learn only 3 things about them ...
Moults have clear eyes, break open easily and don't have
a bad smell.
All arthropods moult, including the more familiar insects.
is delicate process that is dangerous for the animal.
are there so many 'dead' 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 exoskeleton in order to grow bigger. Called moulting, this also
allows the crab to regenerate lost limbs. The description of moulting
below generally also applies to other crustaceans.
skinned: The exoskeleton is produced by the tissue layer
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
Bursting out: When the new exoskeleton
is ready, the crab swallows water to expand its tissues and the old
exoskeleton splits along predetermined lines, usually near the abdomen.
The crab then pulls out of the split, slowly and carefully so it does
not tear any limbs.
When it emerges, its new exoskeleton is soft and wrinkly. The crab
continues to swallow a lot of water to stretch the new exoskeleton.
When the new exoskeleton hardens in a few hours, there is space in
the new exoskeleton 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
Happy moult day! Moulting is an
important milestone in the life of most arthropods including crustaceans
like crabs. As a crab develops from the larval stage, it changes its
body shape with each moult. Some of the shape changes can be very
drastic. Much like the way caterpillars moult to develop into the
pupae and moult into a butterfly.
The crab reaches reproductive maturity when it emerges from a moult
with functioning reproductive organs. Unlike insects, some crabs continue
to moult after achieving reproductive maturity. But the time between
moults becomes longer as the animal gets older. Some crabs, however,
stop moulting once they reach reproductive maturity.
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 subsequent
Moult 'n' mate: In some crabs,
mating takes place just after the female moults. Thus, often the male
crab will 'protect' a female that is just about to moult in order
to ensure that he is the one to mate with her. She is able to store
his sperm, some for quite a long time, and use the sperm to fertilise
her eggs when she develops them. Older female crabs may have to mate
again to fertilise her eggs when her stored sperm is depleted.
A freshly moulted crab (top right)
with the moult (lower left).
Sentosa, Jul 04
The moulted exoskeleton 'opened' up.
Changi, Jun 05
Moult of a sea slater?
Sisters Island, Jul 06
Flower crabs mating. Male inserting sperm
into the female which may store it.
Pulau Sekudu, Oct 05
female already carrying eggs under her belly, about to mate.
Pulau Sekudu, May 04
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.
A hermit crab moult outside the shell
with the original inhabitant inside the shell.
Changi, Jul 05
The eyes of the moult are transparent.
Changi, Jul 05
A newly moulted
Pasir Ris, Dec 08
The moult has transparent eyes,
the eye stalks are transparent too!
This is the crab in its new exoskeleton.
Archives by Steve Zinski: a fabulous site packed with info
and photos on a relative Portunid Callinectes sapidus found
in the US, with details on moulting
and a wonderful set
of photos showing how a crab moults.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963.