Frequently Asked Questions
Shells and the Animals That
Make Them
Why are some
shells so shiny? Does someone varnish them? Or do they get polished
by the sand?
Someone may indeed varnish a shell, but this is not the usual
way. Some kinds of shells are shiny because of the way the animal
lives, but the sand isn't the polishing agent -- the animal
itself is. When it is extended from its shell to crawl or feed,
its body slides up around the shell like a cape; indeed, it's
called a "mantle." Some mollusks cover their entire
shell, others only a part. Since the mantle that covers the
shell is also the part that actually manufactures shell, it
can add tiny amounts of shelly substance as it enfolds the shell,
smoothing and shining it, keeping its surface free of growths
and encrustations, and from dulling by chemicals in the water.
I've seen
purple, bright pink, even bright yellow and green! Where does
the beautiful color come from?
The shells are not, as is often thought, painted or dyed. The
colors are sometimes determined by heredity but in some species
can also be influenced by the diet of the animal. Color in mollusks
often serves as camouflage, but some pigments are primarily
structural in function, serving to strengthen the shell. The
yellows and reds of beta carotene are an example. Other colors,
usually iridescent "mother-of-pearl" hues, are due
to light refraction in combination with the actual structure
of a translucent shell material.
How do they
make all those beautiful patterns?
The patterns and designs on shells are produced naturally by
the animal that makes the shell, and the pattern is actually
part of the shell, not a surface ornamentation. Each species
has some patterns that are common to its members.
How many
different kinds of shells are there?
There are between 50,000 and 200,000, species of mollusks, depending
on who's doing the counting. These figures are based on the
number of species that have been described, and which of those
the count accepts as valid, and the estimates of the number
of undiscovered and undescribed species remaining on earth.
Why are
there so many different shapes of shells?
Mollusks' shapes are a product of heredity combined with habitat
and life style for the most part. Shell shapes have evolved
to make their lives easier. A snail that burrows through sand
needs a shell that will move through wet sand easily -- best
is a smooth and slender and gradually tapering shell, narrow
end at the front, with no impeding projections, but mollusks
have worked out many variations on this theme. A shell that
needs lots of camouflage may have evolved a spiny or irregular
surface which will catch and hold all sorts of camouflaging
encrusting organisms. Spines are also helpful for discouraging
predators and, in some arrangements, for life on mud...broad
weight distribution.
How big
do shells get?
The biggest marine snail is Syrinx aruanus, the Australian Trumpet,
at 77.2 cm. (over 30"). The biggest American marine snail,
the Horse Conch, Pleuroploca gigantea, is just over 2'. The
world's biggest clam is of course the reknowned Giant Clam of
the southwest Pacific, Tridacna gigas. The largest one on record,
in the American Museum of Natural History, measures in at almost
55". (Incidentally, their favored diet is not divers but
algae they farm within their own bodies.) These sizes are taken
from Wagner and Abbott's World Size Records, published by American
Malacologists and edited by Barbara Haviland of St. Petersburg,
Florida.
Why can you
hear the ocean in a seashell?
You can hear the sound of the "ocean" not just with
seashells, but also with a coffee cup or glass, or even by cupping
your hand loosely over your ear. The usual explanation is that
the seashell amplifies the sound of blood moving in your ear.
Two pieces of evidence suggest that this is not correct. First,
the sound is the same before and right after exercise (try running
up and down stairs or doing jumping jacks), but it should be
louder after you exercise since blood would be moving faster.
Second, the sound is not drowned out by loud noise such as that
heard next to a window mounted air conditioner. A better explanation
is that the shell (or glass) acts as a resonating chamber, bouncing
surrounding sounds back and forth, jumbling and amplifying them.
This means that you should be able to hear the "ocean"
better in a noisy room than in a very quiet one.
Taken from an article in
the American Conchologist, March 1995 (p. 21), by Gary Rosenberg,
Academy of Natural Sciences of Philadelphia
