Kraken: a new book about squid

Here's an exclusive excerpt from the nature book, Kraken: The Curious, Exciting, and Slightly Disturbing Science of Squid, by Wendy Williams.

One of the biggest problems cephalopods face is how to live safely in a 3-D world. When you imagine swimming in the deep ocean, you have to rethink human-oriented concepts of "up" and "down." As rather large surface animals who live on the continental crust, we usually need only be aware of animals living on the same plane that we do: Will we be attacked by a lion? Trampled by an elephant? Usually, "up" and "down" are not words that hold terror for us. We don't fear giant birds swooping down from above to scoop us up and carry us away, and we don't fear giant worms bursting out of the earth's crust to grab us and drag us underground. We only need to be aware of enemies that, like us, are firmly rooted to life atop the soil.

But surviving in the ocean is more complex. An animal living in the sea needs to have the responses and defenses of a fighter pilot. The enemy can come from anywhere, from the left or from the right, but also from above or from below. It's a three-dimensional world down there. Skeleton-free cephalopods are particularly at risk, since predators don't need to worry about the bones. "The creatures are really just rump steaks swimming around," Australian scientist, Mark Norman once explained. They need special protection.

In response, the animals have evolved an impressive tool kit of tricks. Bathyscaphoid squid, named in honor of a self-powered sea exploration vehicle that was developed after the 1930s bathysphere of naturalist William Beebe, is a family of squid that spends its early life, when it is most vulnerable and most likely to turn into someone else's dinner, at the ocean's surface, where there are plenty of small tidbits for a tiny animal to eat. As the Bathyscaphoid squid develop, they descend deeper and deeper into the water. These squids have evolved a body that's translucent and nearly completely invisible. At the top level of the ocean, the water's rich with nutrients. It's easy for them, as predators, to find food. Unfortunately, it also easy in the sunlight to become pretty to other predators. But with a body that's almost transparent, these young squids are ghostlike, nearly invisible. Being nearly invisible when tiny is quite convenient. The young squid at the sea surface can easily sneak up on its even tinier prey without being noticed. A prey animal might perceive what seems to be a twinkle of sunlight at the sea surface, only to find itself enveloped in a mass of squid arms and tentacles.

Locating your enemy in the ocean is a 24/7 task. Color and luminosity are both armor and weaponry. Many animals developed the ability to change shape and color to blend in with their surroundings. Some fish can do this, as can some frogs and, of course, chameleons. But no group of animals is as sophisticated in this strategy as are the cephalopods, nature's best now-you-see-them, now-you-don't masters of quick change. When we watch these animals zip through a myriad of psychedelic displays in only seconds, we stare, transfixed. But the basic organization of this magic show is simpler than you might think: It's done with three layers of three different types of cells near the skin surface — a layer of chromatophores, a layer of iridophores and a layer of leucophores.

The top layer of cells, the chromatophores, contains the colors yellow, red, black, or brown. The colors present are species-dependent. The color in a chromatophore cell sits near the cell's center in a tight little ball with a highly elastic cover. When the muscles controlling the chromatophore are at rest, this ball of color is covered over and can't be seen. When a chromatophore is showing, what you're seeing is this little ball, stretched out into a disk roughly seven times the diameter of the at-rest ball.

To operate properly, one chromatophore cell has a number of support cells, including muscle cells and nerve cells. The arrangement is cunningly elaborate. Anywhere from four to twenty-four muscle cells might attach to only one chromatophore. When these muscles contract, pulling on the chromatophore cell, the elastic sac is stretched out, revealing the color inside. When the muscles relax, the ball returns to normal size and the color disappears.
There's a simple way to envision this: Imagine a small, circular sheet of red paper. Crumple it into a tiny, tight ball. The color red is now only a pinpoint. Using your hands — and the hands of up to eleven other people if they're around to simulate the twenty-four muscle cells — stretch the paper out so that it's flattened to its full size. Then crinkle the paper into a tiny ball again. Do that umpteen times a second, to simulate flashing. On an infinitely smaller scale, that's how a cephalopod operates one individual chromatophore.

This is enormously elaborate engineering, requiring a considerable amount of coordination and support. The muscles surrounding the color-containing cells are controlled by nerves that interact with other nerves. Some scientists think that this complicated system may be one explanation for cephalopod intelligence, since the system requires the interactions of so many neural cells.

Just below the layer of chromatophores is another layer of cells, the iridophores. This layer of cells shows a different array of colors — metallic blues, greens, or golds. The iridophores do not open and close. Instead, they reflect light. They are sometimes used to camouflage an animal's organs, like eyes, by shimmering and drawing attention away from the organ. Some scientists have studied this strategy as a way to improve camouflage for soldiers on the battlefield.

Underneath this layer is the final layer, a layer of leucophores, flattened cells that passively reflect the color of background light, increasing the animal's camouflage.
When I first watched cephalopods showing off their artistic genius, some of their techniques seemed familiar. I knew I had seen this use of color and light somewhere else. Then I remembered Claude Monet's many paintings of water lilies, of haystacks, and of a cathedral at Rouen. Monet could paint the same scene many times, but each painting is different because the master could so expertly show the differences created by only slight shifts of light.

Cephalopods are the original Impressionists. I often wonder if the French painters didn't quietly study the cephalopods' techniques. Both the Impressionists' and the cephalopods' light shows provide the illusion of great depth by using luminosity — the reflection of light. Both skillfully use thousands of points of light and color to trick the observer.

But not all cephalopods enjoy equal artistic talent. Cuttlefish, which live nearer the ocean's surface where light still penetrates, are outstanding in their Impressionistic skills. Humboldt squids, on the other hand, are quite limited. With their highly honed predatory abilities and their large size, they don't need to devote so much energy to disguising themselves. Moreover, since so much of their lives is spent in dark ocean depths, all the Humboldt needs is red chromatophores, which allow it to disappear quickly.

Roger Hanlon, a cephalopod researcher at Woods Hole's Marine Biological Laboratory, is studying cephalopod camouflage abilities that may have military applications, along with the Air Force Research Laboratory in Dayton, Ohio. Recently, the Department of Defense awarded the MBL scientist $(removed) million for a study of "Proteinaceous Light Diffusers and Dynamic 3-D Skin Texture in Cephalopods." The Ohio lab is studying some of the proteins involved in cephalopod camouflage, to see if some of those proteins might somehow be used to help soldiers become less visible on the battlefield.