Pass the popcorn — scientists are playing 3D movies to cuttlefish

Strapping coloured glasses to a marine mollusc and showing it 3D movies of shrimp seems like a bizarre experiment.

But it gave biologist Trevor Wardill a unique insight into how cuttlefish see, and how the cephalopods are able to use their vision to guide their precise strikes on moving prey.

"I have to say most people thought this wouldn't work because the cuttlefish would just take the glasses off and not be very happy with me putting them on in the first place," Wardill told Quirks & Quarks host Bob McDonald.

"And to be honest they were right."

Cuttlefish are related to octopus and squid, and like squid, have eight arms and two longer tentacles. They hunt by rapidly shooting out their tentacles at unsuspecting prey and capturing them with the suckers attached at the end. 

"They're a bit like cats. They'll kind of sneak up very slowly and then they will move backwards and forwards to judge the depth," said Wardill.

These cephalopods have to have very precise depth perception to know just how far away to shoot these tentacles. If they're off in any way, they miss out on a meal.

Testing depth perception with 3D movies

Wardill wanted to understand how they can see to act with such precision, and what mechanism they use to perceive distance. One possibility was that their depth perception was similar to ours, and depends on the brain quickly analyzing the small differences between the images each eye produces of an object in front of us — what's known as stereo vision.

This is the system that is exploited by 3D movies which use special glasses so that each eye sees a slightly different version of the movie to produce the illusion of depth.

Inspired by recent experiments studying praying mantis vision, he decided to build the cuttlefish an underwater 3D cinema to test their stereo vision in his lab at the University of Minnesota.

That was the easy part. The hard part was fitting the cuttlefish with their own 3D glasses.

"It was very tricky to convince them to wear the glasses, but we found some glue that worked relatively well and eventually falls off," said Wardill. "And then we could use that glue to attach some velcro. The idea here was that if the cuttlefish really didn't like its glasses, it could take them off."

It wasn't just the design that was tricky. When they first tried handling the cuttlefish, they would squirt ink at the researchers, contaminating the tank.

"They use that as the defence mechanism but it's also a good demonstration that they're pretty annoyed with the situation," he said.

Training cuttlefish to wear glasses — one shrimp at a time

Over the span of a week he was able to train some of the cuttlefish to get comfortable wearing the glasses by rewarding them with their favourite snack.

"I basically used the same strategies I do for looking after my daughter," he said.

"Every time we did something — like I moved them from one tank to another tank — I would give them a shrimp as a reward. And then if we put the glue with the velcro on, we'd put them back in the tank, we'd give them a shrimp for a reward."

Soon the cuttlefish were watching 3D movies starring a shrimp scurrying back and forth. Sure enough, they would alter their attack depending on how far they perceived the shrimp to be from them, proving that they do see in stereo, just like humans.

"Like us watching Tyrannosaurus rex come out of the screen to attack us, and you know you lean back in the seat," said Wardill, "it'll back up — it'll swim backwards so that it can shoot out its tentacles at the appropriate distance," Wardill said.

The results were published this week in the journal Science Advances.

Going on the hunt may make the cuttlefish vulnerable

While most hunters have stereo vision, cuttlefish are unique, in that they can also move their eyes independently from one another. 

"The surprising thing with cuttlefish is they can do this trick of stereopsis but they also can independently move their eyes, and that gives it almost a 360 degree view of the world," said Wardill. This is a valuable ability as it allows the cuttlefish a wide field of view to track potential predators.

When it goes to hunt, the cuttlefish focuses on its target and loses some of that wide field of vision.

"So it rotates its eyes the same as we would if we looked at something at a certain distance but here then it's losing receptive field behind it. So while it's busy hunting a shrimp something could sneak up on it because it can't see behind it right at that moment."

Watch the Cuttlefish 3D movie experiment in action: