Autotomy & Regeneration
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.)
Amazingly, the blue crab will grow a completely new limb by a process known as
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.