To sense sound, this earless organism relies on a mechanism found across many species, new study finds

The new findings began with an observation that the size of the speaker they used to test auditory sensation seemed to affect the worms’ responses— even when the sound frequency and intensity remained the same.

“We were modifying the speaker diameter during a behavior experiment, when I discovered that large speakers no longer caused the worms to avoid sound,” recalls Can Wang, a researcher in the Xu lab and one of the study’s lead authors. “This sudden loss of response was unexpected, as I had ensured the sound intensity reaching the worm was identical regardless of speaker size.”

To figure out why the worms weren’t responding, the team tested sounds emitted from different sizes of speakers. While the worms moved away from sounds from the smaller speakers, they showed no response when sounds came from the larger speaker.

“The first time we saw these results, it was quite surprising,” says Xu, who is also a professor of molecular and integrative physiology at the U-M Medical School. “Common sense may lead us to think that the volume, or the sound intensity, would be the determining factor for sensing sound. If it’s too soft, we wouldn’t hear it. But our findings show that it’s not that simple.”

The researchers speculate that the worms may have evolved to sense sounds from smaller sources based in part on the size of their natural predators, which are primarily small animals such as insects and centipedes. 

“They don’t necessarily need to be attuned to larger sound sources, because large animals are not their immediate threat,” Xu says. “Sensing all that background noise would use too much of their energy, so it makes sense that they would evolve to respond only to sounds that are relevant to their survival.”

The effects of the varying speaker sizes are tied to the way the sound waves exert pressure on the worms’ bodies. C. elegans sense sound waves by behaving much like a whole-body cochlea, the fluid-filled spiral in the inner ear of vertebrates. Sound waves vibrate the skin of the worms — themselves a fluid-filled tube — and activate auditory neurons underneath the skin.

But to have any effect, the sound waves must come from a source that is localized enough to create a pressure gradient. If uniform pressure surrounds the worm, such as pressure coming from too large of a source, the liquid will not be able to flow no matter how much pressure is applied. 

“Imagine trying to apply pressure to a water balloon,” Xu explains. “If all the pressure is equal on the entire surface of the balloon, you won’t see any movement because you cannot change the absolute volume of liquid.”

But if the sound waves hit the worm starts more intensely in one localized spot, the change in pressure can trigger vibrations and, ultimately, an auditory response. 

This mechanism of sensing sound as a pressure gradient is found in more complex insect species such as katydids, and even in the cochlea of mammalian inner ear, demonstrating a phenomenon called evolution convergence.

Rather than sharing a common sound-sensing ancestor and maintaining that trait while evolving across generations into separate species, the researchers say these species all developed sound sensation independently — yet arrived at the same basic sound-sensing mechanism.

“We've potentially found evolutionary convergence of a mechanism of auditory sensation from worms to mammals, which is really exciting,” says Elizabeth Ronan, Ph.D., another lead author of the study. “But it is also an example that sometimes what we may think of broadly as one sensation can actually sometimes be broken down into even more specific details or components that we can investigate to learn more about what neurons are actually sensing.”