Vision: it lets us navigate around objects, know the location of the things we want, and read informative blog articles about vision. When we think about our vision, it’s usually all about what images we see, images that our eyes and brain create for us. But for some animals, this is just the beginning.
Cephalopods (squid, octopus, and cuttlefish) are known for their incredible color changing abilities, which they use to blend in with their surroundings. Unsurprisingly, their eyes are also very complex. In fact, cephalopod eyes are very similar in their anatomy and abilities to our eyes. But what is surprising is that scientists have recently discovered that squid have photoreceptor cells not just in their eyes, but also in their skin.
So first off, what is a photoreceptor cell, and why does it matter that squid have them in their skin? Photoreceptor cells are basically light detectors. Having a lot of photoreceptors of different types is what makes our brains able to create the complex images that we see every time we open our eyes. As far as we know, mammals (like us) only have light sensing photoreceptors in our eyes. But by looking at the genes in cephalopod skin, researchers have discovered that cephalopod skin makes proteins called opsins, which make up photoreceptor cells. But this doesn’t mean that cephalopods can actually sense light with their skin, because opsins can also be used for other things. There are other proteins needed in order for the opsins to be able to contribute to vision, just like a camera lens is completely useless without all of the other equipment to help it form an image. In order to figure out if the opsins in cephalopod skin are actually able to sense light, a research team at the Marine Biological Laboratory in Woods Hole first had to determine if these other necessary pieces were also present in the skin. They went about answering this question by looking for the genes of these components in the skin and comparing them to the genes found in the eye. If the genes were the same, then so were the components.
As it turns out, this is in fact the case. In a study published in the Journal of Experimental Biology, the authors documented that the components needed for vision in the eye are also present in the skin of the species of squid and cuttlefish the researchers studied. Not only that, but there are a lot of these photoreceptor pieces in the color changing organs (called chromatophores) that cephalopods use to create amazing color patterns on their skin. This means that cephalopods might use information from both their eyes and their skin to determine their body color patterns.
Wait, so squid can see with their skin? Not exactly. On their own, individual photoreceptors cannot really do much except tell you if there is light or not. But cephalopods derive a lot of information from where light strikes, especially because the photoreceptors in their skin respond to specific wavelengths (meaning colors) of light. Researchers at UC Santa Barbara found that not only do pieces of octopus skin change color when any color of light is shone on them, but they also react most quickly to blue light. This is the same color of light to which octopus eyes are most sensitive.
But the octopus skin didn’t change color immediately. In fact, it took several seconds for it to completely change color. That might seem pretty quick to us, but cephalopods are known to change their body color in a matter of milliseconds. So why is the response to light so much slower in skin pieces than in a living cephalopod? For one thing, it’s very unlikely that cephalopods are relying only on the light information their skin gets; they are probably still using their eyes to inform most of their color displays. After all, they have these big fancy eyes, they ought to use them!
But there is a benefit that comes from relying on two different sources of visual information. Imagine if your survival depended on constantly making your body match your background at a moment’s notice. As anyone who’s ever painted a landscape knows, a lot of factors go into determining the right colors and patterns to recreate an image perfectly, and one of the most important factors is light. Gathering information about the quality of light beyond what is visible to the eyes alone would certainly help improve the cephalopods’ body patterning camouflage.
Light sensing has other uses too. Cousins to the cephalopods, such as clams, mussels, and snails, have also been shown to sense light with their skin. In this case, they often use it to tell if there is a predator swimming overhead. When you’re a tasty little snail living on the seafloor, most of your predators will probably approach you from above, casting a shadow on you right before they attack. Therefore, by sensing changes in light and withdrawing into your shell when the light suddenly decreases you reduce the risk of being someone else’s snack.
So whether you’re trying to color your body to perfectly match your shadowy surroundings, or just trying to know when danger is near, you need all the help you can get. Why only use two eyes when you can use your entire body to help you "see" the light?
Kingston, A.C.N., Kuzirian, A.M., Hanlon, R.T., and Cronin, T.W. (2015). Visual phototransduction components in cephalopod chromatophores suggest dermal photoreception. Journal of Experimental Biology. 218: 1596-1602.
Ramirez, M.D. and Oakley, T.H. (2015). Eye-independent, lightactivated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. Journal of Experimental Biology. 218: 1513-1520.