The mismatched eyes of the "cockeyed" squid Histioteuthis heteropsis have long puzzled biologists since the discovery of the creature more than a century ago.

Findings of a new study, however, have revealed that having one normal eye and a giant, bulging yellow eye helps the deep-sea dweller survive in its cold and dark habitat.

Two Different Sources Of Light In The Deep Sea

In a new study published in the journal Philosophical Transactions B on Feb. 13, researchers looked at more than 150 undersea videos of the cockeyed squid and found behavioral evidence showing that the lopsided eyes of the creature evolved to spot two different sources of light that are available in the deep sea.

These observations along with visual simulations showed that the squid's large eye is specifically adapted to gaze upwards and see the shadows of fellow sea creatures that would be otherwise difficult to see against the fast fading sunlight while the small eye is adapted to gaze downward to scan the deeper and darker water for flashes of bioluminescence.

Study researcher Kate Thomas, from the Duke University, noted that in the deep sea, the kinds of eyes needed to see bioluminescence are different from the ones needed to see basic ambient light. It appears that the cockeyed squid evolved to have one eye for basic ambient light and another for bioluminescence.

Big Eye Improves Sensitivity To Dim Light

The monochromatic blue light that penetrates this deep part of the ocean is extremely dim and travels straight down from above. Histioteuthis lives 200 to 1000 meters below the surface in a region of the ocean called the mesopelagic or "twilight" zone.

Footages revealed that the squid tends to swim with its tail end pointing upwards but tilted so its large eye is consistently oriented towards the sky. Based on the measurements of the squid's eyes and the levels of light that they would be exposed to, the researchers calculated that the big upward pointing eye of the creature vastly improves their sensitivity to dim sunlight.

Some of the creature's prey which include shrimp and lanternfish have luminescent undersides which is why they are camouflaged from sunlight when seen from below. The yellow pigmentation in the lens of the large eyes of the squid may help the creature distinguish between sunlight and bioluminescence.

There appears to be no need to increase the size of the downward-pointing eye though because this would have little impact on the eye's ability to spot bioluminescent flashes against the dark background. More often, the bioluminescent flashes coming from other sea dwellers which could either be a predator or prey are brighter compared with the ambient sunlight.

While having two gigantic eyes may seem to be the best strategy to survive the deep dark sea, sparse resources can make big eyes difficult to maintain. The cockeyed squid managed to come up with an ingenious solution to this dilemma.

"Eyes are metabolically expensive to grow, maintain, and use, so while larger eyes can improve both sensitivity and resolution, selection probably favours an eye just large enough to perform a necessary visual task but no larger," Thomas and colleagues wrote.