3D movies are a love-hate experience. You absolutely adore the feeling of something plummeting out of your screen so close you think you can nearly touch it, or the whole thing completely freaks you out. Either way—the only way it works is being able to use both of our eyes at the same time to capture the image and perceive its illusory depth.
But as a new study shows cuttlefish experience this phenomenon as well. When equipped with little 3D glasses and placed in front of a screen with a 3D movie of a shrimp passing by, they actually tried to grab it with their tentacles.
These new findings are laid out in a study in Scientific Advances, and it demonstrates more than just a cuttlefish’s ability to “hunt” virtual prey—it show’s that their vision systems are capable of stereopsis or “binocular vision”.
Binocular vision is the way that the brain uses images from both of our eyes to create a perception of depth. Humans have this ability—it’s how we know when something is about to smack us in the face, or if we need to reach out to grab something. For a while, it was thought that only primates and people could manage this because of our front-facing eyes. But it turns out that quite a few other creatures judge distance this way as well.
One other invertebrate, the praying mantis, evolved this way, as proven in their own tiny 3D glasses study from about a year ago. A cuttlefish is another complex invertebrate, so author Trevor Wardill, an ecology professor at the University of Minnesota had the idea of using them to further figure out stereopsis.
“To be fair when we proposed the project … they thought it was a little bit crazy,” Wardill says of first proposing the project to his partners at the Marine Biological Laboratory in Massachusetts. “They did not really expect it to work, but we were pretty convinced that we should try.”
The initial act of getting cuttlefish to willingly wear the glasses without trying to take them off and actually watch the screen was tough enough, Wardill says. This process required gluing velcro to the top of its mucus-covered body, placing the glasses on their heads, and keeping the creatures happy and distracted enough to not mess with them (or ink all over the experiment).
But when the animals finally focused on the screen, creating an illusion of depth that can only be seen when using binocular vision, they accurately “hunted” the shrimp on screen.
“Putting little glasses on a slimy, tentacled invertebrate may sound both adorable and funny, but it’s actually an amazing accomplishment,” says Kate Thomas, a visual ecologist and postdoctoral researcher at the Natural History Museum in London who was not involved in the study.
This illusion of depth is created by using two different colored images that are seen through the two different lenses, which the brain then calculates the distance between. Even though cuttlefish are colorblind, the colored filters in the glasses send the accurate color from the monitor to the right eye. They only see the image in each eye as a variety of greyscale intensities, Wardill adds.
Strangely enough, most animals with stereopsis have “yoked” eyes—meaning they look at the same thing at the same time. Cuttlefish’s eyes move separately, except for the moment they notice their prey.
The cuttlefishes weren’t great at “yoking” their eyes, Wardill says, but they still were able to hunt. It’ll take more investigation on what the cuttlefish’s eyes are actually doing to see how they use cues and spacial information to capture their meals.
Their lack of yoked eyes is not the only thing that’s different about a cuttlefish’s stereopsis. The study found that critters can also detect the distance from an “anticorrelated” stimulus, where the image seen in each eye was completely the opposite of the other (think black on white in one eye, white on black on the other).
Humans need the brightness in each image to match up in order to tell distance, so the contrast in this kind of stimulus makes it tough for us to perceive. But the teensy brain of a praying mantis can see depth in these stimuli easily.
This study demonstrates convergent evolution, Thomas says, as these creatures are so far off the evolutionary pathway from humans, but are in some ways similar to us. “I think it’s fascinating that an animal more closely related to a clam than to us has not only evolved eyes similar to ours, but also processes images from those eyes in a similar way to produce depth perception.”
Though they’re interesting results, they shouldn’t be super surprising—and they actually make a lot of sense in the context of the cuttlefish’s daily needs. Binocular vision makes predation easier, Wardill says, since you can see your prey in front of you without moving around or making a bunch of attempts to capture a meal. After all, a cuttlefish probably wouldn’t be catching very many shrimp if it was continually scaring them off.
If anything, Wardill says, it was more surprising that the animals kept their glasses on.