Shh . . . they can hear us

A study on hearing in squids has surprising results that may be useful in treating human hearing loss some day

by Josh Dehaas

Shh . . . they can hear us

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In the basement of the Woods Hole Oceanographic Institute in Woods Hole, Md., researcher Aran Mooney spent much of the last year lowering squid into metre-long tanks, attaching electrodes to them and blasting noise through the water. “Squids are fun little animals to work with because they’re so basic and primitive,” he says. “They’re almost like little wind-up toys. If you put one in a tank, it will just keep swimming and hitting its head on the wall of the tank over and over again.”

Primitive they may be, but Mooney’s research has settled the debate over whether Loligo pealii (think calamari) are sophisticated enough to hear. For decades, marine biologists wondered about that, but no one knew of a sedation method that could keep the animals alive long enough for in vivo tests to prove it. Squid don’t respond to dolphin clicks, so it was assumed they could not hear at all. It turns out dolpin clicks are just at the wrong frequency.

Squid may not be as good at hearing as humans (who can hear up to 20,000 hertz), but Mooney has shown they can detect low frequencies (up to 500 hertz) like the wave of a hungry whale swimming at them. And although the squid “ear” doesn’t likely share a common ancestor with our own, it works similarly enough that Mooney believes the research can have human applications. (Squid are already used to research human neurology, simply because they have large, primitive structures.) Mooney says hair cell loss is a key reason we as humans lose hearing. “We could look in squid and maybe find a way to maintain or regenerate them,” he says.

Mooney’s first breakthrough was in sedating cephalopods for up to five hours. To test squid hearing, he sedated his samples and attached electrodes just under their skin near a sac-like organ called the statocyst. Then, he played a range of tones 1,000 times each and recorded the electrical response. The result? The nerves attached to the statocyst lit up when he played low frequency sounds.

The statocyst, explains Mooney, is like an inside-out tennis ball, with tiny hairs facing inward. A small grain of calcium gets knocked forward or backward, up or down, when sound waves hit the squid’s body. The location of the hairs that bend tells the squid which direction the noise is coming from.

Mooney decided to study squid after his graduate work showed that U.S. Navy sonar could potentially deafen dolphins or cause them to change migration routes. His new theory is that low-frequency sounds from shipping and oil and gas production could send the squid in unnatural directions too. Sound pollution has been a growing concern for marine biologists, as human-made ocean noise has doubled every decade over the past 50 years, according to the San Diego, Calif.-based Scripps Whale Acoustic Lab. A new study by Cornell University’s bioacoustics research program found that in Cape Cod Bay, right whales’ ability to communicate has been severely reduced due to noise.

Knowing whether squid are being affected by ocean noise matters. Squid make up an enormous biomass in the ocean and are food for everything from dolphins to humans. “They’re a keystone species,” says Mooney. “If you remove the squid, you drastically change the whole ecosystem.”

Ironically, Mooney says his next research could be used to do just that. He plans to find out how well the human-sized Humboldt squid can hear. The Humboldt is an invasive species from the south Pacific that has been moving north as far as Alaska and decimating fisheries off the coast of California along the way. If it can hear, he says, underwater noise may be an effective deterrent to its northward march.




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