Autotomy & Regeneration

Autotomy

A few years ago I was holding onto a crab by one of its claws. As I was holding it, I heard a strange "hissing" and "popping" sound and suddenly the entire claw fell off in my hand. This struck me as odd since I was not exerting any force on the claw.

What I observed was one of the blue crabs natural escape mechanisms, called autotomy, which is the voluntary breaking of appendages (or pereopods [legs] in the case of a blue crab.)

The dictionary defines autotomy as a reflex separation of a part from the body. In other words it's the crab's ability to spontaneously self-amputate a limb in order to escape from a predator. In my particular situation the crab could not escape because I had a firm grip on its claw, so the crab "dropped" it in an effort to flee.

Autotomy (derived from the Latin words autos for "self" and tomos for "cut") occurs when a break develops along a special fracture plane located at the appendage's distal base. When blue crabs are subjected to mechanical stimulus, such as a predator attempting to remove them from rocks, they respond by either fleeing or fighting. When choosing to fight, crabs will pinch the predator with its powerful claws hoping that the action will make the predator lose its grip. When choosing to flee, crabs will autotomize the limb(s) being held onto in order to escape. Other reasons to autotomize a limb might be to reduce blood loss from a distal wound, a response to physiological stress, ill health or the presence of pests or bacteria.

Study the photo above left by looking closely at the base of the claw near the movable joint where it attaches to the main body. You will see a thin oblique groove, or fracture plane, in the shell (photo above right shows the fracture plane highlighted in red.) A crab can cause this fracture to break instantaneously, separating limb from body. This plane is specialized for this function and the animal can lose its limb, at this plane only, without trauma or significant blood loss (a blood clot forms immediately to stop bleeding.)

Regeneration

Amazingly, the blue crab will grow a completely new limb by a process known as regeneration.

After the limb is cast off, a membrane quickly closes the wound where the limb was attached and this membrane becomes the capsule of the new limb bud which holds the new appendage during regeneration.

It will take two or three successive molts in order for the limb to return to its normal size (see photo at left showing a crab regenerating its cheliped.)

Autotomy and regeneration are very common. One survey shows that nearly 25% of all blue crabs are either missing or regenerating a limb.