> I was wading in the local creek (central Indiana) when I noted a
> single valve in
> the foot-deep water. Hoping to stir up its mate, I came up, instead,
> with a
> live clam. At a mere 3/4-inch across, it was rather small,... to
> someone who
> has seen three foot wide Tridacna gigas in situ.
>
> Since it is illegal in this state to disturb or molest ANY mollusk, I
> tossed it
> back after noting that the valves were dark brown at the hinge but
> quickly gave
> way to an algae-green color other the rest of the shell. I estimated
> about 15
> "growth rings" on it.
>

Sounds too big for a sphaeriid (fingernail clam).  If the "growth
rings" were regular concentric sculpture, then you had Corbicula.
Despite most of the literature, the main species of Corbicula invading
the U.S. is probably C. leana.  A second species is known only from
the southwest (also invading Europe and South America), which may be
true fluminea (of which manilensis is a synonym).

Unionids may show bands of color or weak irregular growth lines, but
no regular concentric sculpture in our species.

As sculpture rather than growth rings, they form fairly quickly and
without direct connection to environmental factors.  Tiny juveniles
already have several concentric ribs.

Corbicula range from oval (not very elongate) to nearly triangular in
some large old individuals in lakes.  Most unionids are more
elongate.

--
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA

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