Sylvia and other Conchlers,
 
The number of molluscan species is only significant when the number is put
to use somehow, so I think it's fair of me to ask, "Why did you want to
know?" Here are some reasons why I'd like to know.
 
First off, obviously, we all want to know how many species there would be
in an imaginary Museum of All the Mollusca. It would be well worth knowing
how complete are the world's greatest collections, who collected the most
species in a lifetime, and how they did it.
 
I'd like to know how many of those species live in each major type (biome)
of biological environment (deepsea, coastal, freshwater, land, etc.). Which
one has the greatest diversity of mollusks overall? Which one has the
greatest diversity of mollusks per area? (The deepsea environment is the
largest one on the planet; the freshwater biome is comparatively small.)
 
How diverse are the mollusks compared to other major groups (phyla) of
animals, such as echinoderms, arthropods, and chordates? What has made the
mollusks so successful compared to other animals? Does their success depend
on particular features of their biochemistry? behavior? anatomy? Was their
success inevitable in Earth history, or was it a matter of luck (being
there at the right time and place)?
 
Have the mollusks always been this diverse, or has their number of species
increased or decreased through time? Why?
 
Are the mollusks equally diverse in every climatic zone? Of course not, but
where is the diversity the highest, and how did that happen?
 
Which kinds of mollusks are the most diverse: those that burrow in the sand
or mud, those that bore into rock, or those that lie on the surface of the
seafloor?
 
Which classes and families within the Mollusca are most diverse? On the
whole, are the rare species more diverse, or the common ones?
 
Given two molluscan provinces on either side of a continent such as North
or South America, do these provinces have the same diversity?
 
In the Permian Period, there was only one large continent, Pangaea. Today
the continental land masses are broken up and dispersed around the globe.
Does the arrangement and number of continents make a difference in
molluscan diversity, even if the land area is the same?
 
How many of the world's mollusk species are recently extinct or threatened
with extinction? And are these mollusks concentrated in any particular
region, biome, or climatic zone? And again, why?
 
Well, Conchlers, you have now gone through a list of questions that might
be presented to a doctoral candidate in malacology at his or her oral and
written examinations. The books of Peter Dance, Geerat Vermeij, and James
Valentine answer most of them. Of the Conchlers, Gary Rosenberg has spent a
great deal of his career answering some of these questions. I don't intend
to answer the questions on-line, since these other people have already done
a great job in print, but I just want to suggest some of the reasons why
the question, "How many species of mollusks are there?" is a lot deeper
than it sounds at first. It really is worth an answer.
 
Now, how does someone go about counting all the mollusk species that have
ever been named? It would help if someone had prepared a checklist. As it
turns out, the Zoological Record has attempted to do exactly that on a
yearly basis for ... hmm... if memory serves, about 150 years. Only about
half of the way back to Linnaeus, but a far way nonetheless, and probably
including most species that have ever been named. No one has yet compiled
their annual lists into a single long list, as far as I know, but the ZR is
being placed on-line and it will not be many more years before such a list
can be compiled electronically, and very quickly. Check out their Web site.
 
Such lists have been compiled for various regions, however. These lists
could be collated to make a "master list" of Mollusca. Tucker Abbott, in
"American Seashells" (1974), recognized 6500 species from the seas off
North America. Gary Rosenberg has posted a regional list for the Western
Atlantic on the Web. It may be too big for some people to access (I know my
old computer couldn't do it). Does anyone care to announce the number here?
 
Abbott also compiled a master list of Land Snails, but I don't have that
book here at the office. (Anyone have it handy?)
 
The Treatise on Invertebrate Paleontology is of some help, because this
series attempts to describe every genus of fossil invertebrate. Genera, not
species, but that's something. So far, there are six volumes on Mollusca,
some of them 30 years out of date, but only one volume has been published
yet on the Gastropoda. That's a lot of fossils. Warren Allmon, who heads
the Paleontological Research Institution in Ithaca, New York, is also the
head of a massive, long-term project to complete the part on Gastropoda.
Anyway, one could simply count the valid genera in the indices; the invalid
ones are conveniently italicized and the valid ones are not. Sepkoski has
compiled the numbers of genera per family for each invertebrate group
through time from the Treatise. This compilation is flawed because the
Treatise is flawed, but the data exist. (Sorry, the University of Alabama
didn't buy that book; I've never seen it!)
 
Finally, some museum collections have been cataloged in such a way that the
number of species can be obtained without difficulty. (Kurt, if you're
lurking out there, this is your chance to show us how it's done!) At least
in principle, it will not be many tens of years before most museums have
such catalogs, and this is another potential source of information on
numbers of species form particular regions.
 
The best such lists are those that are compiled by a single person, or a
small team, from the specimens themselves, not from lists in books, not
just because so many (half?) of the species names are synonyms (tip of the
hat to Helmut), but because a great many museum specimens have old and
inaccurate labels (thanks, Kurt).
 
So it will take a lot of work to compile even a rough checklist of the
world's molluscan species. But we will probably have that within a few
years, as the Zoological Record digitizes its back issues.
 
Have a great weekend, shell maniacs!
 
Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA