He then lowers the squid into a shallow, 3-foot-wide tank. Also in the tank is a speaker that can emit a broad range of sound frequenciespure tones repeated about 1,000 times for each frequency. He then records the 1,000 responses to each. Averaging those 1,000 responses reduces the natural, random electrical noise in the body yielding the electrical signals, in millivolts, that occur along the nerves after each tone. This hearing test method is similar to those used to checking hearing in human infants.
His preliminary findings indicate that nerve responses showed the squid "actually do hear," he said. "But they only hear up to a certain frequency, about 500 Hz, which is pretty typical of a lot of fish that don't hear very well." Humans hear from about 20-20,000 Hz. Squid also do not detect the very high frequency sounds of dolphin echolocation clicks.
That may help explain why squid are such a prolific food source: They may not always hear well enough to get out of the way of approaching predators. But when Mooney and his post-doc advisor [name?] put the squid in a CT scanner, they found that squid may avoid predators in another way: they are almost the same density as water. That is, when squid were scanned in water, the CT could not image the squid body, illustrating that squid are nearly transparent to sound. This would likely make them very difficult for echolocating predators to detect. So, perhaps, squid could not take the evolutionary leap to adapt ears to detect very high frequencies, but being close in dens
|Contact: Media Relations|
Woods Hole Oceanographic Institution