Woods Hole (Bernd F. Laeschke – November 2010): Ordinary squid may provide clues about the origin and evolution of the sense of hearing. Little is known about how well squid hear and whether they rely on hearing to navigate, sense danger, and communicate with each other. Scientists have now learned that the squid hearing system has some similarities and some differences compared to human hearing.
Squid have a pair of organs called statocysts, balance mechanisms at the base of the brain that contain a tiny grain of calcium, which maintains its position as the animal maneuvers in the water. Each organ is a hollow, fluid-filled sac lined with hair cells, like human cochlea. On the outside of the sac, the hair cells are connected to nerves, which lead to the brain. The calcium grain, called a statolith, enables the squid to sense its position in the water, based on which hair cells it touches at a given moment.
When a squid moves quickly as it does when it flees an approaching predator, the calcium stone lags behind slightly before catching up to the hair cells. “Kind of like your stomach on a roller coaster,” says T. Aran Mooney, a biologist at the Woods Hole Oceanographic Institution (WHOI). “The hair cells are very sensitive and can detect the calcium statolith lagging behind, then catching up.”
According to Mooney, who began his hearing research while working on his Ph.D. at the University of Hawaii, the squids hearing organs are structurally analogous to auditory system of humans. He believes that the organs are on its way to becoming an ear like the more familiar organs of vertebrates.
In a serious of experiments, the scientist did lower squids into a shallow, 3-foot-wide tank. Also in the tank is a speaker that can emit a broad range of sound frequencies - pure tones repeated about 1,000 times for each frequency. The responses were recorded and the preliminary findings indicate that the squid actually can hear. “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,” Mooney says. “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 hear well enough to get out of the way of approaching predators. However, squids have another defense mechanism: when the researcher put them in a CT scanner, he noticed that squid have almost the same density as water. The CT could not image the squid body, illustrating that the species is nearly transparent to sound. This makes them very difficult for echo locating predators to detect.
“It’s been suggested that a primary evolutionary drive behind hearing is to locate where the sound source is,” Mooney said. “If there’s a predator coming you’d better darn well know where that predator is coming from so that you can get out of the way.”
He also thinks squid hearing organs can tell scientists a lot about how ears originated and evolved. “Humans, fish, and lots of animals use hair cells to detect sound and movement. Their hair cell structures are similar to squid, but also quite different. There is probably a basic structure which evolved millions of years ago, but vertebrates and invertebrates have taken quite different evolutionary paths since. By learning more about squid hearing and squid hair cells, we might learn what is important in human hearing and human hair cells, or other animals for that matter.”