Squid Brains, Eyes, and Color

Invertebrates, which are animals without backbones, are often considered simple and dumb, with no brains at all. But the cousins of clams and oysters, the cephalopods (octopuses, squids, cuttlefish), have complex nervous systems and behaviors, as well as excellent vision. You may even have heard that octopuses are the brains of the invertebrate world. Are squid just as smart?

Unfortunately, it's much more difficult to study intelligence in squid, because they're more difficult to keep in the laboratory. Octopuses live naturally in dens on the seafloor, an environment that is fairly easy to recreate in an aquarium, but most squids live naturally in the open ocean and need a great deal of space to move around. Furthermore, since octopuses are used to crawling on the ground and manipulating shells and rocks, it's easy to give them mazes and puzzles to solve. Squids live in an entirely different environment, so their intelligence is probably quite different—and more difficult for humans, who don’t spend time swimming in a watery world bounded only by light and darkness, to understand.

Humboldt squid eyes. Photo by Danna Staaf.

A dissection allows you to take a very close look at the nervous system of a squid, asking and answering questions about how it might work. Let's start with the brain, which comes in three parts: two optic lobes and a central ganglion. If you very carefully pull the eyes out of their sockets, you'll see the optic lobes, one behind each eye. They are yellowish white, soft and fleshy. Between them in the middle of the head, and somewhat more difficult to identify, is the central ganglion, a collection of very soft nerve tissue that actually surrounds the esophagus--every bite the squid takes goes through its brain!

As you cut into the head of a Humboldt squid, you'll notice that the brain is protected by a tough braincase that looks like it's made of cartilage. It's not proper cartilage like humans have, but it's a similar protein. When scientists use sonar to detect squid in the water, this braincase is one of the major parts of the body that reflects the sound.

Don't forget to look at the eyes themselves. On the side facing out, you can see the clear lens, which works just like the lens in our eyes, to focus light. It's quite hard, though it is only made of protein and you can dig into it with a fingernail. If the lens is in very good condition, you can actually place it over text and use it as a magnifier. On the eyeball all around the lens, you can see a reflective mirror-like coating. That's part of the squid's camouflage. Its eyes are very large and easy to see, so it tries to hide them from predators by reflecting light away from them. But it also needs some light to see, so it can't reflect all the light.

Now look at the back of the eye after you've pulled it out of the eye socket. The yellowish white strands running along the back of the eye are optic nerves, bringing visual information back to the optic lobe. One thing that makes the cephalopod eye so remarkable is the position of these nerves. In the eyes of vertebrates like us humans, the optic nerves actually block some of the photoreceptors that detect light, creating a blind spot. Our brains are very good at coping with this blind spot, so we almost never notice it. But cephalopods don't have a blind spot at all, because their eyes are arranged so their nerves never block the light.

Speaking of photoreceptors, the reason that we humans can see in color is that we have different kinds of receptors, called cones. Squid don't have these, and so we're pretty sure they can't see in color. But it is hard to believe, since they are so colorful themselves, and able to change their color quickly to match the environment. How do they do it?

Look very closely at the squid's skin and see all the tiny dots. They're called chromatophores, and they're like the pixels on a computer screen. Each chromatophore can be turned on or off by a signal from the nerves and muscles around it. When it's on, it shows a color, and when it's off, the skin looks white. Chromatophores can only make yellow, red, brown, and black, but underneath them is a whole different set of elements called iridophores and leucophores that reflect light and can make blue, green, and white.

What about that color blindness? Well, we think that squid skin might actually be able to detect some light on its own, and control its color changes without even needing the eyes. One of the really neat things about cephalopods is that even though they have a centralized brain, they also have a highly distributed nervous system. Lots of processing, even “thinking,” can happen throughout the body. Octopus arms can control much of their own movement, and the body of the squid may be able to control itself.

Stellate ganglion. Photo by Judit Pungor.

Look for the stellate ganglia (singular ganglion) on either side of the squid's liver. Pull the liver to one side until you see a star-like spread of nerves that looks similar to the ones behind the eye. You may have to scrape some connective tissue gently away before you can see them clearly. Squid stellate ganglia contain the largest nerve cells on the planet, which gave early neuroscientists their first glimpses into the inner workings of the brain and continue to be used in research today. They are part of the distributed nervous system that the squid uses to contract its mantle and to jet away from predators at tremendous speed.

References

Brains: Budelmann BU (1995). The cephalopod nervous system: what evolution has made of the molluscan design. Pp. 115–138 in The Nervous System of Invertebrates: An Evolutionary and Comparative Approach. O. Breidbach, and W. Kutsuch, eds. Birkhauser Verlag, Basel.

Eyes: Holt AL, Sweeney AM, Johnsen S, & Morse DE (2011). A highly distributed Bragg stack with unique geometry provides effective camouflage for Loliginid squid eyes. Journal of the Royal Society, 8(63):1386-99.

Chromatophores: Messenger JB (2001). Cephalopod chromatophores: neurobiology and natural history. Biological Reviews, 76:473–528.