By Elizabeth Zubritsky

by Elizabeth Zubritsky


[The following article was originally published by the University of North Carolina at Chapel Hill in the Spring 1998 issue Endeavors magazine.]

For generations, the monstrous-looking squid has inspired legends, tabloid headlines, and science fiction. But when you get right down to the meat of the matter, this lightning-fast creature is even more amazing than it looks.

By most accounts, squid shouldn't be able to move. But try telling that to the shrimp who just got snatched.

"The squid's prey strike is amazingly fast," says Bill Kier, associate professor of biology. "It literally happens in the blink of an eye."

The odd thing is, squid lack bones, which ordinarily are a necessity for movement.

The other necessity is muscles, but they can't do the job alone because they have one fatal flaw. Under their own power, they can only contract.

Of course, for some movements, contraction is enough. You can bend your arm by contracting your bicep muscle. But to straighten your arm, you need the tricep muscle and the upper-arm bone.

Like kids on a seesaw, the bicep and tricep work together to move your forearm up and down. The pivot point in this case is at your elbow, where the upper-arm bone meets the forearm. To raise your forearm, the bicep contracts. To lower your forearm, the tricep -- which is on the opposite side of the upper-arm bone -- contracts, letting the bicep relax. Without the upper-arm bone, there would be no pivot and no way to lower the forearm.

So muscles, because they only know one trick, need bones to be their levers and pivots. Those parts are so critical that creatures without bones have some kind of substitute. Crabs and insects have shells -- external skeletons. Earthworms and sea anemones have a pressurized fluid system, called a "hydrostatic" skeleton.

"According to what I was taught, and what we still teach in many introductory classes, those are the only options," Kier says. "Animals must have one of those skeletons. But squid don't."

The eight-armed, two-tentacled creatures have nothing but muscle. So do their tentacleless cousins, octopuses, the most famous cephalopods. Yet they move.

That impossibility -- movement without a skeleton -- captured Kier's imagination 16 years ago. Still a graduate student at Duke then, he began filming squid with a high-speed movie camera. He couldn't see the action with a regular one because squid are too fast. He discovered just how fast: A squid's tentacles can double in length in just two-hundredths of a second.

"What's so amazing is that this animal's body is about twenty centimeters (eight inches) long, yet the tentacles can move with acceleration twenty-five times that of gravity," he says. "It's not surprising when you consider that the animal's next meal depends on it. But what allows it in the first place?"

To find out, Kier looked closely at squid muscles and realized that they don't use our seesaw system. Instead, squid have two sets of crisscrossing muscles. One set runs the length of the limb and another runs along the diameter. As one set of muscles contracts, the other is relaxed. And no matter which set contracts, the length and diameter of the limb change.

"It's really a matter of geometry," says Kier, who came to Carolina in 1985. "A squid's arm or tentacle is a cylinder that holds the same volume no matter how long it is." That's the same as saying that a beer is 12 ounces whether it's in a short, fat can or a tall, skinny bottle.

A squid makes its arm or tentacle skinny by shortening muscles along the diameter. As the limb becomes thinner, it lengthens to compensate. Retracting the lengthwise muscles instead makes the limb shorter and fatter.

There's no need for levers and pivots here, but Kier says squid still have a skeleton -- one made entirely of muscle. It's called a "muscular hydrostatic" skeleton, and it's just as good as having bones. Better, in some ways.

"Squid don't just get by with this system," he says. "They're capable of remarkable control and precision of movement. Unlike us, they aren't limited to bending at a few places such as elbows or knees."

The two sets of muscles, he says, let squid bend their arms and tentacles at any point, in any direction. Squid even have a third set of muscles that spirals around their limbs to help with twisting.

But squid aren't the only ones who can do all this, as Kier and his colleague and wife, Kathleen Smith, an anatomy professor at Duke, found in their research. Elephants' trunks work the same way. So do tongues, whether they belong to humans, cats, snakes, or other animals.

"There's no reason to believe there are close evolutionary relationships between these animals," Kier says. "Tongues, trunks, and squid muscles evolved separately, essentially solving a single problem -- the need for complex movement -- the same way several times."

But knowing how squid arms and tentacles work didn't tell Kier why tentacles are so speedy. After all, squid arms aren't nearly as quick as tentacles, though the two are so similar that biologists think tentacles evolved from arms. Knowing that the squid's ancestor had ten arms, instead of eight arms and two tentacles, makes that seem even more likely.

"Replacing two arms with two tentacles probably gave squid a tremendous advantage in hunting," Kier says.

But that ability doesn't show up right away. When squid first hatch, their tentacles aren't much different than their arms. They certainly aren't any faster, and the nearly microscopic young animals grab prey with their arms.

As the squid develop, though, their tentacles go through a kind of mini-evolution. By the time the animals are four or five weeks old, the muscles are built for speed. Then the young begin hunting like adults, flaring their arms and snatching prey with their tentacles.

"There's an old principle in biology that an animal's evolution is mimicked in its development," Kier says. "When studied closely, it's rarely true. But it's true in general terms here."

Still, squid aren't the only ones whose muscles become specialized for speed. Ours do, too. We have dutiful "slow-twitch" muscles, which have a lot of endurance but don't contract quickly -- the kind that hold our backs upright. And we have "fast-twitch" muscles, the quick-response kind that let us sprint or move our fingers rapidly.

Any muscle -- slow or fast, human or squid -- has the same basic mechanism. It's packed with bundles of fibers. A single fiber contains shorter, finer strands called "filaments." Thick filaments alternate with thin ones like interlaced fingers, and, when they slide together, the muscle contracts.

In our muscles, filaments may slide quickly or slowly. It depends mainly on the thick ones, whose building blocks -- proteins called "myosins" -- come in fast and slow varieties. The muscle speed depends on the type of myosins made by the cells.

Once again, squid do things differently. Thick filaments in squid tentacles are made from the same building blocks as those in squid arms. Rather than changing the proteins, squid make a change at a higher level: They shorten the thick filaments.

Because thick filaments aren't as long in fully developed tentacles as in squid arms, there are more units -- called "sarcomeres" -- contracting. More units shortening at once means faster contraction. And faster contraction of the muscles along the diameter of the tentacle leads to faster extension. That's the secret behind the speedy strike.

At least, that's one of the secrets. Kier and his Dutch colleague Johan van Leeuwen think there might be more. The researchers recently created a mathematical model of tentacles, and it suggests that the filaments at the tip of the tentacle are even shorter than those at the base -- an arrangement that would provide the fastest possible extension.

"It would be wonderful if it were true," Kier says, "because nobody's seen that kind of variation within a single muscle before."

In the coming months, Kier will be looking at tentacle muscles to see if the prediction holds up. And he'll be trying to explain something he found years ago: that neighboring thin filaments in tentacle muscle line up like pickets in a fence. So do the thick filaments. The arrangement is called "cross striation" because it creates dark bands where the two fences overlap and light ones where they don't.

While most mammalian muscle is cross-striated, most squid muscle is not. Their filaments are almost always staggered -- a pattern called "oblique striation." The developed tentacles are the only exception.

In theory, it shouldn't matter whether the filaments in tentacle muscle are lined up or staggered, Kier says. As long as the filaments are short, the muscle should be fast. So Kier doesn't yet know why tentacle muscle is cross-striated.

"But what makes all of this research so exciting," Kier says, "is that we don't find the answers to, `Why are squid so fast? Why are they built this way?' in the usual places. We don't find them by studying the basic components -- proteins and genes -- or by applying what we've learned about mammals. It's by going outside those boundaries and by relating what we learned at each level -- muscle organization, behavior, evolution -- that we found out how unique and truly amazing squid are."

Kier's research is funded by the National Science Foundation.

For her story "The Long Road to Rural Health," (Endeavors, Fall 1997) Elizabeth Zubritsky won the 1998 Peter Lars Jacobson Award in medical journalism from the School of Journalism and Mass Communication.

Article by Elizabeth Zubritsky.
Feb 22, 2015
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