Watching 3D films at the cinema might not be to everyone's taste, but it's hard to deny that the way the images appear to leap from the screen feels like technological movie magic. However, this visual trick relies more on stereopsis, a biological strategy by which some animals, including humans, use a pair of forward-facing eyes to measure how far away something is. Until a few years ago, it was thought that only vertebrates possessed this perceptive power and only very recently did scientists discover the use of stereoscopic vision in the first invertebrate species – predatory praying mantises. This revelation spurred Trevor Wardill and a team of visual ecologists from the University of Minnesota, USA, to investigate the possibility of stereopic vision in a much larger and more complex invertebrate carnivore – the cuttlefish.
Cuttlefish are unique amongst cephalopods, the class that they share with squids and octopuses, in that they have a substantial overlap in the range of vision between their left and right eyes. They are ambush predators, lurking close to their prey before rapidly grabbing them with tentacles and injecting them with deadly toxins. Since a mistargeted tentacle strike can result in their supper scarpering, cuttlefish rely heavily on precisely aiming and timing their strikes – a trait that Wardill felt made cuttlefish likely suspects for having stereopic vision.
To test his suspicions and confirm whether the common cuttlefish (Sepia officinalis) indeed use stereopsis for hunting, the team built miniature pairs of 3D glasses consisting of either red–blue or blue–green lenses, allowing them to present the cuttlefish with simulated images of prey set at specific distances. As cuttlefish lack a nose and ears for securing spectacles before their eyes, the team glued small pieces of Velcro to the sides of the animals so that they could attach the 3D spectacles with ease. Fortunately, a small number of the animals were unfazed by their new eyewear and carried on swimming and hunting as usual. Once accustomed to their new accessories, the team presented the cuttlefish with 3D movies of walking grass shrimp, a cuttlefish delicacy, on a TV monitor inside their experimental tank to lure them into striking.
The cuttlefish wearing the 3D glasses consistently positioned themselves correctly and extended their tentacles to just the right spot in order to land successful strikes at the shrimp movies. To test whether the cuttlefish could also judge distance without stereopsis, they were also shown images of shrimp that appeared in only one of the lenses of their 3D glasses, which removed the image's stereopic depth. This revealed that while the single-lensed cuttlefish were equally able to locate the shrimp, they took much longer to position themselves for the tentacle strike and would misjudge the distance to their fake food more often. While this result demonstrates the importance of stereopsis for speedy and accurate tentacle strikes, it doesn't seem be the only mechanism for depth perception in the cuttlefish's arsenal, as some of the single-lensed shrimp were still easily able to hunt successfully.
Despite their alien appearance, this curious case of convergent evolution goes to show that cuttlefish aren't quite so different to humans as once thought, but they do possess an extra stereoscopic skill that makes human eyes seem much less than picture perfect. For mammals, stereopsis and eye movement are closely linked, but surprisingly, Wardill's cuttlefish were moving their eyes independently like a chameleon for much of their hunting process; some were even still able to strike while their eyes were looking in different directions! When it comes to hunting for shrimp, seeing truly is believing – for cuttlefish, at least.