The ‘cockeyed’ squid’s mismatched eyes help it see different contrasts of light in the deep ocean, a new study in Philosophical Transactions B reports.
The species (Histioteuthis heteropsis) lives down in the ocean’s mesopelagic or “twilight” zone some 200 to 1000 meters below the surface. The odd creature gets its name because, unlike most animals, it has two different sized eyes. One is small, while the other is large and yellow. While this contrast has puzzled scientists for years, the new study sheds light on the difference.
To study the creature, researchers from Duke University observed over 150 videos of the squids moving through the water. This showed that the slow-moving creatures prefer to drift through the sea with their heads down and tails in the air. Such an orientation allows them to have their big eye pointed up at their surface and their small eye looking to the depths, Phys.org reports.
Sunlight comes only from directly above in the murky water. As a result, the smaller eye would have a rough time pointing out silhouettes against the light. By increasing the size of the eye staring at the surface, it raises the squid’s sensitivity to dim sunlight. However, increasing the eye looking towards the dark ocean would do little to improve its ability to find the bioluminescent creatures that call the area home.
This shows that the large eye is specifically adapted to look up into the fading sunlight, while the small eye is better suited to scan the darker water below.
“The deep sea is an amazing natural laboratory for eye design because the kinds of eyes you need to see bioluminescence are different from the kinds of eyes you need to see the basic ambient light,” said study co-author Sönke Johnsen, a professor of biology at Duke University, in a statement. “In the case of the Histioteuthis, this cockeyed squid, they chose one eye for each.”
This is the first study to explain how the squids’ lopsided eyes evolved. Though it may seem better to just have two large eyes, it is actually not as efficient from an evolutionary standpoint.
“Eyes are really expensive to make and maintain,” added lead author Kate Thomas, a biologist at Duke University. “You want eyes just big enough to do what you need to do, but you don’t want to have any bigger eyes because then you are just wasting resources.”