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Petrolisthes eriomerus (Porcelain crab)
Copyright ©
2005 Mary Jo Adams
Look
for this species low in the intertidal on beaches where rocks rest
on sand or fine gravel and there is a fair amount of wave action.
It is also sometimes seen in mussel beds or among kelp holdfasts.
Petrolisthes eriomerus is a small crab with
a carapace reaching a diameter of only about ¾ inch.
The carapace is round and compared to other crabs, the claws appear
huge in relation to the rest of the crab. If the claws are
open, you may see that the base of the moveable "finger" has a blue
spot. This species has been found to depths of 280 feet.
A
similar porcelain crab, Petrolisthes cinctipes tends to
be more of an outer coast species and is often found in clumps of
the California mussel ( Mytilus californianus ).
It has a red-orange spot at the base of its "finger", can be found
somewhat higher in the intertidal, and avoids areas where the rocks
are set in gravel or sand.
These
species are filter feeders, collecting diatoms and other nutrients
from the water. They also have tufts of hair on their claws
that they use to brush bits of food up from rock surfaces.
Porcelain crabs have a unique defense mechanism in which if they
are threatened or disturbed, they may drop one of their appendages.
The lost limb will eventually grow back.
Porcelain
crabs are not true crabs, but belong to a group called anomurans,
making them more closely related to hermit crabs and king crabs.
One easily observed distinction between true cabs and anomurans
is that in anomurans, only 4 pairs of legs are normally visible
where as in true crabs, 5 pairs can be seen.
This page was created by Mary Jo Adams on 12/5/05.
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