"Mysterious cowries of New Caledonia" by R. Pierson Et G. Pierson, is
probably the primary book on the subject of Melanism and Rostration.  It
is wonderfully illustrated showing very many combinations of these two
abnormalities, and ponders the cause at great length. Most causes are
excluded.
 
One interesting thing is that photos show that the mantles of melanistic
Cypraea are different (primarily in color) from normal specimens. The
mantle base is more black than normal.
 
Both abnormalities occur separately and in combination.  They affect
only a small portion of a population in the same area.  At one time, I
spent a week in the primary melanistic area, and found one melanistic C.
arabica but about a hundred normal specimens.  I think this is confirmed
as typical.
 
Pierson lists the species that have the various combinations of
abnorality present.
 
Pierson says that juvenile specimens are never effected (they never
found a melanistic or rostrate juvenile) so these traits are added after
the shells form a base, or after they are adult.  Does anyone have
specimens to refute this?
 
Pierson says that they never found a melanistic or rostrate individual
laying or associated with eggs.  This intimates that they don't
reproduce so that the trait is not handed down genetically.
 
As to minerals being the cause, they are present in many places in New
Caledonia that do not have melanistic or rostrate Cypraea.  In Noumea,
there are the main refining plants, with considerable transfer of metals
to the ocean, but there have never been found melanistic Cypraea mappa
though they are fairly common and the same with other species.  Pierson
eliminates the metal as the sole cause.
 
Pierson lists 18 species that have never been found to be melanistic or
rostrate that are found in the areas involved in 38 other species, and 4
species that have never been found melanistic or rostrate that are found
only outside the melanistic/rostrate areas.
 
Pierson's photo of a very black C. arabica has a mantle that has normal
papillae (to me) but the base of the mantle is more grey than normal.
 
Pierson didn't finish with a final cause, but eliminated a lot.  He was
a medical doctor and makes comparisons with human abnormalities.
 
Work still remains to be done.  Evidently nothing will be proved by
taking a normal Cypraea from other areas and introducing the suspicous
metals into the aquarium.  What experiments would you propose for some
of our malacology students?  Perhaps organizations such as HMS,
COA, AMS, and WSM could be induced to offer grants for that specific
research?
 
Cones spring to mind as having melanistic specimens.  Occur in New
Caledonia in all degrees, and in Apia, Samoa where I have collected
melanistic C. marmoreus.  What other species are melanistic?  What would
compare with Cypraea rostation, and what species have that abnormality?
--
                     Aloha from Wesley M. Thorsson
Editor of Internet Hawaiian Shell News, a monthly Internet Publication
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