There's a fair amount of chromosomal data on mollusks, though I don't
know to what extent the sex chromosome(s) are identified.  Haploid
numbers range widely and can be very useful taxonomic markers (for
example, Amphipeplinae in Lymnaeidae is characterised by having fewer
chromosomes than other lymnaeids).  Total chromosome numbers can reach
over 200 in the highly polyploid sphaeriids; perhaps even stranger is
the fact that most freshwater corbiculids manage to have a degree of
sexual reproduction (though essentially clonal) as triploids.  Ploidy
can be a species-level indicator, as in some of the African planorbids
where shell form varies little but chromosome number tells you if it
can carry schistosomiasis.

Sex determination probably varies a good deal, given the range of
patterns.  Probably the majority of mollusks have separate sexes, but
there are several large groups that are characteristically
hermaphroditic throughout their life (e.g., at least most of
Euthyneura, Anomalodesmata, plus individual species such as
Utterbackia imbecilis and Toxolasma parvum), some that are mostly or
entirely parthenogenetic (e.g., many thiarids), and some that normally
change sex during their lifetime, including not only Crepidula but
also Ostrea edulis, which normally switches sex during the
reproductive season (i.e., each reproductive individual produces one
type of gamete and then switches to the other).

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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