|The case for polarized communication|
Some cephalopods can change the polarization
of the skin and may use it for stealthy signaling
The underwater world is polarized and many of its inhabitants are aware of it. They have polarization vision (P-vision). But can any of them actively control the polarization of light and use it for signaling? Maybe so.
In1960 Moody and Parriss discovered P-vision in octopuses and soon after others found it in the rest of the octopods and decapods (sweet words in the ears of a shoemaker). Indeed, if you have eight or ten arms and a big head you've probably got P-vision. Could it be the secret of how the Giant Squid avoids encounters with eager underwater photographers? Nobody has yet been able to study those creatures of Jules Verne's nightmares, but their smaller relatives are much more amenable for experimentation (and a favorite of neuroscientists).
The European Cuttlefish (Sepia officinalis) is a relative of the squid with a calcified internal shell. They live in coastal waters throughout the Mediterranean Sea and eastern Atlantic from England to North Africa. They can be quite cute or cute-less, depending of the profile they show the camera (but beware of hurting their feelings: they may ink you), as seen in these photographs from the National Resource Center for Cephalopods, in Galveston, Texas.
The fact that cuttlefish have both P-vision and also polarized patterns in the skin, doesn't necessarily prove those have behavioral significance. That's what Shashar, Rutledge and Cronin set out to prove. They took advantage of the cuttlefish vanity (or lack of it). They placed mirrors in front of them to see how they reacted to their own image. Generally they tended to retreat from the mirror. The cuttlefish behavior changed markedly when a transparent filter was placed between the mirror and them. The filter didn't change the visible image but only distorted the polarization. In this last case, they stayed put and were not scared by their own face!
The polarization pattern is more intense when the cuttlefish cruise or hover in the water and when they are alert at the bottom even if not moving (the researchers determined alertness by the tracking of the eyes). It disappears when they camouflage on the bottom and are not alert. During mating behavior the male's pattern first diminishes and then reappears. The polarization pattern also disappears when a female is laying eggs, before and during attacks of prey and during extreme aggression between males.It is well established that the diverse body colors and patterns in cuttlefish have behavioral significance and are used in intraspecific (pal to pal) communication and for camouflage. Shashar et al. suggest that: "Polarization may provide cuttlefish with a channel concealed from some of their predators". They point out that vertebrate predators of cuttlefish, like sharks, cetaceans and seals are not known to posses P-vision.
In the eye of the Octopus (and the Squid)
So, why is P-vision so prevalent in cephalopods? How do they do it?
Most cephalopods do not migrate or forage over long distances so navigation is probably not their main use for P-vision. Some suggestions include: target recognition, breaking camouflage and increasing detection range. A very good use could be for detecting transparent objects, like those delicious jellyfish (I want my jelly!). Against a polarized background, like the one produced by light refracted at the water surface, they could become very noticeable by depolarizing the light or maybe even by acquiring interference colors. The pictures below, due to Dr. Shashar, show the increased contrast very well.
The anatomical basisfor this sensitivity has some similarities with that of insects. The visual pigment rhodopsin orients preferentially in a direction parallel to the microvilli tubes where it is located. The microvilli of each photoreceptor cell are parallel to each other, making the cell much more sensitive to light linearly polarized parallel to that direction. The microvilli of adjacent photoreceptors are perpendicular to each other, thus providing the basis for polarization discrimination. However, as light is usually only partially polarized, a third measurement is needed at another angle to eliminate ambiguity: this may be accomplished by movements of the eye or maybe by variations between different parts of the retina.