Why This Squid Has One Giant and One Tiny Eye

Histioteuthis heteropsis is commonly known the cockeyed squid for good reason: Its eyes don’t match.

These creatures are born with two eyes of the same size and dark color. But during its juvenile years, one of the cockeyed squid’s eyes transforms, growing until it bulges and turns yellow. The exact reasoning behind this mismatching has long puzzled scientists. But now, reports Sam Wong for New Scientist, researchers think they’ve found an answer.

Kate Thomas, a biology graduate student at Duke University, and her team examined remotely operated vehicle (ROV) footage from Monterey Bay to study the creature in detail. They reviewed 161 videos of cockeyed squids filmed over 26 years. While analyzing the footage, she noticed something odd: The cockeyed squid swims in a sideways position. The large yellow eye is constantly scanning the surface above it while the small black eye studies the water below.

Thomas and her team came to the conclusion that the eyes must be functioning independently, writes Laura Geggel for Live Science. The bulging eye is scanning for shadows of marine life above the creature, silhouetted by the sun’s filtered light. Meanwhile, the smaller normal black eye has its own job: It scans the water below the cockeyed squid for any bioluminescence. Bioluminescent creatures emit light as part of an internal chemical reaction. The researchers published their results in the journal Philosophical Transactions of the Royal Society B.

These two different eyes likely developed as a response to the squid’s natural habitat. The creature dwells up to roughly half a mile below the ocean’s surface—a depth with very little sunlight penetration. As a result, creatures who live in this zone have evolved a range of mechanisms to cope with their natural habitat. In the cockeyed squid’s case, its unusual features help it see two different types of light.

A larger eye wouldn’t be needed to see bioluminescent flashes from deep sea creatures because these are often brighter than the sunlight that makes it to these depths. A larger eye, however, would dramatically increase visual perception, which is crucial for seeing silhouettes in the dim light of the ocean depths. Thomas and her team believe the yellow pigment in the larger eye helps it tell the difference between bioluminescent flashes and sunlight, writes Wong.

The black smaller eye would not be able to distinguish shapes because of the sun’s light. But it wouldn’t need to. “Once it is looking for bioluminescence, it doesn't really need to be particularly big, so it can actually shrivel up a little bit over generations,” Sönke Johnsen, an author on the paper, tells Geggel. “But the eye looking up actually does benefit from getting a bit bigger.”