Thoughts on Octopus Hearing
beccalopod;185007 said:
They most definitely can detect sound, but not the same way we do. Since sound is just vibrations, they feel it that way, the same way we can feel a very loud bass. I would just recommend not using a sub-woofer, and not putting it up so loud that things start to rattle.
They definitely feel the sound, but it is more complex than the way that humans pick up loud bass vibrations. That latter is done through touch/pressure sensing, as the vibrations become low-frequency enough to be perceived as sensate motions of tissues.
Octopuses (at least some) can feel vibration as you're describing, but that's not the only method. They also have a hearing organ — or at least an organ that seems to provide hearing along with other functions. As with mammals, hearing is connected to balance. In our case, the three-semicircular loops (one each in the X, Y and Z planes) provide sensing of motion, and we adapted that same fluid to pick up vibrations with fine hairs tuned to different frequencies. As I understand it, the octopus (like cephalopods in general) has a small chamber somewhat similar, with bits of mineral resting on fine hairs that are very sensitive to motion of the central piece.
When the octopus moves, the pressure on those hairs change, giving it a very good sense of acceleration. Orientation, too, is well maintained; the octopus uses this organ to keep its eyes aligned horizontally, so that it can properly judge the polarized light. (This is similar to your polarized sunglasses; when you tilt your head, the light looks "wrong.")
Apparently, at least for some species and some frequency ranges, the octopus can use its statocysts to sense sound vibrations through these same hairs. Those frequencies have been measured around the 600Hz range for some octopuses.
This understanding is relatively recent. Older papers, such as this one from 1960, suggest that octopuses might be deaf:
http://jeb.biologists.org/content/37/4/845.full.pdf
This assumption held true for decades, and at least as late as 1987 they still weren't sure. A paper that year was entitled "Why Cephalopods Are Probably Not Deaf":
Why Cephalopods are Probably Not "Deaf" on JSTOR
The above abstract notes that "hearing" versus "vibration sensing" is not a meaningful difference in water. We define "hearing" in mammals operationally; that sense conducted by the eighth cranial nerve. We don't yet know enough to do this for cephalopods.
But we continue to learn more, and even by the time of the above article they had good clues. In 1988, they had taken the expedient of dissecting out live octopus statocysts, hooking them up to meters, and seeing if they reacted to vibrations, and could discern something about the amplitude and frequency. They did and could:
http://jeb.biologists.org/content/134/1/451.full.pdf
The paper describes the sensitivity of the hairs as being similar to other invertebrates (like crayfish) but nowhere near as sensitive as vertebrates such as fish. Nevertheless, they recognized that they weren't able to measure the primary nerve output, and by this time it was known that octopuses could still react to vibrations even with the statocysts removed.
Later testing showed the greatest sensitivity for one reef-dwelling species around 600Hz, which makes sense in its environment. Another did not respond to this frequency, but responded strongly (by holding its breath for almost a minute!) at frequencies in the low bass range (50-150Hz):
http://www.jstage.jst.go.jp/article/jmasj/34/4/34_266/_article
Some squid species hunted with sonar don't seem to respond to those sonar frequencies, which seems a little surprising.
Incidentally, many octopuses have reported responses at frequencies as low as 50Hz, which is the sort you'd expect to be induced by loud bass speakers. And the experiments are often done with speakers operating in the air near the tank, not directly in the tank itself. In other words, the original poster's speakers might be a problem for the animal, but it seems to me that at reasonable volumes this might not be an issue.
Since the statocyst is moderately deep within the creature's head, surrounded by the animal's flesh, it is unsurprisingly not as sensitive as that of creatures whose hearing organs are closer to or at the surface. Also, it means that larger animals (even larger ones of the same species) might suffer from gradual hearing loss just from the ever-growing amount of tissue (mostly cartilage and muscle) between the statocysts and the surface.
I use this assumption in my novel, but I've seen no research testing it, nor have I seen the speculation anywhere else.
The nautilus, which has been evolutionarily separate from other cephalopods for a long time, has a slightly different statocyst design, which this paper describes (and tests some abilities of):
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692077/