Squid have sensory hair cells across their bodies that detect water movement, revealing surprising similarities to how the human ear senses sound.
A new study has found that a squid’s entire body may work a little like an ear. Scientists have discovered sensory cells covering the animal’s entire surface, tuned to feel water movement.
Since these cells resemble the ones that let humans hear, the finding could shed light on how hearing works, and how it fails.
The research was led by Brian McDermott, an associate professor at the Case Western Reserve School of Medicine.
Scientists have known for years that squid have hair cells on their heads and arms. These cells carry bundles of tiny, hair-like projections.
But nobody knew what the rest of the body had. How does a squid sense water motion along its mantle, the main body?
“Squid are cephalopods with a diverse population of hair cells on the surface of their bodies, which may yield insights not only into how these fascinating animals detect water movement to survive, but also into how hearing and deafness occur in humans,” McDermott said.
The team studied the longfin squid, Doryteuthis pealeii. They chose it because its genome has been sequenced and it is genetically tractable.
They worked with free-swimming baby squid, called paralarvae.
The researchers produced the first full-body map of squid lateral lines – rows or fields of hair cells along the skin.
Fish have lateral lines too, running along the head and body. In squid, they had only been described in detail on the head and arms.
The new map shows something far more elaborate.
Hair bundles cover the whole animal. They form dense lines in some places and broad fields in others.
The mantle alone has four distinct regions: the collar, the body, the fins, and the tip at the rear. Even the siphon, the tube squid use to jet through water, carries hair bundles.
Yet some spots – like the area in front of the siphon – stay nearly bare. The researchers suggest this may keep the sensors from being overwhelmed when the squid jets.
The scientists found the cells using light-sheet microscopy: a laser that forms a thin sheet of light illuminating one plane of the specimen at a time.
That let them build detailed 3D images while minimizing damage to the tissue.
In humans, sound vibrations travel to the cochlea. This snail-shaped organ in the inner ear is lined with thousands of hair cells, each one carrying a bundle of tiny projections called stereocilia.
Those bundles are taller where low pitches are detected and shorter where high pitches are detected. That helps tune each cell to a particular frequency of sound.
The vibrations set the projections in motion. The hair cells connect to the nervous system and send messages to the brain.
There, the signals become meaningful sounds.
Sea creatures have similar bundles on their bodies. Their survival seems to depend on sensing different kinds of water movement.
It works much like the way our ears sort pitches. The researchers concluded that squid skin acts a bit like a human ear.
This is where squid differ. Fish lateral lines have hair bundles that do not vary in length.
Squid seem to adjust bundle length to tune their hair cells to different frequencies, much like the human ear. The measurements back this up.
Hair bundles on the mantle collar had very short projections, about 2.8 micrometers long on average. Those on the head were about five times longer, at 14 micrometers.
The collar sits at the edge of the mantle, where water flows in during breathing.
The researchers think shorter bundles may suit that faster flow, while slower movement over the head and arms may call for longer bundles.
The team also traced where these cells connect. Nerve fibers from the mantle hair cells run to the stellate ganglion – a cluster of nerve cells that controls the mantle muscles.
The signals travel along giant axon pathways, which carry messages quickly.
The researchers also confirmed that these were true sensory hair bundles. Baby squid are covered in short-lived cilia that fall away within a day or two of hatching.
While those cilia have no sensory role, the hair bundles persist into adulthood.
All of this makes squid a promising model for studying human hearing. The cells that let us hear are fragile, and their bundles are often where trouble starts.
“Often, when a child is born deaf or a hearing person loses their hearing, it is the hair bundle that has been damaged,” McDermott said.
“So, studying the squid’s hair bundle holds promise for understanding how hearing loss occurs.”
The study is published in the journal Current Biology.
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