Penn Researchers Shed Light on the Roundworm’s Curious Swimming Behavior

The round worm Caenorhabditis elegans, a nematode, is a puzzling creature.

A previous study at the University of Pennsylvania established that, in some cases, these nematodes are actually counter-current and swim upstream rather than with the flow of liquid as a result of hydrodynamic forces. Another study indicated that they tend to accumulate next to surfaces.

Now, a new study published in the Journal of the Royal Society Interface finds that, rather than settling to the bottom of a pool of water as one would expect for an animal heavier than water, the nematodes seem to “swim happily” through the liquid.

Using two microscopes, one that observed from above and the other from the side, the researchers found that, despite their slow pace and inability to develop lift, the nematodes manage to remain essentially suspended above the bottom of the vessel through constant collisions with it. By continuously bouncing against the bottom, they can maintain their swimming gait.

Nematodes are broadly used in medical science to study the relationship between genotype and phenotype, to study simple neural circuits of behavior and to study the aging process, among other uses. The results of this current study could give researchers a means to sort nematodes based on their ability to swim, which depends on their genes, nerve cells, age, reaction to drugs, environmental conditions and many other factors.

Being able to sort these animals by certain traits allows researchers to screen hundreds or thousands of them for a specific behavior or trait in order to determine which genes are responsible for it.

The research was led by Haim Bau, professor in the Department of Mechanical Engineering & Applied Mechanics in the School of Engineering and Applied Science; alumnus Jinzhou Yuan; and graduate student Hungtang Ko. David Raizen, associate professor in the Department of Neurology in Penn’s Perelman School of Medicine, also contributed to the study.

The findings led to the idea that, since the nematodes interact closely with the bottom of the vessel and if the vessel has some kind of a topology or pattern, the animal will have to follow that pattern.

“This is somewhat unusual for swimmers,” Bau said. “You can imagine if you're swimming in the ocean and there is a trench, you won't dip into the trench and come out; you'll simply glide over it. But the same is not true for C. elegans. They will actually go to the bottom of the trench and try to climb upwards.”

To investigate this, the researchers used an inclined conduit to direct the animal to stay in the direction of steepest ascent or descent. They found that the velocity of the animal decreased when it was going uphill and increased when it was going downhill. Since the gravitational force is known, this provides a relatively easy way to estimate the swimming thrust of the animal.

The researchers could also modify the animals to fluoresce, enabling them to monitor the nematodes’ swimming to get some idea about the topology, or structure of the surface.

The researchers then attempted to sort the nematodes based on their ability to negotiate an inclined conduit and overcome gravity. They constructed a device containing a holding chamber where they put a large population of nematodes. The nematodes would attempt to climb the inclines. The ones able to negotiate the incline would fall into a collection chamber, while the weaker ones would remain in the holding chamber.

By repeating this process and isolating the abler nematodes, the researchers are able to enrich a particular population with a desired property, in this case thrust, and then subject them to genetic analysis to identify the genes that are responsible for their exceptional ability.

This could have applications in identifying drug resistance. The nematodes that are more susceptible to drugs are weaker and therefore less likely to escape the holding chamber. By collecting the drug-resistant ones, researchers can subject them to genetic analysis to try to identify which genes instill the drug resistance.

The researchers also realized that they would be able to force the animal to move in a desired direction.

“To demonstrate that, we fabricated a ratchet, which included a sequence of inclined planes arranged in a torus,” Bau said. “The teeth of the ratchet had two different inclination angles: one relatively mild and the other quite steep.”

The animals would go up the mild incline, drop to the pit between two teeth and then climb the next mild incline. This gave the researchers a sort of “toy” that allowed them to force the animals to move in one particular direction.

​​​​​​​​​​​​​​​​​​​​​The ratchet, Bau said, “goes a little bit more into the science-fiction category.”

If researchers can force the animals to move in the way they would like rather than randomly, the nematodes may be able to carry cargo or generate work or convert energy. For instance, if the scientists designed a ratchet that rotates, the nematodes would be able to push the ratchet instead of climbing over the inclines.

This research is part of a bigger sequence of studies that focus on the swimming behavior of nematodes. The next steps are to investigate a question that arose throughout the course of this research: whether nematodes can sense the direction of gravity.

“We would like to identify whether they sense gravity, and, if so, what is the mechanism that allows them to do this,” Bau said. “This is a very fundamental question and, as far as I know, nobody has the answer.”

This research was supported, in part, by the National Institutes of Health’s National Institute on Aging.

Haim Bau