The shells that hermit crabs seek are made by marine gastropods that secrete
calcium carbonate from their mantel—the organ that covers their soft bodies.
The shell is built up in deposits until the calcium carbonate becomes a crystalline
structure held together via thin membranes of organic material. The univalve-type
shells that hermit crabs prefer to adopt are spiral in shape. This formation
affords the growing gastropod within the hard shell an ever-increasing area in
which to expand.
Hermit crabs are scavengers and often locate these borrowed dwellings by
smell, when the original gastropod inhabitant dies and begins to decay. Once
a hermit crab adopts a shell, it will keep it until the shell is outgrown, carrying
it continuously as a shield, wherever it goes. In order to carry its home, one of
the crab's front claws is completely dedicated to clutching the shell. This claw
bends backward and holds on to the spool of calcium carbonate at the shell's
center. In order to move, the animal must first use this claw to lift the shell and
heave it onto its back. In spite of such difficulties, the drive to remain housed
is so strong in this species that a typical hermit crab would rather be torn limb
from limb than be pulled out of its shell. The only time that the animal will
willingly leave its shell is a) if it locates another, more suitable one, or b) if it
is shedding its exoskeleton—a process which can only be accomplished by
fully exiting its dwelling just long enough to wriggle out of its own exfoliated
shell casing.
When a hermit crab that has grown too large for its current home locates a new
one, it determines the structure's suitability via a process called fondling. During
this activity, the hermit crab will explore the shell's surface and its internal
volume-to-weight ratio by rolling the shell over and gently rocking it back and
forth. Since hermit crabs actually choose the shells that they inhabit, there is a
large body of information concerning shell selection.
Source:
www.cabinetmagazine.com
