A floating Anodonta (or other thin-shelled unionid) is always a dead
unionid.  In our area (Upper Mississippi River) usually the only time you see
a floating unionid is after spring ice-out (in the more lake-like areas or
the backwaters of the Mississippi River).  We seldom see this except in the
spring (~April) of the year.  The only other time you see floating shells (or
decomposing unionid animals, separated from their shell) is after some type
of condition that caused low dissolved oxygen, or perhaps even a chemical
spill from a barge or train, or industrial or agricultural runoff of some
type.
    However, from 1982-1886 we had a considerable "dieoff" over a 400 mile
reach of the Upper Mississippi River.  The die-off continued throughout the
summer for several years.  No one was ever able to figure out the cause (the
die-off was not from low oxygen levels, or low water levels), but eventually
it ended.
    In 1986 there was a Symposium devoted to unionid die-offs in the U.S.
After listening to all of the presentations, it was obvious to me that there
were a variety of reasons for die-offs in different rivers and areas of the
US.  But none of the die-offs were like the one on the Mississippi
River---where the shells, or unionid animals from heavier shelled species,
were floating on the surface of the river.  They do not "float" in order to
find a new location (as I once read).  They only live in, or on, the
substrata.
Marian
Marian E Havlik
Malacological Consultants
1603 Mississippi Street
La Crosse, Wisconsin 54601-4969 USA
[log in to unmask]