Dear Tom and Paul,
Paul's opinion is one possibility which can be observed almost yearly also
in the river Nile,Egypt. During the dry season the water level drops several
meters in the lower course of that river (due to the many dams upstreams).
Both snails and mussels try to dig as deep as possible in the mud in order
to survive such a dry period. However, many succomb and dry out within the
shells. The shells of Corbicula remain tightly closed when they dry out and
are filled in part with air.
The same happens with large prosobranch gastropods which do not survive the
long hot summer. The opercula are usually stuck so deep in the aperture,
that they remain there even after there death. In this way specimens of
Pila, Lanistes, Bellamya and Cleopatra the shells become filled with gasses
too when they dry out.
When water is released again during the rainy period all these dead
specimens start to float and the stream carry them slowly but steadily
towards the Mediterranean Sea. In this way almost each year you can find
such Nilotic molluscs washed ashore in fair numbers on the Mediterranean
beaches of Israel.

An other event may also lead to a wave of floating mussels. In August 1997
thousands of Anodonta cygnea were seen floating in canals in the northern
part of the province of North-Holland, the Netherlands. All specimens turned
out to be dead and partly filled with air. Most probably they died during a
temporary spell of low oxygen contents of the water.
The tight muscles kept the valves closed but the decomposing animals created
gasses within the tightly closed specimens. The moment these mussels became
lighter than water they started to float.

Living mussels are too heavy to disperse themselves by floating on the
surface of streams, lakes, etc.

Best regards,
Henk K. Mienis
National Mollusc Collection
Dept. Evolution, Systematics & Ecology
Hebrew University of Jerusalem
IL-91904 Jerusalem, Israel
E-mail: [log in to unmask]

----- Original Message -----
From: Monfils, Paul <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, June 18, 2001 6:10 PM
Subject: Re: Freshwater Mussel Question


> Here's an opinion based not on personal experience, but simply on what I
> hope is logical deduction.  The density of a mussel (both shell and
internal
> anatomy) is greater than that of water.  So, when placed in water, and
> filled with water, they necessarily sink.  The only possible way a mussel
> could float would be for the shell to be filled with entrapped air.  There
> is no way a mussel could fill its shell with air while submerged.
Therefore
> the mussels in question must have been stranded high and dry at some
point,
> allowing the water to drain out of their shells.  Mussels so exposed are
> likely to expire rather quickly.  Many species can close tightly, forming
a
> watertight seal, and retaining water in the shell until they die, the
> aductor muscle relaxes,  and the shell gapes slightly, allowing the water
to
> leak out.  A mussel that was so stranded, and did manage to survive until
> the water returned would quickly open its valves and fill with fresh
water.
> Therefore my guess is that while the mussels described by Mr. Morgan may
> have been complete (with the soft parts still inside the shell), it is
very
> unlikely they were alive.
>
> Paul M.