> why are so many synonyms are made ? Usually, we say  the author didn't read enough papers, we say the author didn't know the specimem is a young shell.

Even if the author has read a lot of papers, the one he needed to see
might not have been readily available.  Sometimes it's obvious the
author didn't do his homework; sometimes the redundancy takes a lot of
work to track down.

> but i think the main reason is that we don't know how much difference is
> enough to make a species name valid.
> Yes, a criteria exits, that is about hybrids, unfortunately,It doesn't work.

Even among experts in systematics, there is a lot of debate about what
the best theoretical criteria are to distinguish species, much less
about whether particular examples meet the theoretical criteria.
Within any one group, you need to carefully examine variation within
and between species in order to find what seem to be good
distinguishing markers.  The degree of hybridization is a useful
criterion, but as you note, it's often not very practical to test and
even then the results can be unclear.  An easy place to see this is in
comparison of different editions of bird field guides.  Species
recognized change over time as new data come in.  Some species cannot
interbreed.  Some can produce largely sterile offspring.  Some produce
offspring that are fertile but are at a disadvantage because they
don't have just the right moves to really impress a member of either
parent species, or else they aren't as good at surviving in the exact
niche of either parent species.  Some produce hybrids that reproduce
successfully with a parent species, but the parent species largely
stay apart and maintain their differences.  Some produce hybrids that
do not reproduce with either parent but can reproduce theselves as a
new hybrid species.  A lot of those reproduce asexually, a situation
that the hybridizaiton definition can't handle.

Species are continually evolving, so it is no surprise that their
boundaries are fuzzy.

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

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