Shhhh! The squids are listening

Squids can hear and their primitive ears can help us better understand how human hearing works.

The ordinary squid, Loligo pealii, is well known as a kind of floating buffet. “Almost every type of marine organism feeds somehow off squid,” said biologist Aran Mooney from the Woods Hole Oceanographic Institution. Not just fish, but also many birds, seals, sea lions, dolphins, and whales depend heavily on squid.

Researchers attaching electrodes to the sedated squid

Yet, despite their importance in the food web, it is amazing how little we know about the squid – what senses they rely on to navigate, avoid danger, find food, and communicate with each other. Scientists know that fish and other vertebrates can hear, but it wasn’t clear whether the squid could even detect sound.
Finding this out isn’t exactly trivial; it’s not as if you can just play a tone and see if a squid raises a tentacle in response. The researchers had to develop techniques to learn if, and what, squid can hear. In a study published in The Journal of Experimental Biology, they confirmed that squid can indeed detect sound. Now, they have begun to investigate the rudimentary sensory organ to see what it can tell us about hearing in higher animals.


A primitive ‘ear’?
In humans ears the sound waves strike the eardrum and set it vibrating. An array of tiny bones in the middle ear amplifies the vibrations and sends them rippling through fluid in a spiralling tube called the cochlea. The cochlea is lined with hair cells. On one end, they have  hair-like structures that project into the fluid in the cochlea; on the other end, the cells connect with nerve fibres outside of the cochlea. The vibrations generate movement of the fluid, which bends the hairs and triggers nerve impulses.
Squid have two organs called statocysts, which have similarities with cochleas. Located near the base of the brain, each statocyst is a hollow, fluid-filled sac lined with hair cells. On the outside of the sac, the hair cells are connected to nerves, which lead to the brain. “It’s like an inside-out tennis ball,” Mooney said, “hairy on the inside, smooth on the outside.”
Inside each sac is a tiny grain of calcium carbonate called a statolith. It enables the squid to sense its position in the water. Structurally, the statocyst “is analogous to our auditory system” and is likely an evolutionary step toward higher-order hearing organs, said Mooney.

Can you hear it now?
The scientists used a technique similar to hearing tests routinely conducted on newborns, who, like squid, can’t readily affirm that they’ve heard something. In those tests, electrodes are placed on the infants’ heads, and sounds are presented to them through earphones. The sounds stimulate electrical activity in nerves that is recorded by the electrodes.
It wasn’t easy to attach electrodes to a squid so the researchers developed a noninvasive method to anesthetise squid for up to five hours. The sedation enhances the potential for squid to become a valuable “lab rat” to help us learn more about  human hearing.
The findings showed that statocysts respond to sounds, but only to relatively low frequencies, up to 500 Hertz; they do not detect the high frequency sounds that dolphins use to find prey. (Human hearing spans a range from about 20 Hz to 20,000 Hz.)
Mooney thinks the statocysts have a lot to teach us about how ears evolved. “Humans, fish, and animals use hair to detect sound and movement,” he said. “Their hair cell structures are similar to squid, but also different. There is probably a basic structure that evolved millions of years ago, but we have taken different evolutionary paths since.”
“By learning more about squid hearing and squid hair cells, we might learn what is important in human hearing,” he said.
“We need to learn more about the basic functioning of squid ears first, but down the road, squid ears and hair cells might be models for examining human hearing.”

Article taken from The Bangalore Mirror