Everyone here is familiar with the incredible ability of cephalopods to change their appearance, right? If you aren’t, review your cuttlefish anatomy and watch this video. A few frames from the video are shown on the right.
This is an amazing ability, and the question is how do they do it? Roger Hanlon has been spending years tinkering with cephalopods, trying to puzzle it out and come up with an explanation. There are a couple of things a master of disguise needs.
A good visual system. To match the background, you need to be able to see the background at least as well as the predator trying to see you.
Fast connections to the effector organs. Cephalopods have motor nerves that go straight from their brains to the chromatophore organs with no synaptic delays along the way.
The hard part: cutaneous chromatophore organs that can change intensity and texture with a fair amount of spatial resolution. Cephalopods have tiny, discrete sacs of pigment scattered all over their body, each one ringed with muscles that can iris shut to conceal the pigment, or expand the sac to expose the pigment. There are also muscular papillae that work hydrostatically to change the texture of the skin from smooth to rough to spiny/spiky.
An algorithm. A set of rules that translate a visual field into an effective skin pattern that hides the animal.
One of the minor surprises of this work is that that last item, the algorithm for generating camouflage, may not be that complex. By studying many camouflaged organisms, they’ve categorized camouflage techniques into just three different strategies.
The three are responses to the coarseness of the patterns in the environment. If the background is fine grained and simple, blend in by generating a uniform skin pattern that matches the average intensity. If the background is a mixture of small objects of varying intensity, take on a mottled appearance to blend in. And finally, if there are relatively large objects with a fair amount of contrast around, instead adopt a disruptive camouflage scheme, which has the function of breaking up the outlines of the body. These three categories are illustrated below.
Hanlon was also testing their abilities experimentally by placing cuttlefish on computer generated backgrounds and challenging them to try and match them. As you can see, they don’t do as well as they do on natural textures, but you can also see how what they’re trying to do fits in with the categories. Note in the bottom right that the animal really isn’t trying to make an exact duplicate of the checkerboard pattern below it—it has created a pale square in its midsection that isn’t aligned with the grid, but in a natural environment would catch the eye of a predator as a white square, rather than a tasty cuttlefish.
Another interesting observation is that cephalopods are thought to be color blind—they have one visual pigment with a peak sensitivity at 492nm—and that was tested with checkerboards of various colors but similar intensity at 492nm, and the cuttlefish can’t see them. A checkerboard of blue and yellow squares that is painfully contrasty to our eyes is seen as a uniform gray by the cephalopod, which then adopts a uniform gray-brown skin color. How they do any kind of color matching is not known, but it may be that they simply don’t: in an environment lacking many bright primary colors, sticking with natural shades of gray and brown may be adequate.
They may not have color vision, but they do have an ability we lack: the ability to see the plane of polarized light. Their skin also contains a class of pigment cells called iridiphores that are under neural control and that reflect polarized light through the overlying chromatophores. Changes in the plane of polarized light would be completely invisible to us, but quite apparent to the cephalopods—they could be sending secret signals to one another right under our noses by subtly rotating their iridiphores.
The lesson is that there are sneaky subtleties in cephalopod capabilities, but the general principles of camouflage aren’t particularly elaborate—the eye is easy to fool, and just a few general strategies are sufficient. Of course, you still need a fairly elaborate array of controllable pigment elements in order to implement those strategies.
Hanlon R (2007) Cephalopod dynamic camouflage. Current Biology 17(11): R400-404.