Snails don’t need slime to get ahead

STANFORD (US) — To move from here to there, a snail crawls using one muscular foot on a layer of self-secreted mucus-like slime. Now it appears the sticky stuff isn’t so essential after all.

It was already known that snails and slugs propel themselves by generating a series of muscular pulses on their feet. These waves of muscle contraction and relaxation travel along the central portion of the foot from tail to head. The waves move much faster than the snail itself, and generate enough force to push the snail forward.

A new study finds these muscular waves are sufficient to propel the snail forward on a flat surface, without needing the special mucus to provide more traction. The mucus does help the snail stick to surfaces, however, and comes in handy when traveling up a wall or across a ceiling, upside down.

The research could help advance work on snail-like robots that could be developed for use in medical applications like endoscopes.

“We were surprised to find that the special mucus properties weren’t essential,” says Janice Lai, graduate student in mechanical engineering at Stanford University. “Mucus is still very important, but we found that there are other mechanisms that the snail uses to generate the traction to move forward.”

Lai shot high-resolution videos of crawling snails and used a laser to measure how the muscle waves moved back and forth and up and down on a snail’s foot. She also measured the forces that a snail generated when crawling on a gel.

Details are reported in the Journal of Experimental Biology.

Researchers discovered nearly 30 years ago that snail mucus has some unusual properties, allowing the animal to stick to a surface while moving, with the mucus changing its characteristics according to how firmly the snail presses on it.

The slime initially acts like glue, sticking the snail to the surface. But when the snail’s foot presses down hard enough on the mucus, it becomes more liquid, allowing it to flow underneath the moving snail. Existing theories assumed that this special characteristic of snail mucus was always necessary for the snail to push off and move forward.

But Lai and colleagues, Professors Juan C. del Álamo and Juan Lasheras at the University of California-San Diego, and Javier Rodriguez of Carlos III University of Madrid, noticed that snails could move horizontally just fine on a thin film of water, and wondered whether the mucus’ special properties were really necessary.

Lai adapted a tool Lasheras’ lab had previously developed to study the movement of cells. Snails are similar to our cells “in that they both have to move and adhere to a surface at the same time,” she says.

Lai used high-resolution videos to show that parts of a snail’s foot lift off the ground as the waves of motion travel through it. If the snail’s foot never lifted off the ground, then the animal would need the special mucus to achieve enough force to push itself across a horizontal surface.

But if, a part of the snail’s foot lifted up as the waves traveled through it, then the animal could produce enough thrust to push itself forward even without the special physical properties of the mucus.

Lifting part of its foot reduces the amount of friction the snail has to overcome to move. This would be similar to a caterpillar, which lifts the middle part of its body up and stretches forward as it moves.

Lai tracked the movement of snails on a horizontal glass surface, using a high-resolution camera placed underneath. She measured the back-and-forth waves of muscle contractions by tracking the movement of distinctive speckles on the foot. She also used a laser to measure the distance that the foot waves moved up-and-down off the glass surface.

To measure forces, she placed the snails on a gel and measured how the gel deformed as the snails moved across it. She already knew how much force it took to deform the gel, so she was able to calculate how much force it took to produce the observed deformation.

Based on their new measurements, the researchers found that the snails didn’t require the special mucus to travel horizontally. The lifting of the snail’s foot as the waves traveled through it produced enough force to propel the animal even without the slime. The slime’s adhesive ability still plays a crucial role, however, in allowing the animal to crawl upside down and up vertical surfaces.

Apart from changing how snail locomotion is viewed, the work has practical applications as well.

Other research groups are making robots that, like snails, can move on vertical walls and upside down, and researchers from Tohoku University in Japan are building an endoscope, a tool that doctors use to look inside the body, that would move like a snail.

Snail-like robots are less complicated to build as “there are no legs sticking out,” Lai says, and their crawling motion allows them to traverse a wide variety of surfaces.

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