About
crabs moulting
Why are there so many ‘dead’ flower crabs? You might come
across what appears to be dead crabs strewn among the seagrass or
on the sand bars. These are often not dead crabs but merely their
discarded skins!
Like
other arthropods, crabs have a hard exoskeleton (external skeleton)
and need to shed their shell in order to grow bigger. Called
moulting, this also allows the crab to regenerate lost limbs.
|
After moulting, a crab can be
two to three times larger
than it was before! |
Double
skinned: The
exoskeleton is produced by tissue layers under it. In preparation
for a moult, the tissue layer separates from the exoskeleton and starts
to produce a new exoskeleton. The old exoskeleton is also partially
broken down from the inside and minerals from the old exoskeleton
such as calcium are recovered and used to build the new one. Thus,
close to moulting, the crab is encased in a double exoskeleton!
Bursting out: When the new exoskeleton
is ready, the crab swallows water to expand its tissues and the old
shell splits along predetermined lines, usually near the abdomen.
The crab then carefully pulls out of the split in its new exoskeleton
which is still soft and wrinkly. The crab continues to swallow a lot
of water to stretch the new exoskeleton. When the new shell hardens
in a few hours, there is space in the new shell for the crab to grow.
Moulting thus makes the crab 'watery' and less valuable as food. The
'soft-shell crabs' that we eat are crabs that have just moulted! Since
crabs are vulnerable during moulting, they usually find a safe place
to hide during this time.
New limbs: To replace a lost limb,
a new one develops under the old exoskeleton and unfolds from a sac
at the time of moulting. At this moult, the new limb is usually not
as large as the lost one. It will gradually grow bigger with the subsequent
moults.
How to tell the difference between a moult
and a dead crab? Unlike a dead crab, a moult is light and
has transparent eyes. It has no inner layers of flesh nor a bad smell.
Most crabs moult throughout their lives. But the time between moults
becomes longer as the animal gets older. Some crabs, however, stop
moulting once they reach reproductive maturity.. |
|
Moult of a female
Flower crab |
