How Worm Balls Avoid the Knotted Step

How Worm Balls Avoid the Knotted Step

MANY ANIMALS find safety in herds, colonies, schools or swarms. But few species opt for the aquatic fibrous worm technique Lumbriculus variegatus, a creature that at a few inches long is much longer than it is wide. In hard times, for example, when water is scarce, tens of thousands of them can quickly writhe into a tangled ball, to seek out a wetter environment and save all but the poor creatures on the periphery of the bubble from desiccation. How they escape this predicament, however, until recently was a mystery to both biologists and those familiar with nodes.

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Saad Bhamla, a professor of bioengineering at the Georgia Institute of Technology, has made his career investigating such unusual talent. He was first intrigued by the movement of these roundworm bubbles. But other behavior was even more seductive. If the bubble is startled in any way, for example by a flash of bright light, it dissolves back into its thousands of constituents in mere milliseconds. The Doctor. Bhamla was perplexed: how can they not tie a knot?

He came across research by Jorn Dunkel and Vishal Patil, two mathematicians at the Massachusetts Institute of Technology who turned their attention to knots (whose abstraction turns out to be a rich choice for mathematicians). He posed the riddle for them, and a truly interdisciplinary team was assembled.

The trio first had to figure out what was going on in the midst of a blackworm superorganism. Then they froze a solid and scanned it with ultrasound. An analysis of the scans revealed that each worm was tangled with an average of 1.7 others. The rapid dissolution of a bubble, then, required that each worm only break free of a few neighbors. But then many more opportunities to tie appeared as thousands more sought freedom.

To analyze this, the team had to consider how worms move in the first place, and that’s where Dr. Bhamla. they snap into a corkscrew shape and spiral at speed – occasionally alternating the corkscrew movement of their bodies from clockwise to counterclockwise and vice versa.

Patil, who now works at Stanford University and presented the team’s preliminary results at a recent meeting of the American Physical Society, found that this technique may be what saves worms from repeated entanglement. He created a computer model of the balls, complete with the physical details of the worms’ bodies. The resulting simulation revealed that they would struggle to get out, either heading straight for the exit, or weaving left and right. What worked best for a quick escape was not just a corkscrew motion, but a motion that periodically switched between clockwise and counterclockwise.

The Doctor. Bhamla says this kind of discovery could be useful in the field of so-called soft-active matter – the study of the collective action of large numbers of self-propelled individuals (biological or mechanical) acting independently. Never mind all that: what is certain is that he is delighted to have unraveled a complicated problem.

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This article appeared in the Science and Technology section of the print edition under the title “The worm turns”

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