Most cephalopods lack gas-filled chambers within their bodies, such as the swim bladders that fish can use to hear. That suggested they could not detect the pressure wave component of sound.

However, sensory physiologist Hong Young Yan of the Taiwan National Academy of Science in Taipei, Taiwan suspected that octopus and squid might use another organ called the statocyst to register sound. In previous research, Yan showed that prawns can use their statocysts to hear

The statocyst is a sac-like structure containing a mineralised mass and sensitive hairs.

Yan's team discovered that the octopus can hear sounds between 400Hz and 1000Hz. The squid can hear an wider range of sound from 400Hz to 1500Hz

"That indicates that squid have a better hearing capability than the octopus," says Yan. "Interestingly though, both species hear best at a frequency of 600Hz."

The discovery could open up a new understanding of cephalopod behaviour. Compared to fish, octopus and squid do not appear to hear particularly well, they can hear as well as invertebrates such as prawns.

But the fact they can hear raises the possibility that these intelligent animals may use sound to catch prey, communicate with one another or listen out for predators.

"The key question which I would like to investigate is what kind of sounds are they listening to?" says Yan. "Perhaps they listen to sound to evade predators and can eavesdrop to sounds made by their prey. Or, perhaps they even could make sounds to communicate among themselves."

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