It may seem like a joke, but a team of researchers from the University of Minnesota (USA) have carried out a somewhat peculiar experiment that involved putting 3D glasses on a cuttlefish and putting it in a kind of underwater theatre where images of tasty shrimp were projected. The aim was to study the vision of these intelligent cephalopods and find out how they manage to determine the distance to their prey.
The results, published in the journal Science Advances, revealed that cuttlefish use stereopsis to achieve total precision in their hunting movements. This phenomenon of visual perception is what allows them to recompose the three-dimensional photo of what is perceived.
Imagine you're a cuttlefish and you have to hunt for prey. To succeed at first and not scare it off, you must be able to position yourself at the right distance: if you are too far away, your tentacles will not be able to reach it, but if you get too close, it may detect you earlier than expected and run away.
To determine how these invertebrates calculate the distance to their prey, scientists developed their experiment in a pond with a screen that projected 3D images of a shrimp. When the cuttlefish was fitted with 3D glasses, it thought the shrimp was in the middle of the pond and was much further away from the screen than when it was not wearing glasses. This reveals that, as we have mentioned, cuttlefish use stereopsis to see and hunt, and are able to integrate the images coming from both eyes to see in-depth. Although this phenomenon was traditionally thought to be something exclusive to vertebrates, studies like this one reveal that some invertebrates also have this capacity, although the process is not exactly the same as that which occurs in humans.
"Although cuttlefish have eyes similar to humans, their brains are significantly different," explained Paloma González-Bellido, assistant professor in the Department of Ecology, Evolution and Behavior at the University of Minnesota's School of Biological Sciences. "We know that cuttlefish brains are not segmented like those of humans. That is, in principle, they would not have a specific part of the brain, like our occipital lobe, dedicated to the processing of vision. Our research shows that there should be an area in their brain that compares the images from the left and right eye of a cuttlefish and calculates the differences," he concludes.
Cuttlefish eyes are highly developed, much more so than those of squid and octopus, and researchers argue that these animals may be the only cephalopods with the ability to use stereopsis to calculate distances. Although the development of the cuttlefish's nervous system is different from that of vertebrates, the final functioning is very similar. "This study takes us a step further in understanding how different nervous systems have evolved to address the same problem," explained Rachael Feord, first author of the research paper. "The next step will be to dissect the brain circuits needed to calculate stereopsis in sepia to evaluate the differences in functioning with the human brain".