What Is A Shell?
by Lynn Scheu
Shells are lovely natural objects,
equals in beauty to any flower or butterfly, they are more than
just pretty baubles found on beaches. They are the exterior
skeletons (exoskeletons) of a group of animals called mollusks.
The word "mollusk" means "soft-bodied;"
an exterior skeleton is very important to these creatures, providing
them with shape and rigidity, and also with protection, and
sometimes camouflage, from predators.
Mollusks are classified into
major groupings according to the characteristics of their shells.
Snails (Gastropoda) have a single shell which spirals outward
and to one side as it grows. Most Cephalopoda (octopi and squid)
have no shell, but the Chambered Nautilus of that group has
a shell. This shell does coil, but it coils flatly, in a single
plane. Tusk shells (Scaphopoda) also have a single shell, but
it does not coil at all; it grows in a narrow and very slightly
curved cone shape. Bivalves (Bivalvia), including oysters, clams,
scallops and mussels, have two parts to their shells that enclose
their tender bodies like the two halves of a hinged box. Chitons
(Polyplacophora) are little armored tanks, with a row of eight
overlapping plates protecting them. The Neopilina (Monoplacophora),
are deep-sea "living fossils;" they have a single
shell which hardly coils at all, but fits over their bodies
like a protective cup. (Some gastropods (the limpets) have shells
like this too, but their body structure is very different.)
Last are the deepsea worm-like Aplacophora, with no shell at
all, but little calcareous spines on their bodies.
Mollusks make their shells from
calcium they derive from their environment, either the food
they eat or the water they dwell in. When a tiny mollusk hatches
from its egg, it comes into the world equipped with a tiny shell.
This shell is actually a part of the animal, growing as it grows,
accommodating its needs. Each different species of mollusk makes
a shell that is, in most cases, unique to it alone. Indeed,
this uniqueness of form is partly what allows amateur shell
collectors (conchologists) and professional scientists who study
mollusks (malacologists) to determine a mollusk's species. Each
species is destined genetically to develop the same type of
shell its progenitors did. But, just as with humans, there are
many distinct differences. Food, climate, environment, accident,
and the mollusk's particular heredity all play their parts in
making each shell an individual.
Just as important as protection
and rigidity, is the assistance a shell renders its maker in
pursuit of the necessities of its life. Some shell shapes are
streamlined for ease of burrowing through mud or sand. Some
bivalves are heavily ridged in ways that help them stay anchored
in the bottom where they live. Others are assisted in burrowing
by special shell sculpture. There are shells that grow long
spines to entrap and encourage growth of camouflaging seaweed
and corals. Others, like cowry shells, are smooth and polished
and highly glossy with bright interesting patterns on their
shells. Often the animals themselves are patterned as well.
When a predator disturbs the animal, it withdraws inside its
shell, revealing a surface that is very different in color and
pattern than the one the hungry predator had its eye on. It
is speculated that quick change artistry has a startle effect
on the predator. (And still other mollusks, like the slugs,
squid and octopi, and those lovely creatures, the nudibranchs,
have given up building a shell altogether, relying wholly on
other defenses.)
Indeed, a great deal may be learned
about a mollusk's individual life by examining its shell. Many
bear old healed-over breaks and chips that speak of a battle
won with some predator. Others show color change which testifies
to a change in diet or water chemicals. Mollusks that are very
old in snail-years may become greatly thickened and dulled in
color. Rough water conditions discourage the growth of long
spines on normally spiny species, while quiet habitats allow
extravagant spiny extensions. (It is now believed that this
effect may have to do with a mollusk's different diet in rough
water.) Sometimes a shell may have so great an encrustation
of marine organisms on its shell that locomotion becomes impossible
and the animal starves to death.
And shells aren't found only
on beaches. They can be found anywhere mollusks live. Both gastropods
and bivalves inhabit rivers and streams and lakes, where they
have made adaptations to freshwater living. Many species have
also adapted to life on land, and can be observed in leaf litter,
in trees, on plants, under rocks and buried in loose dirt. In
the ocean, mudflats and mangrove areas are homes to hundreds
of mollusk species. Some live on the tree roots and branches
just at water line. Others bury themselves in the mud or sand
bottom, emerging to feed, often at night or at low tide. There
are mollusks which live only on coral reefs, and those which
inhabit the ocean depths. Some, like the slit shell and the
Neopilina, were thought to be extinct until they were rediscovered
in these depths. Shells live on seaweed, on coral holdfasts,
under rocks, buried in the roots of undersea grass beds. Some
shells even float throughout their entire lives on a raft of
bubbles. Each has a specialized habitat and an ecological niche.
