olting
Just as we outgrow our clothes, crabs outgrow their shells. Crabs have a hard
outer shell that does not grow as the rest of its body does. As the crab grows
larger, they must shed their shells, in a process called molting.
Right before molting, a crab reabsorbs some of the calcium carbonate from its
old shell, and then secretes enzymes to separate the old shell from its under
skin. Next, the skin secretes a new, soft, paper-like shell beneath the old one.
This process can take several weeks.
A day before molting, the crab starts to absorb seawater and begins to swell
up like a balloon. This helps expand the old shell and causes it to come apart
at a special seam which runs around the body. The carapace then opens up
like a lid. The crab extracts itself from its old shell by pushing and
compressing all of its appendages repeatedly. First it backs out, then pulls out
its hind legs, then its front legs, and finally it comes completely out of the old
shell. This takes only about 15 minutes.
When a crab molts it removes all its legs, eye stalks, antennae, all its
mouthparts, and its gills. It leaves behind the old shell, the esophagus, its
entire stomach lining, and even the last half inch of its intestine. Often a large
group of crabs will molt at the same time, and their old shells wash up on the
beach.
If you find something on the beach that looks like a dead crab, pick it up, open
the lid, and look closely inside. If nobody is home, it is a cast-off exoskeleton.
Molting is a very dangerous time for crabs because they are soft and
defenseless for days afterward. The crab cannot walk because its legs will not
support its weight, and it cannot eat.
While the crab’s new shell is soft it takes in water and expands, stretching its
new skin. After 32-60 hours, the crab is able to walk, however it can be three
to seven days before it starts eating again. After two to three weeks its new,
soft shell fully hardens.
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Special Seam
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Protect
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Exuvia
A crab’s shell splits open at a s