Types of Mollusks
Mollusks have adapted to an amazing range of
environments. Clams and snails can be found
living on mountains, in lakes and ponds,
marshes, bays and estuaries, along sandy
seashores, floating on the sea surface and living
at the bottom of the sea, near hot springs.
There are clams that burrow into wood and
limestone rock, that live attached to the
undersides of sea urchins and the gills of fish. Snails can
be found living in high
trees, in the intestines of sea puddings, and within the arms
of starfish.
Seashells occur in a great range of shapes, colors and sizes.
One of the
smallest seashells is the Pythina clam, a tiny, smooth translucent
clam the size
of a rice grain, that lives attached to the underside of shrimp
and crayfish. At
the other extreme is the largest known seashell -- the giant
Tridacna clam of
the southwest Pacific. This monster's shell consists of two
attached valves
which are four foot long and weigh 500 pounds.
Mollusks first made their appearance 500 million years ago,
during the
Cambrian period. 100 million years later, the six classes
of mollusks living
today could be found in the fossil record. The amazing variety
of shells --
their color, patterns and sculpture -- have been shaped by
millions of years of
environmental changes and genetic mutations.
The largest, most common, and best-known seashells are the
univalves or
gastropods -- conchs, whelks and snails. They have one shell,
which is often
coiled. Single-shelled animals first appeared in the fossil
record 500 million
years ago. Some gastropods, such as limpets and abalone have
flat
saucer-like shells. Snails are the only mollusks to have the
distinction of
colonizing land as well as freshwater and marine habitats.
The Aztecs of
ancient Mexico depicted their rain god, Tlaloc, rising from
a conch shell. The
Greek god Triton, one of Neptune's trumpeters, was depicted
with a large
conch shell that he used to summon river deities around their
king.
Bivalves, such as oysters, clams, and scallops, are mollusks
that have two
movable interlocking shell valves attached at one end by a
strong muscular
hinge. The valves open and close like the front and back cover
of a hardback
book. By retreating and closing its soft body inside its shell,
a bivalve is able
to protect itself from most predators. Bivalves such as the
blue mussel attach
themselves to wharves and rocks with thread-like byssal anchors.
While most gastropods and bivalves are sedentary creatures,
the sprightly
scallop can be quite lively. An adult scallop, when provoked
by a predatory
fish, crab or starfish, will suddenly leap up and zigzag away.
The snapping
action of the valves propels the animal backward. However,
the scallop can
also move forward by suddenly clamping its valves shut, expelling
water from
its hinge side.
The third major type of mollusks are
cephalopods, such as the squid and octopus.
They're also the most active. They possess a keen
intelligence and most have evolved beyond the
need for shells. Their fossil record spans over the
past 400 million years, when many species then
had shells. The only shelled cephalopod that
remains today is the chambered nautilus, which
was once abundant in ancient seas along with its
cousin the ammonite. Ammonites became extinct
65 million years ago, presumably in the same catastrophe that
wiped out the
dinosaurs. The chambered nautilus is a magnificent animal
with a creamy
white shell with broad rusty brown stripes. The inside of
its shell has an
iridescent mother-of-pearl coating. The shell itself is divided
into different
chambers where volumes of gas and water give the nautilus
buoyancy in the
water.
Besides gastropods, bivalves and cephalopods, there are three
less common
types of mollusk. There are the chitons, also known as the
armadillos of the
sea; the tusk shells, resembling miniature elephant tusks;
and the gastroverms
-- small, fragile and primitive animals that weren't even
considered mollusks
until the late 1950s.
