Some cuttlefish absolutely refuse to wear 3-D glasses.
These relatives of squid and octopuses have blimplike bodies that end in a ring of eight arms topped by two prominent eyes. It’s not hard to mount a pair of specs in front of those eyes, but a cuttlefish’s arms are so dexterous that, if it’s displeased with its new accoutrements, it can just yank them off. “And indeed, that happened a lot,” says Trevor Wardill from the University of Minnesota, who spent the better part of a recent summer trying to accessorize the animals. “But about 20 to 30 percent didn’t seem to be bothered. Everyone was very surprised.”
Together with his colleagues Paloma Gonzalez-Bellido and Rachael Feord, Wardill used the glasses to show different images to each of a cuttlefish’s eyes. By doing that, they proved that these animals have stereopsis—that is, their brains can work out how far away objects are by comparing the slightly divergent images perceived by each of their eyes. It is an ability that humans and a few other animals share. But, as is the norm with cuttlefish, they manage the task in an odd and surprising way.
Stereopsis comes so naturally to us that we take it for granted. It’s actually a difficult computation that doesn’t happen automatically for every creature with a pair of forward-facing eyes. “It’s thought that animals need a fancy brain to do that calculation,” says Wardill. Indeed, after scientists rigorously confirmed that humans have stereopsis in 1838, it took another 132 years to do the same for another animal. Macaque monkeys came first, followed by cats, horses, sheep, owls, falcons, and toads.
Wardill suspected that cuttlefish also belong to this exclusive club. Their large eyes have the same resolution as a cat’s, and their brains are sophisticated. They also hunt by shooting out two long tentacles to grab their prey—a technique that demands accurate depth perception. They surely have stereopsis. Wardill and his colleagues just needed some way to test it.
As luck would have it, Jenny Read and Vivek Nityananda from Newcastle University had devised the perfect method. A few years ago, the duo glued 3-D glasses onto praying mantises, and watched as they struck at patterns of moving dots that resembled prey insects. 3-D glasses work by forcing each eye to see a different image, slightly dislocated from the other, which creates a perception of depth—for animals with brains that can perform that particular type of calculation. By manipulating the dots, the researchers were able to control how far or near the insects should appear, if mantises had that ability. Their experiment confirmed that mantises do also have stereopsis, despite having small brains with 100,000-fold fewer neurons than ours.
With Nityananda’s help, Wardill’s team did the same experiment with cuttlefish. They glued a little Velcro patch on the animals’ head, and used that to secure the glasses. (The patch and glue naturally fell off after a few days.) They then trained the animals to strike at screens showing 3-D images of shrimp. By adjusting the overlap between those images, the team could make it look like a shrimp was closer to the cuttlefish than it actually was. The very first time they tried this, “the cuttlefish moved away from the screen and missed it entirely,” says Wardill. “I was jumping up and down.”
The team situated the illusory shrimp at different distances, and every time, the cuttlefish positioned themselves accordingly. If the animals only saw the shrimp through one of their eyes, they took longer to strike and did so from closer range, as if they were less confident about their assessments of distance. They clearly have stereopsis. But their version differs from ours—and from mantises’.
Humans use brightness as a cue to align and compare the two images that our eyes are seeing. If one of those images is a negative of the other—white dots on black, say, instead of black dots on white—our stereopsis completely falls apart. But the small-brained mantises have no problem with such reversals, because they have a unique form of stereopsis that’s based on movement instead of brightness. They can even gauge distance correctly when the images hitting their eyes are not only negatives of each other, but also shifted in space. (“Having an insect outperform our undergraduates on it was quite fun,” Read told me two years ago.)
Cuttlefish are somewhere in between. Unlike humans, they can deal with the negative images. But unlike mantises, they can’t cope if those images are also spatially shifted. This situation fits with their lifestyle. Cuttlefish often hunt transparent shrimp in turbid water and cluttered environments, so it makes sense that their aptitude for depth perception surpasses ours. But if they miss their prey, they can often give chase and try again. Mantises, however, have to snag insects out of mid-air with their grasping arms. If they miss, they don’t get a second chance, so they’ve evolved an even more advanced version of stereopsis. “This fits a pattern of multiple independent evolutionary routes to stereopsis,” says Nityananda. “It’s a complicated picture, one which we’re only beginning to piece together.”
The study “is no small achievement, as cuttlefish are not the most cooperative animals to work with,” says Tessa Montague of Columbia University, who also studies these creatures. “Not only did the [team] provide compelling evidence that cuttlefish employ stereopsis, but they demonstrated that cuttlefish can be trained to wear equipment and respond to virtual stimuli.” This should open up a path to other clever experiments that will explore how they and other cephalopods make sense of the world.
The cuttlefish have certainly given Wardill’s team more mysteries to solve. The animals can move each of their eyes independently, and don’t seem to align them even when striking at prey. During accurate attacks, the angles of their eyes can differ by up to 10 degrees. “For us, that would be catastrophic,” Wardill says. Our eyes move together, and are almost always focused on the same objects. An angular difference of just half a degree would throw off our stereopsis. With a 10-degree difference, “we wouldn’t be able to judge depth at all.”
How do the cuttlefish cope? “We don’t know,” Wardill says. “That’s the next thing we want to work on.”