

The colossal and giant squids that lurk in the ocean depths are truly remarkable creatures. If there's one feature that's really striking, it's their gigantic eyes, which are any times bigger than any other known marine organism's eyes.

Considering the huge discontinuity between the size of these squids' eyes and those of all other oceangoing animals, the natural question is why such eyes evolved in the first place. Now researchers from Sweden's Lund University think they have the answer — the eyes are a necessary early warning system for these squids' only known predator, the sperm whale. Research leader Dan Nilsson explained to BBC News why he and his team set out to solve this mystery:

"We were puzzled initially, because there were no other eyes in the same size range - you can find everything up to the size of an orange, which are in large swordfish. So you find every small size, then there's a huge gap, then there are these two species where the eye is three times as big - even though squid are not the largest animals."

Indeed, for creatures like these huge squid that typically live a mile beneath the ocean surface, big eyes really serve very little purpose. Most creatures down there have developed bioluminescence, which means you don't need particularly large eyes to spot prey.

The one exception is if a really large object is moving towards you — then, such huge eyes could let you spot the approaching beast up to 400 feet away. If a sperm whale is near, these eyes give squid — which, unlike the whale, can't use sonar to detect an approaching adversary — time to take evasive action. The researchers say that this likely also explains the development of similarly big eyes in the prehistoric beast ichthyosaurus, which similarly relied on its huge eyes to spot even larger approaching predators.

Current Biology via BBC News. Mural from Houston Museum of Natural Science; photo by etee on Flickr.