Deep Sea Eyes Evolved to Protect from Predators

A recent study of the eyes of giant and colossal squid reveals they would likely have only given an advantage for detecting the presence of large predators, such as the sperm whale.

Giant squid have possibly the largest eyes of any creature that ever lived. At a diameter of 10.5 inches, with a pupil size of 3.5 inches and hard lenses as big as human eyes, these eyes are disproportionally sized compared to almost any other creature, which raises the question, “Why?” Any time a creature develops a highly unusual structure such as this type of eye, the typical assumption is that some selection pressure has driven its development. Now researchers believe they have found the answer to what selection pressure drove the development of squids’ huge eyes.

It has long been assumed that the huge eyes of giant squids were in some way an adaptation to living in very dark water. Larger eyes are able to take in more light and so presumably would allow the monstrous squid, which measure up to 49 feet (15 m) in length, to see better in dim light conditions. However, researchers found that for most situations, an eye size greater than 4 inches, the size of swordfish eyes, provided no benefit. See the chart below for a comparison of the relative size of creatures’ eyes.

However, there was one situation in which the very large eyes of the squid would be very helpful: the detection of approaching sperm whales. As sperm whales cut through the deep water, they disturb numerous smaller organisms, which give off bioluminescence. The massive eyes of the giant squid species would allow it to detect and interpret the pattern of disturbed organisms to see sperm whales approaching from a distance of nearly 400 feet, which would allow the squid to potentially avoid approaching whales.

Looking at the chart above, we notice that another creature had eyes about the same size as the giant squid, the Temnodontosaurus platydon. Based on the models provided by researchers, it seems likely that the function of these eyes was the same: to detect the approach of a very large predator.

But what ate a temnodontosaurus? Shown above in comparison with a human, temnodontosaurus was one of the largest known creatures in Jurassic seas.

One possible speculation is the presence of a very large pliosaur. Some estimate that pliosaurs, similar to liopleurodon (image below), may have been up to 50 feet long, about half again the length of temnodontosaurus, which may very well have made it a viable predator. Another possibility, though, is the existence of a kraken, a gigantic cephalopod predator, which would not be well preserved in the fossil record.

What is remarkable about these eyes is their extraordinary ability to function under demanding conditions. If your eyes are not properly functioning, please contact a local ophthalmologist to talk about options for improving your vision.