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Squid skin with a mind of its own

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This pattern looks like it was the work of a clever artist, but it wasn't made by an artist - it wasn't even made by a person! This pattern is the work of a squid. Here is a longfin inshore squid, caught off the shores of Woods Hole, Massachusetts. When I approach him, he turns red, signaling his aggression. His brain has total control over what his skin is doing. Inside the squid skin are small pouches of pigment called 'chromatophores'. Each chromatophore is pulled opened by tiny muscles. So in order to turn red, neurons from the squid's brain send signals to these muscles, which pull open all of his chromatophores at the same time. Here, we've cut the nerves that would normally go the small area on the squid's side. Right after we do this, the chromatophores go quiet, which you'd expect since we've basically disconnected the skin from the brain. But they don't STAY quiet - a few days later, this happens. This squid has control over his skin. This one doesn't. But somehow, these chromatophores are opening and closing anyway. Let's take a closer look under the microscope... The chromatophores open and close in beautiful waves of color. These patterns can seem pretty complicated - some people even think octopuses use patterns like these to mesmerize their prey. But these intricate displays emerge on the squids skin even after it's been disconnected from the brain - so what's going on? First, I wanted to measure how the chromatophores grow and shrink, so I wrote a program that measures the size of each chromatophore over time. Let's add some color to get a clearer picture of what's happening. In this program, I've colored the open chromatophores red and the closed ones blue. Look at what this chromatophore's neighbors do right before it opens. Right before it opens, its neighbors open. Let's see that again. What rule could govern this behavior? You might say a chromatophore opens after its neighbors open. But is it really that simple? I'm going to write a program to simulate the squid skin using just that one rule. Here it is! The small dots represent close closed chromatophores and the larger dots represent open chromatophores. Let's run the program. This doesn't look right. In the real squid skin, the chromatophores open and close over and over again. But in the simulation, they just open once and stay open. What if we add another rule? Let's look at the squid again. What about this - after a chromatophore opens, it needs to wait a bit before can open again. This need for pause is pretty common in many cells, including neurons, and it's called a 'refractory period'. Here's take two of the simulation: First of all, the chromatophore open after its neighbors open, and I've added an extra rule, which is that right after the chromatophore opens, it needs to take a break. I'm starting with a line of open chromatophores and a line of chromatophores that are refractory underneath, so they need to take a break before they can open. Let's run it and see what happens. This looks even more like the real squid skin. Here's another simulation where I've included three different colors of chromatophores. I've made the grey ones expand randomly and the yellow and red ones respond to their neighbors with different sensitivities. As you can see, the simulated patterns look very complex even though they are governed by such simple rules. When we cut the nerves to a patch of the squid skin, the chromatophores acted like they had a mind of their own, but they were really just following two simple rules - listen to their neighbors and take a break after they open. From these simple rules, mesmerizing patterns emerge. Even in our complex biological world, some of the most beautiful things can be explained by the simplest concepts.