John,

>  I have never seen gastropods crack up.  Does it occur only in bivalves?

No there are also gastropods that crack. One specific example are members
of the genus Paryphanta.

I suspect that the cause of the cracking is due to the the different
expansion and contraction rates of the periostracum and mineral matrix.
For example, if the mineral matrix is thin and the periostracum relatively
thick, when the relative humidity varies, these layers will contract (or
expand) at different rates. The mineral matrix may break while the thicker
periostracum holds tight.

The converse would also be true. If the mineral matrix was thick (think
Mercenaria) and the periostracum thin, the matrix will not crack yet the
periostracum will crack and flake off.

Regards,
Charlie
.................................................
Research Associate - Section of Mollusks
Carnegie Museum of Natural History
Pittsburgh, PA, USA

Assistant Professor - Family Medicine
Fellow-American Academy of Family Practice
Fellow-Academy of Wilderness Medicine

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