…and this is a 
Whoa, it's been a while since I've said anything about my infatuation with cephalopods (since, like, the last post…). Let's correct that with a nifty paper I found on octopus suckers.
Here's a typical view of a tangle of octopus arms, all covered with circular suckers. The octopus can cling to things, grasp prey and other objects with those nifty little discs, and just generally populate people's nightmares with the idea of all those grappling, clutching, leech-like appendages.
Octopus suckers are actually beautiful little tools, though, with a fair amount of sophistication in their organization. Don't compare them to the simple rubber suction cups on kids' toy dart guns; these have their own elaborate muscular regulatory mechanisms. This diagram illustrates the internal structure of a single octopus sucker.
Schematic cutaway diagram of an octopus sucker. A, acetabulum; AR, acetabular roof; AW, acetabular wall; C, circular muscle; CC, crossed connective tissue fibers; D, dermis; E, extrinsic muscle; EC, extrinsic circular muscle; EP, epithelium; IN, infundibulum; IC, inner connective tissue layer; M, meridional muscle; OC, outer connective tissue layer; R, radial muscle; S1, primary sphincter muscle; S2, secondary sphincter muscle.
There are two main regions, an infundibulum (IN) on the attachment face of the sucker, and a deeper chamber called the acetabulum (A) (if you don't recall any Latin, "infundibulum" just means "funnel", while "acetabulum" is "vinegar cup"—anatomy is littered with funnels and cup-shaped structures, so these are actually very generic names). Both regions are muscular, covered with a dense sheet of radial muscles (R) and rings of circular and meridional muscles (C and M).The whole thing is surrounded by supple sheets of connective tissue and epithelia.
The way it works is that the sucker is pressed against a surface, and the flexible outer margin of skin conforms to it, forming a seal. Then the radial muscles contract. Now muscle is a relatively incompressible tissue; when it contracts, it changes its length, but it cannot change its volume. When you make a muscle in your arm to show off to the girls, you are reducing the length of the bicep, so it has to bulge outwards to maintain a constant volume. This principle is also how your tongue works: when muscles contract to flatten it, the volume has to stay the same so it protrudes.
When the radial muscles in the sucker contract, the walls of the acetabulum and infundibulum get thinner. The muscle volume has to go somewhere, so the circumference of the cup-shaped acetabulum has to increase, increasing the volume of the acetabular chamber. Since the infundibulum is sealed against a surface, water can't get in; so we have the same quantity of water in a larger chamber, which means the pressure is reduced, generating suction. They can release their grip by relaxing the radial muscles, or contracting the circular muscles, which would reduce the volume of the acetabulum.
An octopus can generate a respectable amount of force with this mechanism. At sea level, they can create a pressure differential of 100-200 kPa (kilopascals; 100 kilopascals is approximately equal to one atmosphere), and at greater depths, where the water pressure is greater, they can generate correspondingly greater amounts of force.
A closeup view of the sucker reveals other details.
Scanning electron micrograph of sucker of Octopus bimaculoides/bimaculatus. The radial grooves and ridges are visible on the infundibulum (I) and the orifice that opens into the acetabulum (A) is visible. The infundibulum is encircled by a rim of loose epithelium (E) that is separated from the infundibulum by a narrow groove. The scale bar equals 1.0 mm.
The infundibulum is grooved. This allows the pressure differential to be distributed to the entire surface of the sucker as it is flattened against an object. Further, the surface of the infundibulum is covered with chitinous denticles that provide a fine network of channels that similarly transmit the force everywhere, and also provide a raspy surface that restricts lateral movement (remember how when you shot your rubber-tipped dart gun at a window it would stick, but you could easily slide the dart around? Octopus suckers wouldn't do that—they'd be locked firmly in one place.)
The authors mention that squid have an additional refinement that makes their suckers even more effective. They contain a piston-like structure inside an interior chamber, coupled so that when something tries to pull away from the sucker, it lifts the piston, further decreasing pressure inside and strengthening its grip—like a Chinese finger-trap, the more you struggle, the harder it is to get away.
Kier WM, Smith AM (2002) The structure and adhesive mechanism of octopus suckers. Integr. Comp. Biol. 42:1146–1153.
Comments
Posted by: Dave S.
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May 17, 2006 7:52 AM
*Puts ID cap on*...Since the cephalopod sucker looks designed (and it even looks like something that known intelligent designers designed), clearly that is evidence it is designed. Unless the devilutionist can specifically show step-by-step how it evolved. And we design theorists get to decide if you've done so.
Wow...it's really easy to be a design advocate, no wonder they find it so appealing!
Posted by: Dark Matter
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May 17, 2006 8:08 AM
In the cross-section there I don't see a nerve attachment...
are the suckers innervated to a ganglion somewhere?
Posted by: Torbjörn Larsson
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May 17, 2006 8:44 AM
"the more you struggle, the harder it is to get away."
That seems to be the general attitude towards cephalopods here. Not that I am complaining...
Posted by: ulg
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May 17, 2006 9:17 AM
So, if I'm understandding this article correctly, if one was attacked by an octopus, one would be quickly covered in hickeys ...
right??
Posted by: Jan Andrea
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May 17, 2006 10:15 AM
Thanks for the explanation -- very interesting!
One thing I've wondered for a long time, though -- is there a scientific(-sounding) term for the apparatus, something besides "sucker"? It just doesn't have a very sciencish sound to it :) (Like "truthiness" only with science...)
Posted by: June
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May 17, 2006 10:28 AM
I vividly remember a childhood dinner where I arranged thick slices of octopus sushi to reassemble the suckers. I repeatedly pressed them sucker-face down on the plate and pulled them back up to make satisfying popping-squelchy noises. Very enjoyable.
Posted by: Wabin
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May 17, 2006 10:41 AM
This may have been posted on here, but it is a dramatic illustration of the effectiveness of the suckers. And the rest of the octopus armament (pun intended... sorry).
http://www.pbs.org/wnet/nature/octopus/media_players_blue/shark_hi.html
Posted by: idlemind
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May 17, 2006 10:48 AM
Umm, just a quibble, but if atmospheric pressure is about 100kPa, I don't see how a sucker could exceed this at lea leveleven if it generated a perfect vacuum. Of course, as you say, under increasing water pressure the suction could be greater.
Truly fascinating stuff. An intelligent designer would have put these things on people's fingertips and palms.
Posted by: Hai~Ren
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May 17, 2006 10:48 AM
Very interesting... never knew suckers were controlled by muscular action.
I know some squid have teeth on the rims of their suckers, while others have hooks in their suckers; do these also help provide better grip on prey? And what if a predator has accidentally gotten snagged by the suckers? Can the cephalopod disengage them?
Posted by: Paul W.
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May 17, 2006 11:04 AM
Depending on the octopus, yeah.
Check out the recent posting on dissecting a large squid. Note all the pretty teeth in the suckers. Serious hickeyage potential there.
Which reminds me of several questions:
Does anybody know what structures these things (and the teeth) are homologous to? E.g., did octopi evolve teeth independently, or reuse a tooth-making routine that had been around almost forever? Are these "teeth" homologous to ours, or just "chitinous denticles" that evolved in convergent ways. And do we really know?
(I can imagine that they're homologous in the same way that compound and camera eyes are homologous---using a "toothless" gene like an eyeless gene to do a conserved "subroutine call" to make a hard sharp thing, even if the subroutine itself evolved in very different directions, to make a different kind of hard sharp thing with mostly different final-stage materials.)
Are suckers a variant of some basic grow-an-x-here appendage -generating riff? Is there a different general conserved/reused riff for growing hollow pucker or pit structures with an orifice ("invaginations"?), and dividing it and the area around it up for further specializations? (Is this just analogous to gastrulation, or homologous in some way?)
More generally, how much evo-devo do we know about various stuff like this? My impression is "not much, yet," and we mostly know a few basic things (hox genes and a few others) and relative handful of stunning examples (like eyeless).
Posted by: Torbjörn Larsson
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May 17, 2006 11:15 AM
"Is there a different general conserved/reused riff for growing hollow pucker or pit structures"
A similar and somewhat related question is if sphincter muscles are general too. They are found in the most peculiar places, often centered around pits or orifices.
Posted by: Hal
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May 17, 2006 11:30 AM
I've had the pleasure a number of times of shaking hands with an octopus (O. dofleini), and the control, sensitivity, pure muscularity, and curiosity that they bring to exploration with their tentacles is phenomenal. They reach into remote crevices continually, probably looking for food and memorising their surroundings; for this, the arms and probably the suckers as well are highly innervated. At any rate, they can control finely how hard they grip, whether they slip, which size suckers engage, whether it's a touch or grip or both, etc. They don't leave hickeys, and you can pull them off your skin without much difficulty. Pulling them off a fabric covered dive suit is easier.
Posted by: gecko1
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May 17, 2006 1:02 PM
The suckers work pretty well for holding onto cameras too as my dive buddy Sami found out the hard way a few years ago up in BC.
Octo vs Photographer 22MB QuickTime File.
Clinton
Posted by: corydalus | January 5, 2008 5:24 PM
I looked both in Wikipedia and my venerable copy of Barnes' Invertebrate Zoology and it seems those suckers are called just that.
Posted by: Graham King | February 29, 2008 11:51 AM
PZ, thanks for this! I have always found these suckers fascinating...
Perhaps it began with the James Mason film version of 20,000 Leagues Under The Sea...
Re your description of how the suction is applied: is the acetabular roof (AR) semi-rigid, or is inward movement of the outer circumference of the ring of radial muscle (R) limited in some other way, so as to force enlargement when its walls thin?
Re precision: can the animal activate its suckers individually (ie singly), or do suckers automatically suck-on-contact when the animal 'wills' to grip with that arm; and do the suckers actively orient to achieve best conformal contact (ie presumably normal to - possibly irregular - substrate surface)?
There are a lot of other muscles proximal to the sucker, I am thinking they may do some sophisticated stuff. Is sucking regulated by local nerve network feedback as well as under central nervous system control?
And, erm, anything else you can tell us, really! Like I said, I'm fascinated... ;-)
Posted by: Mark (Monty) Montague | April 21, 2008 5:09 PM
Many species of squids also add a toothed ring around the edge of the sucker, which decreases the sliding even more. Of course, some even have some suckers modified into big hooks shaped roughly like cats' claws... and that includes the colossal squid, Mesonychoteuthis which has hooks big enough to cause some serious damage to whales. (Actually, modified may be a misnomer, in that we don't really know if hooks or suckers came first: nautilus has no suckers at all, and fossil belemnites had hooks, although they're somewhat different in shape from squid hooks.)