Fish apparently use their gas-filled swim bladders to sense the pressure changes associated with sound waves, so can hear relatively well. Cephalopods don’t have gas-filled chambers, which has tended to suggest that they could not hear or at least not well.
However Hong Young Yan of the Taiwan National Academy of Science in Taipei has been looking at another organ, the statocyst. This is apparently a sac-like structure containing a mineralised mass and sensitive hairs. Previous research has shown that fish also use this to detect sounds and so do prawns.
Yan looked at the common octopus Octopus vulgaris and the Bigfin reef squid Sepioteuthis lessoniana. He found that the octopus hears sound between 400 and 1000Hz, whereas the squid can hear from 400 up to 1500Hz. This discrepency could well be explained by the environment in which they live. Squid tend to be free swimming, whereas octopus live along the seabed among rocks and coral. Sounds above 1000Hz would have a wavelength of over 1.5 metres, which would mean they would be deflected by rocks and coral, making them irrelevant to octopus. Both species hear best at frequencies of 600Hz.
The next question is to find out what they are listening for. Is it sounds of predators, or of prey, or are they also making sounds to allow them to communicate amongst themselves.
The photos on this article as well as a full story are on the BBC website.