Squids feel slimy and smooth to the touch because of a mucus film, but under a special microscope, researchers found hundreds of sensory “hairs” on their skin. Even more surprising, Brian McDermott, an associate professor at the Case Western Reserve School of Medicine and the study’s lead author, believes squid skin can offer insights into human hearing. Although scientists were familiar with these sensory cells, they didn’t know how much of the squid skin they cover.
Hair Bundles Impart Hearing
Human ears detect sound in a fluid-filled, snail-shaped organ called the cochlea. When sound vibrates the eardrum, three bones, the malleus (hammer), incus (anvil), and the stapes (stirrup), amplify the vibration. The stapes pushes and vibrates a membrane called the oval window, which covers the cochlea’s opening. This movement creates ripples in the cochlear fluid that bend the stereocilia, or microscopic hairs, of the hair cells. These hair-like structures are arranged in bundles. This triggers an electrical nerve impulse along the auditory nerve to the brain, where it recognizes the sound.
When a child is born deaf, or a person loses their hearing, their hair bundles are likely damaged. Studying squid skin’s hair bundles is analogous to humans’, making it a promising model for understanding how hearing loss occurs. Like us, squid hairs detect movement in the surrounding water as it brushes past them.
Laser Maps the Squid’s Hidden Sensors
McDermott’s team, including graduate and undergraduate students, conducted part of the research focused on how squids hear. Using a special microscopy technique that uses lasers to create a sheet of light to illuminate squid tissue, the scientists constructed detailed 3-D images without causing much harm to the tissue. This technique, known as light sheet microscopy, revealed previously unknown hair cells and let the team map the sensory network across the body. They chose longfin squid (Doryteuthis pealeii) babies for the study because their genome has been fully sequenced.
The hairs on their skin can be seen as a counterpart to human stereocilia (sensory hairs) that register water movement, rather than the sounds our ears specialize in. Fish lateral lines also have hair bundles, but they don’t vary in length like those of squids, making cephalopods an even better model for studying human hearing.
An Example of Convergent Evolution
Squids and humans last shared a common ancestor 500 million years ago, then evolved along different paths. However, to live in a fluid environment (air or water), organisms must track vibrations and mechanical forces, leading both to develop sensory hairs independently. When unrelated organisms develop similar features to survive in similar environments, scientists call it convergent evolution. The hair bundles also vary in length in both humans and squids, making the comparison even more plausible. Fish also possess hair bundles, but they are all of the same length. In the human cochlea, taller bundles detect low pitches, and shorter bundles detect high pitches. The hair bundles are tuned to detect different sound frequencies.
McDermott’s team interprets this variation in length as necessary to detect different frequencies of water movement—the head and arms have taller hairs to detect subtle cues like twitching prey. The squid’s mantle is streamlined for rapid swimming and has shorter hairs to detect high-frequency ripples in fast-moving water. In that sense, a squid’s skin may function somewhat like an ear.
The diverse population of tiny hair bundles across a squid’s body can offer clues about how it senses its surroundings, and probably how damage to these fragile sensory structures affects hearing.
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