Giant squids' football size eyes are for playing defence

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At 25.4cm/10" Giant and Colossal squids have eyes bigger than an official Size 5 football, but the question has always been why?

Now, researchers reporting online in Current Biology, have used complex computations to arrive at a theory they believe to be the answer.

"They're most likely using their huge eyes to spot and escape their predators, sperm whales," says Duke biologist Sönke Johnsen.

The big squids come in two sizes - Giant and Colossal - and are capable of growing to the weight of five men combined.  Even though this puts the squid in large swordfish territory, swordfish eyes generally have a diameter of a mere 7.6cm/3" in comparison.

"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," Johnsen said. "The question is why. Why do giant squid need such large eyes?"

According to calculations, large eyes in underwater environments become of no use for seeing the typically smaller prey – one of the reasons why even the large whales have eyes a little over 8.9cm/3.5".

"For seeing in dim light, a large eye is better than a small eye, simply because it picks up more light" said Dan-Eric Nilsson of Lund University.

"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 - 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."

So, why do giant squid bother with huge eyes?  The eyes allow for better contrast in the depths, which in turn allows the squid to detect the faint bioluminescence given off by plankton when disturbed by a charging whale.  Detecting this bioluminescence might just be enough to allow a giant squid to escape predation by a Sperm whale, something that might be unique to giant squid.

"It's the predation by large, toothed whales that has driven the evolution of gigantism in the eyes of these squid," Johnsen said.

For some researchers, the paper may be considered only speculative. Johnsen says the research is "really just a cool example" the team used to develop a more detailed model of how eyes in general work in the deep sea, at depths inaccessible to humans. The team plans to publish its complete theory on underwater vision later this year.

For more information, see the paper: "A Unique Advantage for Giant Eyes in Giant Squid." Nilsson et al. Current Biology, March 15, 2012. doi:10.1016/j.cub.2012.02.031

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