Can Worms Hear?

“Can worms hear?” is an age-old question, one Darwin attempted to answer in the 1800s by having his son serenade earthworms with a bassoon and seeing if they wriggled away. Darwin’s answer: no. But new research suggests otherwise.

While other complex senses, such as vision, are widespread in the animal kingdom, so far, hearing has been found only in vertebrates and some arthropods. Almost all hearing animals rely on an organ that vibrates when sound waves hit it, firing neurons associated with processing sound. In humans and most other vertebrates, that’s our ear, comprising a delicate eardrum and inner ear.

But C. elegans, a tiny worm that’s ubiquitous in biology research, doesn’t have a specialized hearing organ. Instead, new experiments have revealed, its skin doubles as a sound-sensing membrane, effectively making the worm’s entire body an eardrum. This study, detailed recently in the journal Neuron, presents the first evidence ever found that a non-arthropod invertebrate can sense airborne sound.

The results come from more than a decade of targeted research led by Shawn Xu’s lab at the University of Michigan. Building on others' work that found the one-millimeter worms could smell, taste, and touch, the team uncovered evidence that the worms also had the senses of proprioception—the so-called sixth sense of body awareness—and light detection."(Read how superhuman hearing may someday be possible.)

“And since then, there’s only been one thing missing, and that is the auditory sensation,” says Xu, a sensory biologist. “We’ve been spending all these years searching for this one.”

The discovery, he says, presents a big leap in our understanding of both how organisms can hear and how hearing may have evolved. It also could expand the search for hearing in more organisms that lack obvious ears, such as mollusks and other worms (including Darwin’s earthworms) and shed light on animals whose hearing capabilities scientists are still deciphering, like some salamanders and “earless” frogs.

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