Ross said:
 
>Some ineresting info coming up.  Seems the primary cause is injuries to,
>or abberations of the mantle, at least so far as patterns and pigments
>are concerned.
 
Also shell deformities of shape. and the directions those take are legion!
I've seen some that actually have a high spire and resemble, when viewed
from the dorsal side only, a huge marginella or an obese olive.
 
 Has any anyone, however, done any biochemical or genetic
>work, to discover whether melanism is caused by a mutation (Why do only
>a small minority of any given population ever become melanistic???), and
>what the biochemistry of the phenomenon is?
 
As I understand what I have read of melanism, as I said earlier, some of it
is not melanin, but actually red pigment (beta carotene?), laid down so
heavily that it  looks black. Other species lay down massive amounts (by
normal pattern standards) of one of their pigments. Not all contain red or
black pigment in their shell patterns. So it turns out in these cases that
it is just an excess of pigment. That is probably the case with melanin
too. (By the way, I have read and heard (who knows where!) that the pigment
in shells is metabolic waste product. I guess the mollusk deposits it in
its shell like trees deposit it in their leaves, but trees have chlorophyll
to cover it up til autumn.)
 
What resources I have on hand at the moment indicate that melanin is a dark
pigment  found  in hair, skin, feathers, scales and some internal
membranes, and in the peritoneum of many animals. Our skin has melanin in
varying amounts, and when we spend time in the sun, there is an increase in
melanin pigment resulting in a tanning. The darker the natural skin tone,
the more melanocytes (pigment cells) to start with. Black pigmentation is
advantageous in four ways according to this darn volume of Encyclopedia
Brittanica that seems stuck on my desk for the duration.
1. Acts as a barrier against the effects of ultraviolet rays of sunlight.
2. provides a mechanism for absorption of heat in cold environments by
small cold-blooded animals like insects.
3. Affords concealment at twilight and night.
4. Allows greater visual acuity by absorbing scattered light within the eye.
 
Would any of those advantages apply to Prony Bay cowries?
Melanin is formed as an end product of the metabolism of the amino acid
tyrosine. (Anybody getting bored?  Skip a paragraph. This gets chemical!)
 
Tracing backward in the chain, the amino acid Tyrosine  comprises from 1-6%
of the mixture obtained by the hydrolysis of most proteins. It is
particularly abundant in insulin and in papain, the papaya enzyme. It was
first isolated from casein (in cheese). It is one of the essential amino
acids for many animals (they cannot synthesize it themselves and so must
get it through diet) while others (mammals and fowl) can synthesize it when
needed.
 
So is somebody feeding the cowries in Prony Bay and the Keppel Island
region of Queensland an excess of cheese and papayas in order to give them
artificial suntans?  That solution ought to appeal to you, Ross?  ;-)
Anyway, there you have the extent of my resources at hand.
 
 Also, i have heard nothing
>but  a vast silence regarding the **precise** cause of rostratism, and
>how it is linked to melanism.
 
'Twould be my guess, as I said earlier, and that of other, and far more
recognized and respected students of this group (check out Burgess and
Pierson) seem to be agreeing with me, that senility in the presence of "one
or several substances present in the substrate" causes senile lesions. They
point out that cowries have a "highly developed neuroglandular secretive
system"  and that the substance(s) would "perturb" the regulation of
enzymatic secretion.
 
It also occurs to me that conchs and oysters etc. produce pearls to smoothe
over the irritating sharp edges of a foreign body within their shells.
Cowries have this acknowledged "highly developed neuroglandular secretive
system" and since their mantles are in daily contact with the exterior
surface of their shells, given an irritant perhaps constantly settling on
these shells, might it not be reasonable to assume that the rostration is a
result of the mantle depositing extra shell each time to cover that
irritant, as if its entire shell were a foreign body. And that this would
be most extreme at the head and foot ends where the animal virtually must
emerge to move from place to place? Thus the heavy shells and the
rostration? Not ***precise***, Ross, but it'll have to do til somebody more
knowledgeable in the field comes along. (Lindsey???)
 
These oddities are limited to Prony Bay at the southern end of New
Caledonia, according to B. & P.  AND the melanistic and rostrate cowries
were recorded before the nickel mines were opened. This doesn't address
Queensland' melaninistic/melanotic cowries.
 
By the way, Conch-L list manager Amy Edwards wrote a REALLY interesting
article on "The Color of Shells"  for American Conchologist (V. 22 No. 1,
March, 1994 p. 10) which would help a lot of us understand more about
color. Some of it is way too much physics for me, but Amy makes it really
much easier to understand. Check it out. I keep meaning to get it up on the
Conch-Net on the American Conchologist page, but it never seems to get done.
 
Phew. I've done all the thinking I am capable of this morning. Off for a
walk.
 
Lynn Scheu
[log in to unmask]
Louisville, KY, Home of the 1999 COA Convention where we will serve NO
papayas (but maybe a little cheese) at the Conch-L party.