Cephalopod gnashers

Cephalopods can inflict a nasty bite. On their underside, at the conjunction of their arms, they have a structure called the beak which does look rather like a bird's beak, and which can close with enough force to crush shellfish. Many also dribble toxins into the wound that can cause pain, tissue necrosis, and paralysis. They aren't the best animals to play with.

If you think about it, though, cephalopods don't have a rigid internal skeleton. How do they get the leverage to move a pair of sharp-edged beaks relative to one another, and what the heck are they doing with a hard beak anyway? There's a whole paper on the anatomy of just the buccal mass, the complex of beak, muscle, connective tissue, and ganglia that powers the cephalopod bite.

The beak itself is made up of a combination of chitin (a carbohydrate, the same stuff that makes up insect exoskeletons) and proteins. The buccal mass is a roughly spherical lump of tissues with a fair amount of motility and independence—the beak can be swiveled about at various angles, can protrude and retract, and the whole mass can be dissected out and still function surprisingly well. In at least some species, the isolated buccal mass will continue to chomp away for up to two hours after it's removed. It's like an autonomous set of choppers.

Fig. A drawing of the position of the buccal mass within Octopus bimaculoides. The inset photo shows the left side of the buccal
mass and the orientation of the buccal mass in the descriptions that follow: the dorsal surface is up, the ventral surface is down, the
chitinous beaks (UB, upper beak; LB, lower beak) mark the anterior end of the buccal mass and the esophagus (Eso.) is posterior. Note
that the beaks are in an opened position.

The joints of the beak are unusual in their articulation. Illustrated below are a couple of ways flexible joints can form. One way is to make a bendable hinge (1), for instance as with a clam shell. Another is the familiar ball-and-socket joint (2), as we see in our hips where the femur meets the pelvis. Another particularly versatile way and one that is common in skeleton-less animals like a cephalopod is the muscular hydrostat (3), in which compression in one direction generates a protrusive force in another. We have some muscles that do this sort of thing—the best examples are our tongues. Practically the whole body of a cephalopod uses this kind of action.

Fig. 1. A drawing of a box in which the base and lid are connected by a flexible joint. Note that the lid and base are formed of a
single piece of material in which the hinge represents a thin connection that allows bending.
Fig. 2. A schematic drawing of a ball-and-socket style sliding joint. The terminal end of the right link is a sphere that fits within
the cup of the left link. Such an arrangement allows rotation at the articulation while force is transmitted between links.
Fig. 3. A depiction of the action of a generalized muscular hydrostat. Muscular hydrostats are composed of a 3D arrangement of
muscle fibers. A cylindrical muscular hydrostat such as the one depicted here may change shape due to the selective contraction of the
muscle fibers of a given orientation, but it does not change volume significantly. For instance, change in shape such as that shown in
A can be created by the contraction of fibers arranged radially or circumferentially. Change in shape such as that shown in B is created
by contraction of longitudinal muscle fibers, i.e., fibers parallel to the long axis of the cylinder.

The cephalopod beak doesn't use these methods. The two elements do not hinge directly on one another at all, and don't contact during the bite cycle except at their cutting edges. Instead, they are imbedded in a muscle mass that provides the flexible basis of movement.

The authors sectioned the buccal masses of several kinds of cephalopods—cuttlefish, octopus, and squid—and used computer reconstruction to visualize the relationships of the various pieces. Here, for instance, are the two pieces of the beak, the upper beak in blue and the lower beak in red.

A computer rendering of the upper (blue) and lower (red) beaks of Octopus bimaculoides. The left pair shows how the upper
beak fits within the lower beak. The right pair shows the upper beak separated from the lower. Both sets are pictured in a front right
quarter view.

The illustration below shows the two pieces in the bite cycle. The opening of the beak is to the right in all.

Diagram illustrating movement of the upper beak (blue) with respect to the anchored lower beak (red), of Octopus
bimaculoides, during a stereotypical bite cycle. Position A, resting; B, opening; C, fully open; D, closing;
E, closed and retracted.

When you look at the beak pieces alone, you can see that there is no articulating point between them at all, no joint on which the two pieces swivel. Instead, the hard chitinous parts are suspended on 4 sets of muscles: the superior mandible muscle (SMM), a pair of lateral mandibular muscles (LMM), a posterior mandible muscle (PMM), and an anterior mandible muscle (AMM).

A series of computer renderings of three sets of beaks of Octopus bimaculoides with attached mandibular muscles. The left
set shows the muscles attached to the beaks. The center set shows the upper and lower beaks separated with the mandibular muscles
originating on the respective beaks. The right set shows the beaks and mandibular muscles separated. The color convention is as
follows: the lower beak (LB) is red, the upper beak (UB) is dark blue, the superior mandibular muscle (SMM) is green, the lateral
mandibular muscles (LMM) are purple, the anterior mandibular muscle (AMM) is yellow, and the posterior mandibular muscle (PMM)
is light blue.

Much of the paper focuses on the anatomy and physiology of these sets of muscles to determine their role in the bite cycle. I'll spare you the details, except to say that the LMMs seem to be a pivot point for rotation of the upper beak, and also act as a hydrostat to help open the beak. The AMM is a beak closer, while the PMM has complex functions, depending on the contraction state of other muscles: it can bring the posterior portions of the beak closer together, opening it, or it can close the beak by bringing the anterior parts together. The SMM is a closer and retractor.

These are all part of the predatory apparatus of cephalopods. They lunge forward or reach out with their arms, grasp their prey with suckers, and then deliver the coup de grace with a savage snap of their horny and muscular beaks. It's charming in a grisly sort of way.

Uyeno TA, Kier WM (2005) Functional Morphology of the Cephalopod Buccal Mass: A Novel Joint Type. J Morph 264:211-222.