Sea shells are varied in shape and size
Throughout the summer season, scores of people travel to the shore.
Often in the mornings, you can see folks out combing the beaches to see
what the tide left behind. Seashells are admired by many and often
collected by children. These natural wonders, strewn along beaches like
jewels from the sea, are created by snails, clams, scallops, and other
marine mollusks. For a very long period of human history however, these
shells were far more than the evacuated homes of sea creatures and
objects of childhood fascination. They held a position of value, being
used as money, medicine, precious ornaments and even used in art.
The shells of some snails were also important to our ancestors. In
the16th century, natives of Central America poured Purpura Patula snails
into cauldrons and crushed them. The mashed snails shells would ooze a
purple dye which was used to colour cloth. By 1648, the natives had
started exporting this dye to Spain. There was such a high demand for
the dye, the natives were forced to find ways to maintain their supply
while not endangering the population of snails. By imposing conservation
measures, they instead learned to pluck a snail off the rocks, gently
blow into its shell and collect the dye that trickled out. The snail was
then returned to the rocks unharmed.
Other cultures dyed cloth with mollusk juice. When Antony and
Cleopatra sailed in the battle of Actium, their sails were coloured "tyrian
purple."
This famous colour was derived from shellfish in the Murex family.
The shells were used for this purpose over three and a half thousand
years ago on the island of Crete and possibly as far back as Neolithic
Man.
Seashells occur in an amazing range of shapes, colours 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 feet long
and weigh 500 pounds!
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 colonising 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.
Mollusks can make shells because their blood is rich in liquid
calcium. They concentrate the calcium in areas and separate it from the
blood, forming calcium carbonate crystals. The crystals are deposited in
layers of varying size, shape and orientation. The layered construction
strengthens the entire shell. Colours in shells are created by pigments
found in food.
The formation of spines, grooves and ribs on shells aid in protecting
the inhabitant and in some cases add strength. Production of new shell
material is influenced by several factors: sexual hormones, intrinsic
rhythms, diet, acidity of water and temperature of water.
You can tell a lot about the world a mollusk lives by looking at the
shape of its shell. A shell that's low and wide might indicate strong
currents or many predators. A thinner, more spherical shell probably
came from deep water or from areas around the north and south pole.
These are places that are poor in calcium, unlike rich tropical waters.
On hard seafloors, crawling gastropods have coiled shells or flat,
saucer-like shell cases that allow them to retreat into the shell when
in danger.
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