All the Better to See Sperm Whales With, My Dear.

by Katie Pratt

The colossal squid has the largest eyes on the planet, but it remains a mystery as to why. These soccer ball-sized eyes are almost three times the size of the second largest animal eyes (those of the sperm whale reach 109mm in diameter), and up until now they were thought to have a diameter of 250 to 400mm. However studying them has proved challenging, as these underwater giants are extremely elusive.

Animal eyes are uniquely adapted to the lives of their owners. For example, birds of prey have eyes that can spot small rodents moving as they soar hundreds of feet above them. Grazing animals, however, have excellent peripheral vision and have their eyes positioned on the sides of their heads in order to increase their field of vision and watch out for predators.

But in no other animal than the squid does this adaptation result in a gargantuan eyeball.

A new paper published this month in Current Biology postulates a new theory that might underscore the evolution of this huge organ. Researchers at Lund University, Sweden, and Duke University, North Carolina, began their research by trying to find an accurate measurement of a squid eyeball. They dug up a photograph from 1981 that showed a freshly caught squid with a pupil that measured 90mm in diameter. They also took a direct measurement of the eye of a squid captured in New Zealand, which had a diameter of 270mm. That’s bigger than a human head!

You seen my body anywhere?

The team then used various modeling approaches to test the situations in which this eye might perform well in an effort to find the selection pressure that drove its development. They took into account the fact that these animals live in deep water where there is little or no sunlight, and also the refractive properties of water. They found that while certain properties, such as hunting for small prey, were not aided by a large eye, the ability to spot large objects would be enhanced.

So what large objects would a colossal squid need to spot?

Sperm whales. They like to eat squid, and unlike other whale-bait, giant and colossal squid actually possess enough speed to elude these large predators. But wait…wouldn’t it be too dark to see a sperm whale? Yes, but the squid likely aren’t looking directly at the whale but at the bioluminescence (generated by a number of species of plankton) it disturbs as it approaches.

Sadly this is where the theory falls apart a little, as it would only account for this eye-adaption if colossal squid were usually found inhabiting plankton-rich waters. And even if they were, how would they know the plankton were there without a large animal to disturb them? The authors suggest that perhaps squid are attracted to the plankton for some other reason, such as to find a mate, but that’s unfortunately where the speculation ends.

One thing’s for sure, though; a giant squid would look far more terrifying in grandma’s nightgown than a wolf. Tentacles everywhere.

Katie Pratt is a graduate student in Molecular Biology at Brown University. She has a passion for science communication, and in an attempt to bring hardcore biology and medicine to everyone, she blogs jargon-free at www.katiephd.com. Follow her escapades in the lab and online on Twitter.

Be the first one to mind the gap by filling in the size of the squid eyeball (+/- 10mm) as a comment and get your name in the blog along with a sweet new BenchFly mug!

UPDATE: Congratulations to Veena Thomas – winner of this week’s Mind the Gap!

About the winner: Dr. Veena Thomas is a postdoctoral fellow in Vijay Pande’s lab at Stanford University. Her research interests include computational simulations of protein aggregation diseases, for which she was awarded an NRSA fellowship, and ligand-based drug design for neglected diseases.

About the prize: In addition to fame and glory beyond their wildest dreams, winners receive our new hot-off-the-presses large (15 oz) BenchFly mug to help quench their unending thirst for scientific knowledge… or coffee. Check out where the mug has traveled – will you be the first in your state or country to win one?