Caenorhabditis elegans can sense and respond to sound, despite having no ear-like organs, according to a new study from the University of Michigan Life Sciences Institute. The findings, published today in Neuron, offer a new biological tool for studying sensory biology.
Shawn Xu and his team have been using Caenorhabditis elegans to study sensory biology for more than 15 years. When his lab began this work, these millimeter-long worms were thought to have only three main senses: touch, smell, and taste. Xu's lab has since established that worms have the ability to sense light, despite having no eyes, as well as the ability to sense their own body posture during movement.
"There was just one more primary sense missing—auditory sensation, or hearing," said Xu, the study's senior author. "But hearing is unlike other senses, which are found widely across other animal phyla. It's really only been discovered in vertebrates and some arthropods. And the vast majority of invertebrate species are thus believed to be sound insensitive."
The scientists discovered, however, that worms responded to airborne sounds in the range of 100 hertz to 5 kilohertz—a range broader than some vertebrates can sense. When a tone in that range was played, worms quickly moved away from the source of the sound, demonstrating that they not only hear the tone but sense where it's coming from.
The researchers conducted several experiments to ensure the worms were responding to airborne sound waves, and not vibrations on the surface worms were resting on. Rather than “feeling” the vibrations through the sense of touch, Xu believes the worms sense these tones by acting as a sort of whole-body cochlea.
The worms have two types of auditory sensory neurons that are 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, which then translate the vibrations into nerve impulses. And because the two neuron types are localized in different parts of the worm's body, the worm can detect the sound source based on which neurons are activated.
"This opens a whole new field for studying auditory sensation, and mechanosensation as a whole," he said. "With this new addition of auditory sensation, we have now fully established that all primary senses are found in C. elegans, making them an exceptional model system for studying sensory biology."