Abstract
The functional morphology of the suckers of Octopus
rubescens were studied by computer models of the acetabulum. The
suckers undergo a shape change when contracting, forming a shape
that is yery resistant to pressure and therefore adaptive tor
adhesion. However, when the octopus wants to let go, it relakes
the muscles of the sucker and the acetabulum collapses around
"hinges" to a shape that is not at all resistant to pressure,
allowing the sucker to release.
Introduction
Suckers are found in all cephalopods except nautilus. (Dilly
and Nixon, 1977) Suckers are a very powerful and sensitive organ. For
example, suckers may be torn from the living animal before
detaching from the substratum (Lane, 1957). A more comprehensive
study found that Octopus vulgaris can exert a "holding tension'
of up to 100 times its body weight ( Trueman and Packard, 1968).
Many studies have examined the morphology of suckers (plate 1)
and the different mechano- and chemoreceptors in the infundibulum
(Dilly and Nixon, 1977, Wells, 196Ja), but no work has been done
on the actual mechanics of how the sucker adheres.
It is generally assumed that suckers adhere by the use of a
preszure difference adhesive (i.e. suction). For example,
consider Figure 1. When no force is exerted by the spring (a),
there is no pressure difference between the inside of the
cylinder and the outside and no force holds the cylinder to the
surface. When the spring contracts (b), the pressure decreases
inside the cylinder. This pressure difference between the inside
and out result in a force that pushes the cylinder down on the
surface. The seal between the cylinder and the surface is
important because any leaks would negate the the pressure
difference.
Materials and Hethods
Experiments were conducted on a 90g OOctopus rubescens at
Hopkins Marine Station, Facific Grove, CA.
Tenacity: Measurements of the force per area a sucker
could resist were made as follows: a small,roughened circle of
plexiglas attached by a string to a spring scale was placed on
submerged suckers of a live octopus and the diameter of the
suckers on the plexiglas was measured with calipers. Ihe octopus
was narcotized in a 17 solution of ethanol because an animal that
is not narcotized will not hold onto the plexiglas. The arm of
the octopus was restrained while the spring scale was slowly
pulled away. The maximum force before the sucker released was
recorded, and the force per sucker surface area (infundibulum) was
calculated. In many cases it was probable that the octopus
released of its own volition rather than the force exceeding the
sucker's hold.
Histology: A small segment of an arm was excised and put
either directly into a fixing solution (507 sea water, 507 a
misture of 27 formalin and 2 gluteraldhehyde) or into a relaking
solution of isotonic magnesium chloride, and then into the fixing
solution. Ofter fixing, the segments were dehydrated in ethanol
and left overnight in an infiltrating solution of JB-4. Segments
were then embedded in JE-4 in a vacuum for two hours, sectioned
(6 microns) with a glass knife, and stained with methylene blue
and chromtrope 2H.
Ig record the shape change of a sucker as it is
contracting, an excised sucker was cut through its center in a
sagittal cut. It was stimulated with an electrode and the
contractions were video taped through a dissecting microscope.
Iracings of the contracted and relaxed acetabulum were
made from histological sections and were incorporated into a
finite element analysis of structural stiffness. The MacNeal¬
Schwendler Corporation MSC/pal stress and vibration analysis
program was used to examine the deformation of suckers in the
plane strain only. The sucker is circularly symmetrical, thus a
planar section as an appropriate, simple model for the strain in
the entire structure. The computer model dealt with the
acetabulum of the relaxed and contracted suckers because it is
the part of the sucker that is creating the pressure difference.
The infundibulum is not directly involved in creating the
pressure because it flattens out against the surface to which the
sucher is holding and creates the seal. Also, analogs structures
were also made for comparison resisting pressure.
Iwo models of the acetabulum were tested; one in which the
nodes at the opening of the acetabulum were fixed and another in
which they were free to slide on the substrate. The contracted
acetabulum with the fixed nodes is more representative of the
actual sucker because the infundibulum is pressed against the
surface and would not slide very easily.
Oll models were built of quadrilateral plates with the
stiffness of contracted muscle(1 N/m, Schmidt-Nielson, 1979).
except for the relaxed acetabulum model which had halt that
stiffness. A force of up to an atmosphere was applied to the
models by assigning a pressure of zero inside the model and
increasing pressure outside. The volume at a given pressure
divided by the original volume (no pressure) was plotted to
determine thestiffness of the structures.
Results
The average tenacity of the suckers was J.95 10' N/m" with
a range from O to 1.015 10P N/m. One atmosphere of pressure is
equal to 1N/m.
Tracings from the video tape of the shape change the sucker
undergoes when stimulated by an electrode (figure 2). Shape (a) is
the relaxed form, "hinges" on which it can collapse are evident.
In the contracted form (d), the "hinges" are almost gone and the
acetabulum is approximately spherical.
The relative change in volume was plotted verses increasing
pressure for all the models (figure 3).The sphere and the
hemisphere are are the most resistant to pressure, at one
atmosphere, retaining 607 of their original volume. The model of
the contracted acetabulum with the "fixed" nodes was the next
most resistant structure, deforming only 50 at one atmosphere.
The quarter of the sphere retained only 157 of its original
volume at one atmosphere. The relaxed acetabulum and the
contracted acetabulum with the bottom nodes free to move on thex
axis collapsed totally before a 20"' of an atmosphere had been
applied.
he shapes of the pressurized models are shown in figures 4
-9.
Discussion
The abject of this study was to ascertain how suckers can
resist the atmosphere of pressure that had been exhibited in the
tenacity tests. The octopus sucker undergoes a shape change when
it contracts. The contracted shape is very resistant to pressure,
because like an arch, it is very resistant to compression forces.
This is important to the octopus because the sucker, when
contracted, must resist the pressure difference required tor
adhesion. However, when the octopus wants to let go, it can do so
easily by relaxing the muscles of the sucker. Not only does the
material become less stiff, but the sucker acetabulum can
collapse very easily around the "hinges". This relaxed state is
not at all resistant to pressure, so that the sucker can not hold
any negative pressure and the sucker releases easily.
The values used here for material properties (stiffness) of
the contracted muscle may be low, especially if the muscles are
reinforced with collagen. However, it will be difficult to
measure the stress and strain curve for the acetabulum muscle
because f the small size.
Most of the octopus suckers studied to date are very
similar to those of a rubescens. The suckers of the squid and
cuttlefish do not have infundibula proportionally as large as the
actopodia suckers, but the acetabulum muscles are the same sise
or larger and have the "hinges", so it would be reasonable to
assume that they work the same way.
Ocknowledoments
I would like to thank Mark Denny for his constant support
and adyice, without which this project would not have gotten oft
the ground and through to completion; and for constantly helping
and explaining to me what exactly biomechanics is. I also like to
thank Chris Fatton for his help with the histology. My special
thanks my fellow students in BIO 175H and to Mark, Stuart, Chuck,
and Gilly, who made this quarter the most enjoyable one of my
years at Stanford.
References
Dilly, P.M., and Nixon, M. (1977). Sucker surfaces and prey
capture. pp. 447-511 in Nixon, M. and Messenger, J.B.
(eds.) The Biology of Cephalopods Zoological Society of
London 38 Academic Fress
Dilly, P.N., Nixon, M., and Packard, A. (1968) Forces exerted by
Octopus vulgaris Fubbl. Stn. Zool. Napol. 34: 86-97
Gorden, J.E. (1980) Structures; or Why Things Don't Eall Down,
Fequin Books. New York.
Lane, F. (1957) Food p.24 The Kingdom of the Octopus: the life¬
history of the Cephalopoda Jarrols Fublishers, London
Olexander, A.MCN. (1968) Fressure, density, and surface tension. Ep.
183-186. Aminal Hechanics, University of Washington Fress,
Seattle.
Parker, G.H. (1921) The power of adhesion in the suckers of
Octopus bimaculatus Verrill. J. ENB. Zool. 33: 391-394
Schmidt-Neilson, K. (1979) Aminal Fhysiology: adaption and
environment. 2 edition. Cambridge University Fress,
Cambridge. p.5
Trueman, E.H., and Fackard, A. (1968) Motor performances of some
cephalopods. J. ERB. Biol. 49: 495-507
Wells, M.J. (1963a) Taste by touch: some experiments with
octopus. J. ENP. Eiol. 40: 187-19
Figure Legend
Plate 1 - Morphology of the sucker of O. rubescene.
Figure 1 - Diagram of a cylinder containing a piston on a
surface.The F represents a force that pulls on the
piston and is representative of the radial muscles in
the ectopus.
Iracings from the video tape of the sagittal section
Figure 2
of the sucker contracting under stimulation by an
electrode.
- Graph of the relative change in volume of the si?
Figure 3
models yerses increasing pressure.
Disgram of the contracted acetabulum with the
Figure 4
inside, bottom nodes "fixed" in position, collapsing
under increasing pressure.
Disoram of the contracted acetabulum with the inside,
Figure 5
bottom nodes free to slide on the  adis.
Diagram of the relawed acetabulum contracting under
Figure 6
pressure. The model has a stiffness of one half the
contracted muscle value.
Diagram of the sphere model deforming under pressure.
Figure 7
Ihe model looks like a hemisphere but experiences the
same forces asphere would.
Figure 8 - Diagram of the hemisphere model deforming under
pressure.
Figure 9 - Diagram of the quarter of a sphere deforming under
pressure.
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s sphincter muscle
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