Cuttlefish Tricks

This strange looking cephalopod in the MRC uses polarized light, but for what?

Common throughout the world's oceans but strangers to New England waters, cuttlefish came to the MBL with Roger Hanlon when he assumed the directorship of the Marine Resources Center in 1995. Sepia officinalis, the doe-eyed cephalopod hovering in the tanks in the MRC, is related to the squid. An ancient mollusk with eight arms (or legs) growing out of its head, the cuttlefish looks like something a science fiction writer would imagine. The animals have elaborate patterns of stripes and a large, oddly elephantine head. They can change colors in an instant and can create dozens of speckled as well as striped patterns. Although it is invisible to us, they can produce patterns of polarized light, apparently at will. When threatened, they can suddenly produce false eye spots that are plainly visible and frankly ominous. And they can also squirt ink like their cousins the squid and octopus. Little wonder the strange animal is one of the favorites of tourists who visit the MRC in the summer.

But for biologists, interesting is as interesting does - and the cuttlefish has a lot of interesting behavior. The animals change color and body pattern not only for camouflage, but also in reproductive behavior. The males, for instance, show each other their arm stripes and darken their faces when they compete for dominance. Males will go through the same routine when they see their reflection in a mirror.

Hanlon, who has literally written the book on cuttlefish (Cephalopod Behaviour), is interested in the social and sexual behavior of the animals, how they use sensory information, and the question of why they have such big brains. A good part of their large brains is devoted to vision, which they use for hunting, camouflaging themselves, and communicating and courtship.

Many marine animals are sensitive to polarized light, but it is not clear what information they get from this light. Last year, a group in Maryland showed that light reflecting off the bodies of cuttlefish and octopus is polarized - and that the pattern of polarization changes and appears to be related to behavior. One of those Maryland biologists, Nadav Shashar, has come to Woods Hole to work with Hanlon on unraveling the role polarized light signals play in marine animals.

"We know cuttlefish have the structures in their eyes to pick up polarized patterns," Hanlon says, "but no one knows why they use it." And no one knows why they create their own polarized patterns. They may, Shashar and Hanlon suggest, use polarized patterns as part of their camouflage to confuse predators that can see polarization. Their own ability to see patterns of polarization may help them find the transparent plankton and small fish they eat. Or, perhaps most interestingly, they may be using polarization to signal each other.

"It may be a hidden means of communication," Hanlon says. While we cannot see patterns of polarization with our naked eye, we nonetheless can employ it. For example, a camera with polarization sensitivity designed by Shashar and colleagues is already being used to test for rust on the bottom of ships. The camera, which is mounted on a remote device to inspect ships in the water, can detect polarized light that shows where rust is beginning before the damage it does is apparent to the naked eye. Other potential technological payoffs included improved machine vision and sensors for climatological studies, which are already under development.