Earless worms respond to sound through their skin

Common model species can sense sound waves without ears, offering a new tool to study auditory sensation.


Earlier, Caenorhabditis Elegans species of roundworm- was believed to have three main senses: touch, smell, and taste. Despite having no eyes, they can sense light. Also, they can sense their body posture during movement.

The species doesn’t have any ear-like organs. Still, they can sense and respond to sound, suggests a new study from the University of Michigan Life Sciences Institute.

The study offers a new biological tool for studying the genetic mechanisms underlying the sense of hearing.

Shawn Xu at the Life Sciences Institute said, “There was just one more primary sense missing—auditory sensation, or hearing. But hearing is unlike other senses, which are found widely across other animal phyla. It’s only been discovered in vertebrates and some arthropods. And the vast majority of invertebrate species are thus believed to be sound insensitive.”

During the study, scientists played a tone with the range of 100 hertz to 5 kilohertz. They found that worms quickly moved away from the source of the sound. It means the earworms do not only hear the sound but also sense where it’s coming from.

Through several experiments, scientists ensured that worms are responding to sound, not vibrations.

Instead of feeling the vibrations through the sense of touch, scientists believe that the worms sense these tones by acting as a sort of whole-body cochlea, the spiraled, fluid-filled cavity in the inner ear of vertebrates.

Scientists found two types of auditory sensory neurons tightly connected to the worms’ skin. When sound waves bump into the worms’ skin, they vibrate the skin, which in turn may cause the fluid inside the worm to vibrate in the same way that fluid vibrates in a cochlea. These vibrations activate the auditory neurons bound to the worms’ skin, translating the vibrations into nerve impulses.

Both neurons are situated in different parts of the worm’s body; the worm can detect the sound source based on which neurons are activated. This sense may help worms detect and evade their predators, many of which generate audible sounds when hunting.

Xu said, “Our study shows that we cannot just assume that organisms that lack ears cannot sense sound. It raises the possibility that other earless animals with a soft body like the roundworm C. elegans—such as flatworms, earthworms, and mollusks—might also be able to sense sound.”

Journal Reference:

  1. Adam J. Iliff et al. The nematode C. elegans senses airborne sound. DOI: 10.1016/j.neuron.2021.08.035
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