Hello Ashley,

Shell spines and other external scupture are secreted by the same organ
which secretes the rest of the shell, the mantle.  Formation of external
sculpture occurs at the same place as linear growth of the shell body, at
the lip of the shell, where new shell material is secreted by the mantle.
This is true of both gastropods and bivalves. Once a spine, nodule, rib, or
other sculptural structure is created at the lip, shell secretion continues
along the lip, causing the lip to grow out beyond the spine.  Once the lip
progresses beyond a sculptural structure, that structure does not grow
further.  If a spine becomes broken after that point, the animal cannot
repair it because it is beyond the reach of the mantle.  The mantle also
secretes on the interior of the shell, but that secretion results only in
increased thickness of the shell body, not linear growth and not external
sculpture.

In a gastropod, the spiral growth pattern of the shell results in sculptural
elements effectively "rotating" around the shell.  The spines which form
along the right side of the shell (the lip) at the end of one growth period
will move to the dorsal position during the next period of growth, and those
same spines will further rotate to the left side of the shell during the
growth period after that.

Paul Monfils



----- Original Message -----
From: "Ashley Spring" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, December 04, 2005 11:35 AM
Subject: Spine formation


> Conchologists and Malacologists,
> How are spines formed in mollusc shells?  Is there a body part that pushes
> the calcite or aragonite up?  Is there a difference in the calcite or
> aragonite structure of spines compared to the structure of the rest of the
> shell?
>
> I have been searching this subject for a while and there seems to be no
> literature examining spine formation in molluscs.
>
> Thank you,
> A. Spring
> PhD student of marine biology
>
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