Visual Perception

This Squid May Look Cockeyed, But It's Actually A Masterpiece Of Evolution

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The deep sea is full of really weird looking creatures. You might think Histioteuthis—also known as cockeyed squids—look bizarre, but they fit right in with their deep-sea brethren. (Seriously, have you ever seen an anglerfish? We rest our case.) And anyway, there's a reason for their strange appearance. In 2017, researchers from Duke University published evidence for why these creatures have one big eye and one small eye: each one is suited for its own purpose.

Related: The Male Anglerfish Fuses To Flesh During Mating

The cockeyed squid Stigmatoteuthis dofleini has one normal eye and one giant bulging eye. New behavioral evidence collected by biologists at Duke and the Monterey Bay Aquarium Research Institute (MBARI) show that cockeyed squids' lopsided eyes evolved to spot two different sources of light available in the ocean's 'twilight zone'.

Strawberry Squids Forever

Also known by the considerably sweeter moniker "strawberry squid" for its pink hue and seed-shaped photophores—little spots on its body that produce light—the cockeyed squid lives 200 to 1,000 meters below the ocean's surface in a region known as the mesopelagic or "twilight" zone. It's deep enough so that whatever sunlight does penetrate from the surface is dimmer than the bioluminescent flashes of the creatures that live there.

Related: Dive Down Into The Ocean's Unexplored Twilight Zone

To figure out what was going on, biologist Kate Thomas combed through 30 years of footage collected by the Monterey Bay Aquarium Research Institute so she could watch how the askew cephalopods behave. She noticed that they swim in an unusual position, with their head pointed down and tentacles pointed up in a nearly vertical orientation. Their big eye is constantly looking up toward the surface, while their small eye is constantly looking down toward the ocean floor. That was a clue that ultimately led Thomas to her conclusion.

The cockeyed squid Histioteuthis heteropsis has evolved an ingenious solution to a deep-sea ocular conundrum. With one large eye adapted for seeing the shadows of fellow sea creatures, and a second small eye adapted for spotting bioluminescent flashes, it makes optimal use of meager illumination.

All The Better To See You With, My Dear

Thomas suspected that the big eye facing upward was adapted for spotting silhouettes against the dim sunlight, while the small eye facing downward was better suited to spotting bioluminescence. One piece of evidence that supported her theory was the fact that the larger eye in many of the squid they examined had a yellow pigment to its lens, which could help break through the kind of counterillumination camouflage its prey might use to hide their silhouette from above.

Related: Every Human Is Bioluminescent

For more evidence, Thomas used visual modeling to see how changing the sizes of the squids' eyes might change the way they see the world. If making the smaller eye any bigger helped the squid see bioluminescent prey more clearly, it might show her hunch was wrong. Sure enough, her simulations showed that increasing the size of the small eye had little impact on its ability to spot bioluminescence, but increasing the size of the big eye greatly increased its sensitivity to dim sunlight. "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," Sönke Johnsen, professor of biology at Duke University and senior author on the study, said in a press release. "In the case of the Histioteuthis, this cockeyed squid, they chose one eye for each."

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The cockeyed squid Histioteuthis heteropsis, also known as the 'strawberry' squid for its pink color and smattering of seed-like photophores, has evolved a mismatched set of eyes: one large eye for seeing the shadows of fellow sea creatures above, and a second small eye for spotting bioluminescent flashes below.

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