Histioteuthis heteropsis or "strawberry squid." Photo: Kate Thomas.

The cockeyed squid (Histioteuthis heteropsis) was first discovered more than a century ago, and its intriguing eye anatomy has been puzzling researchers and fisherman alike ever since.

Histioteuthis heteropsis is born with two small, black eyes, but once it reaches juvenile development, the left eye grows to be twice the size of its counterpart.  It can enlarge so much that it pushes the head out of alignment with the mantle in some species. The larger eye takes on a yellow pigment, and tends to keep its focus upwards, while the smaller eye keeps track of fellow marine life below.

Duke University researchers, led by biologist Kate Thomas, analyzed more than 150 underwater videos of the squid in Monterey Bay, recorded by ROVs over the course of 30 years by the Monterey Bay Aquarium Research Institute to determine the purpose of the two different eyes.

They noticed that the squid swims in a unique “upside-down” position, with its head down, and tail facing up. The large eye likely keeps its gaze upwards to search for shadows, while the small eye captures flashes of bioluminescence in the deep sea. The team also found that 65 percent of adult H. heteropsis had yellow pigmentation in the lens of the larger left eye, which may be used to break the counterillumination camouflage of their prey.

H. heteropsis live in the ocean’s “twilight zone” – or mesopelagic region, which is from 600 feet to 3,200 below the water’s surface. The lack of sunlight in this area has allowed other unique sea creatures to develop bioluminescence. The mesopelagic region also contains the highest diversity of visual adaptations in the sea.

“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 Sönke Johnsen, professor of biology at Duke University and senior author on the study. “In the case of the Histioteuthis, this cockeyed squid, they chose one eye for each.”

The study has been published online in the journal Philosophical Transactions B.