Why Does the Strawberry Squid Have Two Completely Different Eyes?

The strawberry squid (Histioteuthis heteropsis) has two completely different eyes because it lives in an environment where the light above and the darkness below demand entirely separate visual systems — a mystery formally explained in 2017 by Duke University researchers using decades of underwater video from the Monterey Bay Aquarium Research Institute (MBARI).

What Is the Strawberry Squid?

The strawberry squid is a deep-sea cephalopod found in the ocean’s mesopelagic zone, commonly called the twilight zone, at depths between roughly 600 and 3,300 feet (200–1,000 meters). Its reddish, bumpy skin gives it the “strawberry” name, and while that color appears vivid at the surface, it’s effectively a deep-sea camouflage: red is the first wavelength seawater absorbs, so at depth, the squid appears completely black to any predator or prey nearby.

The Two Eyes: One Large, One Small

The most striking feature of this animal is the radical asymmetry of its eyes — and every detail is functional.

The large yellow eye points straight upward. Its job is to scan the faint, diffuse glow filtering down from the surface above. This dim silhouette-light is just enough to reveal the outlines of prey swimming between the squid and the sky. The yellow pigment in the lens acts as a filter, cutting out competing blue wavelengths and sharpening the contrast of those silhouettes against the murky glow.

The small blue eye points straight downward, into pitch-black water. Here, there is no ambient light to scan for silhouettes — but there is something else entirely: bioluminescence. Living things in the deep ocean frequently produce their own flashes of cold light to communicate, lure prey, or startle predators. A flash of bioluminescent light against absolute darkness is easy to detect, and it requires far less optical machinery to catch than a faint silhouette. A small, sensitive eye is perfectly adequate — and energetically cheaper to grow and maintain.

How Did Scientists Figure This Out?

The 2017 Duke University study, led by researchers analyzing MBARI’s extensive archive of remotely operated vehicle (ROV) footage, confirmed what had long been suspected: that the asymmetry is not a developmental accident or a random mutation that stuck around. It is a precisely tuned, evolutionarily stable solution. By reviewing thousands of hours of deep-sea video, researchers could observe the squid’s natural orientation — consistently swimming with the large eye up and the small eye down — and correlate that behavior with the light physics of the twilight zone.

Why This Is Remarkable

Most bilateral animals — including humans — have two eyes that are functionally identical and work together to create depth perception. The strawberry squid abandoned that plan entirely. Instead, it runs two independent visual systems in a single body, each optimized for a radically different task. This is convergent evolution operating within a single organism: one animal, solving two separate problems, simultaneously.

The Twilight Zone Light Environment

Understanding why the eyes differ requires understanding the light environment of the mesopelagic zone. Sunlight penetrates seawater unevenly. By 600 feet, most visible wavelengths have been absorbed, leaving only a faint blue-green glow. By 3,300 feet, even that is gone. Any animal living across this entire depth range must cope with a gradient — dim directional light from above, total darkness below — and the strawberry squid’s eyes are a precise physical response to that gradient.

A Blueprint for Extreme Adaptation

The strawberry squid is one of the clearest examples in marine biology of how evolution can produce highly specific, non-intuitive solutions to complex environmental challenges. It doesn’t hedge its bets with two general-purpose eyes. It commits fully — one eye for each world it inhabits. That kind of specialization, confirmed through both optical analysis and behavioral observation, makes it one of the most studied animals to come out of MBARI’s deep-sea research program.