A fishy line of sight

EMMETT WARREN emmett.warren@photonics.com and JOEL WILLIAMS joel.williams@photonics.com

As if cuttlefish weren’t cute enough, a University of Minnesota (UMN) research team recently equipped 11 of them with specialized 3D glasses for use in an underwater theater built just for them. The researchers did this not just so the mollusks could enjoy the viewing experience; the team wanted to investigate how cuttlefish determine the distance from which to strike moving prey. The research ultimately revealed that the mollusks use stereopsis — stereo or binocular vision — to perceive depth when hunting a moving target.

Cuttlefish catch their prey by deploying their tentacles, but to be successful they must compute depth in order to position themselves at the correct distance from their target. If they are too close, the prey may get spooked and leave; if too far, their tentacles will not reach.

To test how the cuttlefish brain computes distance to an object, the team trained a group to wear 3D glasses and then strike at moving images of two shrimp — the ultimate interactive media experience. Each shrimp was given a different color and displayed on a computer screen at the Marine Biological Laboratory in Woods Hole, Mass. Researchers used a Velcro patch to apply the glasses to the cuttlefish (with one side of the Velcro temporarily glued to the mollusk's head). The team was then able to determine the mollusk's method of perceiving based on its position in the tank relative to the screen.

Researchers placed 3D glasses on cuttlefish to determine how the cephalopods hunt prey. Courtesy of R. Feord/University of Minnesota.

The images of the shrimp were offset, which allowed the researchers to determine whether cuttlefish compare images between the left and right eyes to gather information. Depending on the offset, a creature using stereopsis to calculate distance would perceive the target — in this case, a shrimp — to be either in front of or behind the screen. As predicted, the cuttlefish consistently struck too far or too close to the screen.

“How the cuttlefish reacted to the disparities clearly establishes that cuttlefish use stereopsis when hunting,” said Trevor Wardill, assistant professor in the Department of Ecology, Evolution and Behavior at UMN’s College of Biological Sciences, in a university press release.

“When only one eye could see the shrimp, meaning stereopsis was not possible, the animals took longer to position themselves correctly. When both eyes could see the shrimp, meaning they utilized stereopsis, it allowed cuttlefish to make faster decisions when attacking,” he said. The researchers also found that the mechanism that underpins cuttlefish stereopsis is likely different from that of humans.

“While cuttlefish have similar eyes to humans, their brains are significantly different,” said Paloma Gonzalez-Bellido, UMN assistant professor in the same department. “We know that cuttlefish brains aren’t segmented like humans’. They do not seem to have a single part of the brain — like our occipital lobe — dedicated to processing vision. Our research shows there must be an area in their brain that compares the images from a cuttlefish’s left and right eye and computes their differences.”

It was once thought that complex brain computations such as stereopsis were exclusive to higher-order vertebrates, but studies such as this are leading scientists to reconsider the capabilities of invertebrate brains.

“This study takes us a step further toward understanding how different nervous systems have evolved to tackle the same problem,” said Rachael Feord, the research paper’s first author. “The next step is to dissect the brain circuits required for the computation of stereopsis in cuttlefish with the aim of understanding how this might be different to what happens in our brains.”

The findings were published in Science Advances.