The cuttlefish hovers in the aquarium, its fins rippling and large, limpid eyes glistening. When a scientist drops a shrimp in, this cousin of the squid and octopus pauses, aims and shoots its tentacles around the prize.
There’s just one unusual detail: The diminutive cephalopod is wearing snazzy 3-D glasses.
Putting 3-D glasses on a cuttlefish is not the simplest task ever performed in the service of science.
“Some individuals will not wear them no matter how much I try,” said Trevor Wardill, a sensory neuroscientist at the University of Minnesota, who with other colleagues managed to gently lift the cephalopods from an aquarium, dab them between the eyes with a bit of glue and some Velcro and fit the creatures with blue-and-red specs.
The whimsical eyewear was part of an attempt to tell whether cuttlefish see in 3-D, using the distance between their two eyes to generate depth perception like humans do. It was inspired by research in which praying mantises in 3-D glasses helped answer a similar question. The team’s results, published Wednesday in the journal Science Advances, suggest that, contrary to what scientists believed in the past, cuttlefish really can see in three dimensions.
Octopuses and squid, despite being savvy hunters, don’t seem to have 3-D vision like ours. Previous work, more than 50 years ago, had found that one-eyed cuttlefish could still catch prey, suggesting they might be similar. But cuttlefish eyes often focus in concert when they’re hunting, and there is significant overlap in what each eye sees, a promising combination for generating 3-D vision.
Dr. Wardill, Rachael Feord, a graduate student at the University of Cambridge, and the team decided to give the glasses a try during visits to the Marine Biological Lab in Woods Hole, Mass. The logic went like this: With each eye covered by a different colored lens, two different-colored versions of a scene, just slightly offset from each other, should pop out into a three-dimensional image. By playing a video on the tank wall of a scuttling pair of shrimp silhouettes, each a different color and separated from each other by varying amounts, the researchers could make a shrimp seem closer to the cuttlefish or farther away. If, that is, the cuttlefish experienced 3-D vision like ours.
To test this hypothesis, the team let the cuttlefish get hungry, then placed them in the tank with the video projected on one wall. The first cuttlefish spied the “shrimp,” and very clearly put itself into reverse, backing up a bit before shooting its tentacles at the mirage.
The cuttlefish backs up before firing its tentacles at a projected shrimp.CreditCredit…By Feord Et Al
“I was ecstatic,” said Dr. Wardill. “We were sort of jumping up and down.” (The cuttlefish was immediately given a real shrimp as a reward.)
That motion of backing up was telling because cuttlefish spring their tentacles at their prey from a specific distance. They rely on suckers at the tips of the tentacles to grip and pull in their meal before biting it. If they are too close or too far, the tentacle tips won’t make contact. The researchers also projected the shrimp image to appear at a distance that made the cuttlefish move forward.
The cuttlefish creeps forward as it spots the projected shrimp.CreditCredit…By Feord Et Al
If cuttlefish were so obviously using 3-D vision to gauge the distance to their prey, why could they still hunt with a single eye, as earlier work had shown? Rather than blinding the animals, the researchers played a video of a single shrimp silhouette, its color chosen so that it would be invisible to one bespectacled eye. While cuttlefish would still launch their tentacles at this shrimp, they paused longer before doing so, suggesting they were not exactly sure if they were at the right distance. Perhaps cuttlefish without depth perception still succeed enough for it not to seem significant to earlier researchers.
Still, if the cuttlefish now joins the praying mantis as one of the few invertebrates known to see in 3-D in this way, the scientists have a long list of questions about how exactly they are doing it. Already, the study suggests that there is something peculiar about the way they transform visual information into depth perception. For instance, it does not bother them if one of the two overlapping images is much brighter than the other, something that humans don’t handle well. They also don’t always line their eyes up in precisely the same direction. Sometimes one of their eyes wanders, in a way that in humans would impair depth perception.
“Their brain layout is very different,” said Dr. Wardill. They likely have evolved ways, in their small, shimmering heads, of generating perceptions that are very different from ours — even if they are capable of wearing 3-D glasses.