I really think the dark color on the area around the Strombus raninus was
due to age. When strombs mature they grow their flaring lip and then don't
grow any more. However, constant rubbing of the parietal face when the
animal moves makes it rough, which the animal doesn't like, so it lays down
more shiny shell material - over and over. Eventually the shell in that
area may be nearly an inch thick. Each layer of nacre has some dark pigment
in it so the more there is the darker the shell looks. Very old shells,
called "sambo" forms, have almost black parietal shields.
 
Incidentally, as the animal ages, it can't grow any more and since it's
still laying down shell material the mass of the animal itself actually
reduces in size and weight. Therefore, the most efficient collecting from
the conch fisherman's point of view is just before or just after the lip
has flared, when the animal is at its largest and the shell is still
relatively light in weight. The trouble is, the critter hasn't had much (or
any) chance to lay eggs by then, so this contributes to the decline of the
species.
 
Peggy Williams
 
                     http://www.mindspring.com/~shellelegant
                                  Peggy Williams
                                   Shell Elegant Tours
                 PO Box 575     *     Tallevast FL 34270
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