Based on the fact that squid are heavily predated in the sea, a study in the US has chosen the Longfin squid (Loligo pealeii) to look at how hearing has evolved.
Predator avoidance is a key pressure for evolving hearing capabilities as if you can hear your predators approaching; you have a better chance of avoiding them.
The team from Woods Hole Oceanographic Institution (WHOI) analysed impulses from a pair of structures known as a statocysts found in the base of the squids’ brain which act in a similar way to the auditory system in higher organisms. Electrodes were attached just under the skin near to the nerves from the statocysts to see if they responded to sound. The squid was then placed into a tank where sounds at different frequencies were repeated 1,000 times each and the responses recorded. This hearing test method is similar to those used to checking hearing in human babies.
T. Aran Mooney the principle scientist in this study said that preliminary findings indicated that 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 which may explain why they are a favourite food source.
Mooney now plans to conduct studies to establish whether squid rely on sound to interact, migrate and communicate.
Mooney also thinks squid statocysts 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.
"Down the road, squid ears and hair cells might be models for examining human hearing. But that's just speculative right now. We need to learn more about the basic functioning of squid ears first.
For more information see: T. A. Mooney, R. T. Hanlon, J. Christensen-Dalsgaard, P. T. Madsen, D. R. Ketten, P. E. Nachtigall. Sound detection by the longfin squid (Loligo pealeii) studied with auditory evoked potentials: sensitivity to low-frequency particle motion and not pressure. Journal of Experimental Biology, 2010; 213 (21): 3748 DOI: 10.1242/jeb.048348