The Histioteuthis heteropsis, also known as the cockeyed squid, spends its days drifting through the ocean, eyes on alert for signs of predators or prey. Squid are intriguing creatures in general, but it’s the eyes of Histioteuthis heteropsis that draw you in. Or, rather, the contrast between the eyes—a large, bulging, yellowish one on one side and the significantly smaller, more traditional looking eye on the other side.
The cockeyed squid Histioteuthis heteropsis, also known as the ‘strawberry’ squid for its coloring, 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.
Image Credit: Kate Thomas.
From the groundbreaking flat lens we recently featured on Physics Buzz to the innovative James Webb Telescope, scientists are creating some pretty amazing tools for studying light. But it turns out that animals— especially those that have adapted to life in extreme conditions—have some pretty unique optical tools too. With all of the exciting technologies on the horizon it can be tempting to focus solely on the future. By doing so, however, we risk missing out on creative solutions to challenging problems that nature has already found—like using two different kinds of eyes to navigate a complicated light environment.
In a recent study, researchers Kate Thomas and Sönke Johnsen, from Duke University, and Bruce Robison, from Monterey Bay Aquarium Research Institute, studied the behavior of Histioteuthis heteropsis and Stigmatoteuthis dofleini, two species of cockeyed squids, in hopes of finding out more about their mismatched eyes. Their work was published this week in Philosophical Transactions B, a life sciences journal published by The Royal Society. (There’s also a fun related video by Duke University.)
Cockeyed squid live in the midrange depths of the ocean. Called the mesopelagic or “twilight” zone, this region ranges from 200-1,000 meters below the surface. Some sunlight reaches this area, but it is diffuse and not enough to facilitate photosynthesis, or really to see anything beyond shadows and outlines. The water is dim and gets darker quickly with depth.
Another kind of light exists here, too: bioluminescence. Many sea creatures emit flashes of bioluminescence from their bodies to attract mates, scare off predators, camouflage their bodies, or entice prey to come a little closer. Animals have developed interesting adaptations in response to these light conditions. With its eerily asymmetrical eyes, the cockeyed squid is no exception.
At first, scientists suspected that the different eyes were optimized to the different levels of light in shallow and deep water. This didn’t match well with how squid behave though, and another hypothesis has since emerged. Scientists now think that the larger eye may be adapted for looking up, searching for silhouettes of creatures against the sunlight, and the smaller eye is adapted for looking down, scanning for signs of bioluminescence.
To see whether squid behavior supports this hypothesis, Thomas, Johnsen, and Robison went to the source—footage of cockeyed squid taken by remotely operated vehicles in Monterey Submarine Canyon. The videos included 161 sightings recorded over more than 25 years. For each sighting, they studied the orientation of each eye, the orientation of the squid’s body, and the pigment of its eyes.
The cockeyed squid Stigmatoteuthis dofleini has one normal eye and one giant bulging eye.
Image Credit: Sönke Johnsen.
Their work supports this hypothesis, and shows that squid prefer to be in the head down position with the large eye pointed upward and the small one pointed slightly downward. By combining their observations with computer models that consider the physics of the light-eye interactions under these conditions, the researchers show that the hypothesis makes sense from an optical point of view too.
The large eye can have a diameter up to twice as big as the small one. Why? More area means that you can gather more light—just like with a telescope. The large eye takes on a kind of tubular shape as it develops, reducing its field of view but increasing its sensitivity, a reasonable trade-off in this situation. In the majority of the adults observed in this study, the large eye had yellow pigment while none of the small eyes did. According to the researchers, this is common among deep-sea fish that look upward, probably because it acts like a kind of filter that blocks out background light and makes it easier to detect animals that try to camouflage their silhouettes with bioluminescence.
The smaller, downward looking eye is well adapted for detecting bioluminescence. This eye is shaped like a hemisphere, with a larger field of view but less sensitivity than the big eye. In this case, the larger field of view is preferable because it enables the eye to scan a large area quickly and find the source of a flash. The dark backdrop of the ocean below provides excellent visual contrast for a bioluminescent signal, reducing the need for sensitivity.
Generally speaking, a bigger eye is always better because you can gather more light. However, computer models show that although increasing the size of the eye significantly improves a squid’s ability to see dark objects when looking upward, the payoff is much smaller for a downward looking eye. In terms of biological resources, eyes are expensive. If you’re an animal already fighting for food, you want eyes just big enough to do the job. That, it seems, is just how the cockeyed squid evolved.