Being able to continue displaying to members of the opposite sex, while actively hiding from predators, sounds like a tall order. However, Lydia Mäthger and Roger Hanlon from The Marine Biological Laboratory, Woods Hole suggest that this might be possible for squid because of the remarkable anatomy of their skin.
Cephalopods – squid, octopus and cuttlefish – perceive information about the polarization of light using their highly ordered photoreceptors. Because light tends to become polarized when it's reflected,one valuable use of this ability is that cephalopods can see silvery fish from below in high contrast: they are otherwise very difficult to see as they reflect whatever is above them and appear to blend into the sky.
Cephalopods also use polarized light perception for communication,elegantly demonstrated in cuttlefish by Nadav Shashar and his colleagues in 1996 (). Using a camera capable of spotting the orientation of polarized light, they observed pretty polarization patterns on cuttlefish arms and forehead. These polarization patterns were produced in under one second using specialized cells called iridophores, which are a type of chromatophore or pigment-containing cell. The pigments in iridophores differ from standard pigments in that they are iridescent, reflecting light that is polarized, with a colour dependent on the viewing angle.
Shashar also showed that polarization patterns mean something to the cuttlefish: on seeing a reflection of themselves in a mirror that did not distort their own polarization pattern, cuttlefish were much more likely to move away than if they were faced with their reflection from a mirror that distorted the polarization signal. From their observations that polarization patterns can be quickly controlled and mean something to the animal, and the fact that many cuttlefish predators can't detect polarized light, Shashar and colleagues introduced the concept of polarization patterns as a channel of communication concealed from predators.
Mäthger and Hanlon take this possibility one step further and show that not only can polarization patterns be a concealed communication channel,they can be a channel of communication while concealed. By studying the light patterns reflected from squid skin samples, they confirmed the two expected characteristics of iridophores: that the colour reflected depends on the viewing angle and that the reflected light is polarized. They also show that the overlying chromatophores act as colour filters but leave the polarization unaltered. This means that the squid has the potential to express itself freely using polarization patterns while doing its best to hide from predators by changing colour – as long as the predators aren't polarization sensitive too. Fortunately, predators such as sharks, dolphins, whales and seals appear not to be.
Another implication of this finding is that squid can use filtered reflected light to blend into their surroundings, matching their background more closely. Fish are stuck with appearing `silvery' when light reflects off them; squid, however, can use the iridophores to reflect up to 90% of the light back through the chromatophores, changing its colour. This means that chromatophores can control a squid's appearance both by determining the reflectance, or colour bouncing directly off, and the properties of light allowed back through after bouncing off the iridophores, or transmission. In effect, this opens up a new way of decoupling a squid's colour from the colour's brightness, potentially enhancing camouflage, giving the chance for a squishy, tasty, slow mollusc to survive in the open seas. This part-reflection camouflage system does start to sound like `how to build a Bond car', and so perhaps it isn't surprising that some of the funding for this work comes from DARPA, the American military research agency.
