Good question here... My thinking (as a layman) would be that they are naturally adhesive, given their shape... they are essentially suction cups, and as such, when something of that shape and texture (I assume they're kinda like cartilidge?) pushes up against a surface, the air (or water in this case) escapes but the natural recoiling of the cup that ensues causes it to attach to that surface, because the cup pulls back, yet there is no opening for water to enter back into that space... so it pulls against the surface it's attached to.
Then, because those cups are attached to the octo's muscular arm, the octo can pull its arm, and because the object is attached to the cups the object is pulled along with it... but the cups themselves are nerveless and naturally preserve their form given their composition.
That's my guess, and boy, I really wish someone else would have responded to this question...
Don't feel too bad, Tony...it's my understanding that the science of adhesion and suction is deeper and more complex than most of us realize. Check out some of the news stories floating around now about "Gecko adhesive" or "Gecko gloves." Most of 'em aren't worth the paper or electrons they're written on, but the POINT is, we don't really know how most things that stick (even tape, glue, etc.) actually stick. Gecko feet prove that it's not as simple as you'd think. They don't use glue or other secretions, they don't use hooks, they stick like crazy, and they can lift their feet any time they want.
ive always been interested in herpetology, and used to keep lizards... i remember reading a couple years ago that if the a gecko used all of its pads at once, it could hold several kilos....but of course, they roll the pads, otherwise they probably would get stuck...sticking, i think is the easy part, making the sticking useful is the trick.... but then again this has come up a couple times in the last 10 yrs (but never this widespread) so my guess is to wait another decade :|
Well...here's my halfhearted crack at explaining the gecko foot thing, as I somewhat remember reading:
Gecko feet are a lot like hook-and-loop fasteners (velcro.) Without the hooks. And the loops are microscopic, and solid, so not really loops at all, but rather just little "papillae" or "nubs." Think of your tongue with longer, taller taste buds on a microscopic scale. Hey, gimme a break, I'm tryin' here! :P
The theory is that these eensy little flexible nubbins of gecko skin are the key element, thus efforts to reproduce gecko feet in the lab have focused on mimicking the tiny nubs. The research has confirmed that these are indeed what's working. Less certain is why. Speculation is that this is molecular interaction between the skin and the surface--something on the order of hydrogen bonding or such. Or perhaps friction. The little nubs may serve to drastically increase surface area, magnifying a weak little adhesive force dramatically, and/or perhaps allowing the force to be broken easily.
The reason this is so cool isn't so much the sticking (though the sticking is quite respectable) as it is the instant on/off nature of the sticking. Slap the foot down, it's stuck...no need for glue to set or cure, etc. To remove the foot, peel up one edge of it--comes off like it's not stuck at all. The little nubs each have a terribly weak adhesion, so peeling an edge up only pulls on a few at any given time--thus they let go like nothing happened. Pull on them all at once, however (by, say, climbing the walls) and they hold fast. Thus, you've got an adhesive that's instant on/easy off and totally reusable. Sort of like post-it notes, but infinitely better.
I love stuff like this, sci-fi stuff popping up for real. Even if I'm totally off in remembering what I've read on it, I don't care...it's just too cool.
To grab a crab, an octopus draws up the centers of its suckers to create a vacuum. Octopuses have tremendous gripping power. It takes a 40-pound (18-kg) pull to release the grip of a three-pound (1.4-kg) octopus.
That's one HUGE Sucker!!! I wonder if the circle in the back of the sucker is what helps them to release at will? Seems like if it was totally like the rest of the surface, they would have a problem. Like when we try to remove a suction cup from something without first breaking the suction.
I know this thread is really OLD but I just stumbled over this thread while browsing through the archives.
Anyway, here is a more scientific description of how the suckers work.
This text was published at BioOne.org.
It goes like this :P :
Octopus suckers consist of a tightly packed three-dimensional array of muscle with three major muscle fiber orientations: 1) radial muscles that traverse the wall; 2) circular muscles arranged circumferentially around the sucker; and 3) meridional muscles oriented perpendicular to the circular and radial muscles. The sucker also includes inner and outer fibrous connective tissue layers and an array of crossed connective tissue fibers embedded in the musculature. Adhesion results from reducing the pressure inside the sucker cavity. This can be achieved by the three-dimensional array of muscle functioning as a muscular-hydrostat. Contraction of the radial muscles thins the wall, thereby increasing the enclosed volume of the sucker. If the sucker is sealed to a surface the cohesiveness of water resists this expansion. Thus, the pressure of the enclosed water decreases instead. The meridional and circular muscles antagonize the radial muscles. The crossed connective tissue fibers may store elastic energy, providing an economical mechanism for maintaining attachment for extended periods. Measurements using miniature flush-mounted pressure transducers show that suckers can generate hydrostatic pressures below 0 kPa on wettable surfaces but cannot do so on non-wettable surfaces. Thus, cavitation, the failure of water in tension, may limit the attachment force of suckers. As depth increases, however, cavitation will cease to be limiting because ambient pressure increases with depth while the cavitation threshold is unchanged. Structural differences between suckers will then determine the attachment force.