> Now I'm wondering why do shells mature and grow teeth years earlier.  Is
> this a need
> for population ?
>

Probably there are a range of triggers.  In many species, temperature
plays a role.  Under normal conditions, this provides a cue for
seasonal patterns.  However, if a larva settles outside the normal
range, or if the temperature regime is changed, then that can throw
off the development and produce anomalies.  For example, the
frequently large size of may species in Bermuda has been attributed to
the somewhat cooler conditions there in comparison to the Caribbean
(where larvae come from to Bermuda, thanks to the Gulf Stream).  If
it's never quite warm enough to tell the animal "start gettign ready
to reproduce", then it keeps putting energy into growth rather than
reproduction.  In freshwater, dams generally alter the temperature
regime.  Deep lake water is cold year-round; shallow impounded water
is warmer than shallow flowing water; and downstream of the dam could
be hot or cold depending on how the dam releases water.  None of those
are good for proper reproduction of freshwater mollusks that are
waiting for a seasonal temperature cue.

The presence of conspecifics can play a role, although I don't know of
many cases where this is well-documented.  Stacking Crepidula is
perhaps the best-studied example, where settling on a new spot versus
on top of someone else or getting settled upon determines the
development.

--
Dr. David Campbell
4830 University Blvd E H4
Tuscaloosa AL 35404
"I think of my happy condition, surrounded by acres of clams"

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