FunNature & AnimalThese earless worms listen through the skin

These earless worms listen through the skin

The Caenorhabditis elegans worm is a nematode widely used in biological research and, traditionally, it was thought to have only three main senses: touch, smell, and taste. Long ago, it was discovered that these animals also have the ability to feel light despite the lack of eyes, and they have also been shown to have a sense of proprioception (feeling one’s body posture during movement).

“Only one more primary sense was missing: auditory sensation or hearing,” explains Shawn Xu, a researcher at the University of Michigan and lead author of the study. “But hearing is different from other senses, which are widely found in other phyla animals. Actually, it has only been discovered in vertebrates and some arthropods. Therefore, the vast majority of invertebrate species were believed to be insensitive to sound “.

However, the results presented in this new study published in the journal Neuron indicate that these roundworms can detect and respond to sounds. Specifically, the scientists found that the worms responded to sounds in the air in the range of 100 hertz to 5 kilohertz, a wider range than some vertebrates can feel. When a tone is played in that range, the worms quickly move away from the source of the sound, showing that they not only hear the tone, but also perceive where it is coming from.

A full-body cochlea

The researchers conducted several experiments to make sure that the worms responded to airborne sound waves and not to vibrations at the surface where they rested. Rather than “feel” the vibrations through touch, Xu believes that worms perceive these tones by acting like a kind of whole-body cochlea, the fluid-filled spiral cavity in the inner ear of vertebrates.

Worms have two types of auditory sensory neurons that are closely connected to the skin of worms. When sound waves hit the skin of worms, they vibrate the skin, which in turn can cause the fluid inside the worm to vibrate in the same way that fluid vibrates in a cochlea . These vibrations activate the auditory neurons attached to the skin of the worms, which then translate the vibrations into nerve impulses.

And because the two types of neurons are located in different parts of the worm’s body, the worm can detect the sound source based on which neurons are activated. This sense can help worms detect and evade predators, many of which make audible sounds when hunting.

Our study shows that we cannot simply assume that organisms without ears cannot sense sound, ” explains the expert.

While the auditory sense of worms has some similarities to the functioning of the auditory system in vertebrates, this new research reveals important differences regarding how vertebrates or arthropods perceive sound.

“Based on these differences, which exist down to the molecular level, we believe that the sense of hearing has probably evolved independently, several times in different phyla of animals,” Xu said. “We knew that hearing looks very different between vertebrates and arthropods. Now, with C. elegans , we have found a different pathway for this sensory function, indicating a convergent evolution. This is in stark contrast to the evolution of vision, which, as proposed by Charles Darwin, it occurred quite early and probably only once with a common ancestor ·.

Now that all the major senses have been observed in C. elegans , Xu and his colleagues plan to delve into the genetic mechanisms and neurobiology that drive these sensations.

“This opens up a whole new field for studying auditory sensation and mechanosensation as a whole,” he said. “With this new addition to auditory sensation, we have now fully established that all the primary senses are found in C. elegans, making them an exceptional model system for studying sensory biology.”

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