Actually I had the same request earlier in the week and here is how I replied:

The last worldwide revision on the Order Unionoida was done by Fritz
Haas in 1969.  In his exhaustive monograph he recognized about 835
taxa.  However, of those 835 he recognized 353 North American species
of which we currently recognize only about 300 (so he overshot the NA
species by about 50).  So if you subtract 353 from his total of 835
you get 482 in the rest of the world.  I have NO idea if he is even
in the ballpark for the species that exist outside NA, but no one has
worked on compiling species lists for each continent and comparing
those numbers with Haas' estimate, so long story short, we will run
with his number of 482.  Now we add 300 back to the 482 and we get a
total of 782 worldwide.  As a side note I am compiling a database of
all of the nominal species of mussels worldwide (basically databasing
Haas) and I have well over 3100 names in the file.  Given the crazy
SOB's in Europe describing every three differences in a shell as a
new species (and running up the synonomies into the hundreds for each
species recognized today) there are well over 5000 names in the
literature.  Probably more than you wanted to know.

>
>Date:    Tue, 14 Nov 2000 19:05:15 -0500
>From:    Alan Gettleman <[log in to unmask]>
>Subject: Re: Freshwater
>
>Thomas E. Eichhorst wrote:
>>
>>  Quick question for the freshwater folks.  I have been told there are about
>  > 300 freshwater mussel species in the US.  Is this right?  How about
>>  worldwide?
>>
>>  Thanks,
>>
>>  Tom Eichhorst in New Mexico, USA
>
>Tom,
>
>I'll answer that question if you answer how many land or marine spp. are
>there in the United States or world.  The real musselheads of the world
>Kevin Cummings and Tom Watters and (I praise Harry Lee by also including
>him in this group) can provide more precise data, but here is a start:
>The "best" checklist listing, by my count 305 spp. is Common and
>Scientific Names of Aquatic Invertebrates from the United States and
>Canada: Mollusks. Second Edition.  American Fisheries Society Special
>Publication 26.  Edited by Donna D. Turgeon, et. al. 1998. I beleive the
>cost is about $60. with CD-ROM.
>Non-U.S. numbers are harder to come by......
>
>Now I am curious also.  What other lists are out there for non U.S.
>Unionids??
>
>Alan Gettleman
>Merritt Island, FL
>PLAN TO ATTEND COA 2001 CONVENTION AT CAPE CANAVERAL, FLORIDA July
>7-11,2001.
>


There are many but it is very hard to compile and compare to Haas'
estimte.  Sounds like a good project for this winter!


Kevin
--
Kevin S. Cummings
Illinois Natural History Survey
607 E. Peabody Drive
Champaign, IL 61820
[log in to unmask]
http://www.inhs.uiuc.edu/cbd/collections/mollusk.html